Q U E S T I O N page no: 18
Question: 1. You have been provided with three test tubes. One of them contains distilled water and the other two contain an acidic solution and a basic solution, respectively. If you are given only red litmus paper, how will you identify the contents of each test tube?
Answer: Identifying Solutions Using Red Litmus Paper
To determine the contents of the three test tubes using only red litmus paper, follow these steps:
1. Test the First Tube:
– Dip the red litmus paper into the first test tube.
– Observation:
1. If the litmus paper remains red, the solution is either distilled water or an acidic solution.
2. If the litmus paper turns blue, the solution is basic.
2. Test the Second Tube:
– Move on to the second test tube and repeat the process with the red litmus paper.
– Observation:
1. If the litmus paper remains red, this indicates that the solution is acidic.
2. If it turns blue, then this solution is basic.
3. Test the Third Tube:
– Finally, test the third test tube with the red litmus paper.
– Observation:
1. Again, if the litmus paper remains red, the solution is acidic.
2. If it turns blue, the solution is basic.
– Conclusion
Identifying Solutions:
1. The test tube where the litmus paper turns blue contains the basic solution.
2. The test tubes where the litmus paper remains red contain the acidic solution and distilled water. To differentiate between these two, you can use another indicator or method (like tasting, which is not recommended in practice).
This method effectively utilizes the properties of acids and bases to identify the nature of the solutions in the test tubes.
Q U E S T I O N S page no: 22
Question: 1. Why should curd and sour substances not be kept in brass and copper vessels?
Answer: Curd and other sour substances should be avoided in brass and copper vessels due to the following reasons :
1. Chemical Reaction: Brass and copper are metals that can react with the acids present in sour foods, such as curd. This reaction can lead to the formation of metal salts, which may not be safe for consumption.
2. Toxic Compounds: The reaction between the acids in these substances and the metals can produce toxic compounds, such as copper salts, which can be harmful if ingested.
3. Taste Alteration: The interaction between the acidic components and the metal can also change the flavor of the food, making it taste unpleasant.
For these reasons, it is recommended to store curd and sour substances in glass, ceramic, or food-grade plastic containers, which do not react with acids and ensure food safety.
Question: 2. Which gas is usually liberated when an acid reacts with a metal?
Answer: When an acid reacts with a metal, hydrogen gas (H₂) is typically liberated.
– Example Reaction:
1. A common example is the reaction between zinc and hydrochloric acid: [ \text{Zn (s) + 2HCl (aq) → ZnCl}_2 \text{(aq) + H}_2 \text{(g)} ] In this reaction:
2. Zinc displaces hydrogen from hydrochloric acid, resulting in the formation of zinc chloride and the release of hydrogen gas.
– Testing for Hydrogen Gas:
To confirm the presence of hydrogen gas, you can bring a burning candle or a lit matchstick near the gas. If hydrogen is present, it will produce a characteristic “pop” sound, indicating its liberation.
– This reaction illustrates the typical behavior of metals when they come into contact with acids, highlighting the evolution of hydrogen gas as a common outcome.
Question: 3. Metal compound A reacts with dilute hydrochloric acid to produce effervescence. The gas evolved extinguishes a burning candle. Write a balanced chemical equation for the reaction if one of the compounds formed is calcium chloride.
Answer: When a metal reacts with dilute hydrochloric acid, it typically produces hydrogen gas (H₂) and a corresponding salt. In this case, if the metal is calcium (Ca), the reaction can be represented as follows:
Balanced Chemical Equation: [ \text{Ca (s) + 2HCl (aq) → CaCl}_2 \text{(aq) + H}_2 \text{(g)} ]
– Explanation:
1. Reactants: Calcium (Ca) is the metal that reacts with dilute hydrochloric acid (HCl).
2. Products: The products of the reaction are calcium chloride (CaCl₂), which is an aqueous solution, and hydrogen gas (H₂), which is evolved as effervescence.
– Testing for Hydrogen Gas:
To confirm the presence of hydrogen gas produced in this reaction, you can follow these steps:
1. Collect the Gas: Conduct the reaction in a test tube to capture the gas that is being released.
2. Bring a Burning Candle: After collecting the gas, bring a burning candle or a lit matchstick close to the mouth of the test tube.
3. Observation:
– If hydrogen gas is present, it will ignite with a characteristic “pop” sound. This indicates the presence of hydrogen.
– Additionally, hydrogen is flammable and will extinguish a burning candle if the gas is allowed to escape into the air.
This reaction illustrates the typical behavior of metals when they react with acids, highlighting the evolution of hydrogen gas as a common outcome.
Q U E S T I O N S page no: 25
Question: 1. Why do HCl, HNO3 , etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?
Answer: Explanation:
1. Acidic Character of HCl and HNO3:
– Hydrochloric acid (HCl) and nitric acid (HNO3) are classified as strong acids. When these acids dissolve in water, they dissociate completely to produce hydrogen ions (H⁺) and anions (Cl⁻ for HCl and NO₃⁻ for HNO3). This release of H⁺ ions is what imparts the acidic character to these solutions.
– For example: HCl → H⁺ + Cl⁻HNO3 → H⁺ + NO₃⁻
2. Lack of Acidic Character in Alcohol and Glucose:
– In contrast, compounds like alcohol and glucose do not dissociate to produce H⁺ ions when dissolved in water. Although they contain hydrogen, they do not release it in the form of H⁺ ions. Therefore, they do not exhibit acidic properties.
– For instance, when glucose (C₆H₁₂O₆) is dissolved in water, it remains as intact molecules and does not ionize to produce H⁺ ions.
– Conclusion
The key difference lies in the ability of substances to dissociate in water to release H⁺ ions. Strong acids like HCl and HNO3 do so, which gives them their acidic properties, while alcohol and glucose do not, hence they do not show acidic behavior in aqueous solutions.
Question: 2. Why does an aqueous solution of an acid conduct electricity?
Answer: Explanation :-
1. Ionic Dissociation:
– When an acid dissolves in water, it dissociates into ions. For example, hydrochloric acid (HCl) dissociates into hydrogen ions (H⁺) and chloride ions (Cl⁻): HCl → H⁺ + Cl⁻
– This dissociation produces charged particles (ions) in the solution.
2. Conductivity of Ions:
– Electricity is conducted through the movement of charged particles. In an aqueous solution of an acid, the presence of these ions allows the solution to conduct electric current. The H⁺ ions move towards the cathode (negative electrode), while the anions (like Cl⁻) move towards the anode (positive electrode).
3. Comparison with Non-Electrolytes:
– In contrast, substances that do not dissociate into ions in solution, such as sugar or alcohol, do not conduct electricity because they lack free-moving charged particles.
– Conclusion
The ability of an aqueous acid solution to conduct electricity is due to the presence of free ions that result from the dissociation of the acid in water. These ions facilitate the flow of electric current through the solution.
Question: 3. Why does dry HCl gas not change the colour of the dry litmus paper?
Answer: Explanation :-
1. Molecular State of Dry HCl: In its gaseous form, dry hydrochloric acid (HCl) exists as molecules rather than ions. For a substance to exhibit acidic properties, it must dissociate into ions that can interact with indicators like litmus paper.
2. Role of Water: Litmus paper changes colour in response to the presence of hydrogen ions (H⁺). When HCl gas is dissolved in water, it dissociates to produce H⁺ ions:
– HCl (aq) → H⁺ (aq) + Cl⁻ (aq)
These H⁺ ions are responsible for the acidic behaviour and the resultant colour change in the litmus paper (turning blue litmus red).
3. Absence of Ionization: In the absence of water, as in the case of dry HCl gas, there are no H⁺ ions available to react with the litmus paper. Therefore, the litmus paper remains unchanged in colour.
– Conclusion
Dry HCl gas does not change the colour of dry litmus paper because it does not dissociate into ions without the presence of water. The acidic properties and colour change are only observable when HCl is in an aqueous solution, where it can produce hydrogen ions.
Question: 4. While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?
Answer: Explanation:
1. Exothermic Reaction : The process of diluting concentrated acids is highly exothermic, meaning it releases a significant amount of heat. When concentrated acid is mixed with water, the heat generated is absorbed by the water, which helps in safely dispersing the heat throughout the solution.
2. Risk of Splashing: If water is added to concentrated acid, the heat generated can cause the water to rapidly vaporize, leading to splattering of the acid. This can result in dangerous situations, including burns or injuries from the acid splashing out of the container.
3. Controlled Mixing: Adding acid to water allows for a more controlled mixing process. The water can absorb the heat gradually, reducing the risk of localized heating and potential boiling or splashing.
4. Safety Precautions: Safety protocols in laboratories emphasize this method to prevent accidents. It is a standard practice to always add acid to water, ensuring that the dilution process is as safe as possible.
– Conclusion
To ensure safety and prevent accidents, it is essential to add acid to water rather than water to acid during dilution. This method minimizes the risk of splashing and allows for better control of the exothermic reaction that occurs during the dilution process.
Question: 5. How is the concentration of hydronium ions (H3 O+) affected when a solution of an acid is diluted?
Answer: Explanation :-
1. Dilution Process: When an acid solution is diluted, water is added to the solution, which increases the total volume of the solution. This process decreases the concentration of all solute particles, including hydronium ions (H₃O⁺).
2. Concentration Relationship: The concentration of hydronium ions in a solution is defined as the number of moles of H₃O⁺ per unit volume of the solution (usually expressed in moles per liter, mol/L). When you add more water to the acid solution, the same number of H₃O⁺ ions is now present in a larger volume of solution. Therefore, the concentration of H₃O⁺ ions decreases.
3. Mathematical Representation:
– If the initial concentration of H₃O⁺ in a concentrated acid solution is C₁ and the initial volume is V₁, when diluted to a new volume V₂, the new concentration C₂ can be expressed as: [ C_1 \times V_1 = C_2 \times V_2 ]
– From this equation, it is clear that as V₂ increases (due to dilution), C₂ (the concentration of H₃O⁺) must decrease.
4. Practical Implication: As the concentration of H₃O⁺ ions decreases, the acidic nature of the solution also diminishes. This means that the pH of the solution will increase (moving towards neutral) as the solution becomes less acidic.
– Conclusion
When an acid solution is diluted, the concentration of hydronium ions (H₃O⁺) decreases due to the increase in the total volume of the solution. This results in a less acidic solution, reflected by a higher pH value.
Question: 6. How is the concentration of hydroxide ions (OH–) affected when excess base is dissolved in a solution of sodium hydroxide?
Answer: Explanation :-
1. Dissolution of Base: When a base, such as sodium hydroxide (NaOH), is dissolved in water, it dissociates completely into sodium ions (Na⁺) and hydroxide ions (OH⁻): [ \text{NaOH} \rightarrow \text{Na}^+ + \text{OH}^- ]
– This dissociation increases the concentration of hydroxide ions in the solution.
2. Adding Excess Base: If additional sodium hydroxide is added to the solution, more NaOH will dissolve, leading to an increase in the number of hydroxide ions (OH⁻) present in the solution.
– The concentration of hydroxide ions can be expressed as the number of moles of OH⁻ per unit volume of the solution. As more NaOH is added, the total amount of OH⁻ ions increases, thereby increasing their concentration.
3. Mathematical Representation: If the initial concentration of OH⁻ ions in the solution is C₁ and the volume is V₁, when more NaOH is added, the new concentration C₂ can be calculated based on the total moles of OH⁻ ions in the new volume V₂: [ C_1 \times V_1 + \text{(moles of added OH⁻)} = C_2 \times V_2 ]
– This indicates that as more base is added, the concentration of hydroxide ions (C₂) increases.
4. Practical Implication: An increase in hydroxide ion concentration results in a more alkaline (basic) solution, which is reflected in a higher pH value. The solution becomes increasingly basic as the concentration of OH⁻ ions rises.
– Conclusion
When excess base is dissolved in a solution of sodium hydroxide, the concentration of hydroxide ions (OH⁻) increases. This leads to a more alkaline solution, resulting in a higher pH value.
QUESTIONS Page no: 28
Question: 1. You have two solutions, A and B. The pH of solution A is 6 and pH of solution B is 8, Which solution has more hydrogen ion concentration? Which of this is acidic and which one is basic?
Answer: In the context of the pH scale:
– Solution A has a pH of 6.
– Solution B has a pH of 8.
1. Hydrogen Ion Concentration
– The pH scale measures the concentration of hydrogen ions (H⁺) in a solution. A lower pH value indicates a higher concentration of hydrogen ions.
– Therefore, Solution A (pH 6) has a higher hydrogen ion concentration compared to Solution B (pH 8).
2. Acidic and Basic Nature
– Solution A (pH 6) is acidic because its pH is less than 7.
– Solution B (pH 8) is basic (alkaline) because its pH is greater than 7.
Summary
Solution A (pH 6): Higher hydrogen ion concentration, acidic.
Solution B (pH 8): Lower hydrogen ion concentration, basic.
Question: 2. What effect does the concentration of H+(aq) ions have on the nature of the solution?
Answer: The concentration of hydrogen ions (H⁺) in a solution significantly influences its nature, determining whether it is acidic, neutral, or basic. Here’s how:
Effects of H⁺ Ion Concentration
1. Acidic Solutions:
– When the concentration of H⁺ ions is high, the solution is acidic.
– For example, a solution with a pH less than 7 indicates a higher concentration of H⁺ ions, which contributes to its acidic properties.
2. Neutral Solutions:
– A solution with a neutral pH of 7 has a balanced concentration of H⁺ and hydroxide ions (OH⁻). This means that the concentration of H⁺ ions is neither high nor low, resulting in a neutral nature.
3. Basic (Alkaline) Solutions:
– When the concentration of H⁺ ions is low, the solution is basic or alkaline.
– A solution with a pH greater than 7 indicates a lower concentration of H⁺ ions and a higher concentration of OH⁻ ions, which gives it basic properties.
– Summary
1. Higher H⁺ Concentration: Acidic solution (pH < 7)
2. Neutral H⁺ Concentration: Neutral solution (pH = 7)
3. Lower H⁺ Concentration: Basic solution (pH > 7)
– Conclusion
The concentration of H⁺ ions directly determines the acidic or basic nature of a solution, influencing its chemical behavior and reactions.
Question: 3. Do basic solutions also have H+(aq) ions? If yes, then why are these basic?
Answer: Yes, basic solutions do contain hydrogen ions (H⁺), but their concentration is lower compared to that of hydroxide ions (OH⁻). Here’s a detailed explanation:
– Presence of H⁺ Ions in Basic Solutions
1. Basic Solutions:
– Basic solutions are characterized by a higher concentration of hydroxide ions (OH⁻) and a lower concentration of hydrogen ions (H⁺).
– For example, in a solution of sodium hydroxide (NaOH), when it dissolves in water, it dissociates to produce Na⁺ and OH⁻ ions: [ \text{NaOH} \rightarrow \text{Na}^+ + \text{OH}^- ]
2. Hydrogen Ion Concentration:
– Even though basic solutions contain H⁺ ions, the concentration of these ions is significantly lower than that of OH⁻ ions.
– The pH of a basic solution is greater than 7, indicating that the concentration of OH⁻ ions exceeds that of H⁺ ions.
– Why Are Basic Solutions Considered Basic?
Definition of Basicity: A solution is termed basic or alkaline when the concentration of hydroxide ions (OH⁻) is greater than that of hydrogen ions (H⁺).
This imbalance leads to a higher pH value, which is a characteristic of basic solutions.
Neutralization Reaction:
– In a neutralization reaction, when an acid (which has a high concentration of H⁺ ions) reacts with a base (which has a high concentration of OH⁻ ions), they combine to form water and a salt: [ \text{H}^+ + \text{OH}^- \rightarrow \text{H}_2\text{O} ]
– This reaction illustrates how the presence of H⁺ ions in a basic solution is not sufficient to classify it as acidic, as the OH⁻ ions dominate.
– Summary
1. Basic solutions do contain H⁺ ions, but their concentration is lower than that of OH⁻ ions.
2. The basic nature of a solution arises from the higher concentration of hydroxide ions, resulting in a pH greater than 7.
Question: 4. Under what soil condition do you think a farmer would treat the soil of his fields with quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate)?
Answer: Farmers often treat their fields with quick lime (calcium oxide), slaked lime (calcium hydroxide), or chalk (calcium carbonate) under specific soil conditions to improve soil quality. Here’s a detailed explanation:
– Soil Conditions for Lime Treatment
1. Acidic Soil:
– pH Level: When the soil has a low pH (typically below 6), it is considered acidic.
– Effects: Acidic soils can hinder plant growth by affecting nutrient availability and harming beneficial soil organisms.
– Lime Treatment: Adding lime helps to neutralize the acidity of the soil, raising the pH towards a more neutral level (around 6.5 to 7). This improves nutrient availability and promotes healthier plant growth.
2. Nutrient Deficiency:
– Calcium Deficiency: Soils that lack calcium can lead to poor plant health. Lime provides calcium, which is essential for plant growth and development.
– Improving Soil Structure: Lime also helps improve soil structure, enhancing aeration and water retention, which are crucial for healthy root development.
3. Soil Salinity:
– In some cases, lime can help in managing soil salinity by improving drainage and reducing the concentration of harmful salts.
Summary of Lime Use :
1. Quick Lime (Calcium Oxide): Used to quickly raise soil pH but must be handled carefully due to its caustic nature.
2. Slaked Lime (Calcium Hydroxide): Safer to handle and effective in neutralizing soil acidity.
3. Chalk (Calcium Carbonate): A slower-acting option that gradually raises soil pH.
– Conclusion
Farmers treat acidic soils with lime to neutralize acidity, improve nutrient availability, and enhance soil structure, ultimately leading to better crop yields.
Q U E S T I O N S Page no: 33
Question: 1. What is the common name of the compound Ca(ClO)₂?
Answer: The common name of the compound Ca(ClO)₂ is bleaching powder.
Additional Information:
Chemical Composition: Bleaching powder is produced by the reaction of chlorine gas with slaked lime (calcium hydroxide).
Uses: It is widely used as a bleaching agent in the textile and paper industries, as well as a disinfectant for drinking water to kill bacteria and other pathogens.
Question: 2. Name the substance which on treatment with chlorine yields bleaching powder.
Answer: The substance that, when treated with chlorine, yields bleaching powder (Ca(ClO)₂) is slaked lime, which has the chemical formula calcium hydroxide (Ca(OH)₂).
Additional Information:
1. Reaction: The reaction between chlorine gas and slaked lime produces bleaching powder: [ \text{Cl}_2 + \text{Ca(OH)}_2 \rightarrow \text{Ca(ClO)}_2 + \text{H}_2\text{O} ]
2. Uses of Bleaching Powder: It is commonly used for:
– Bleaching in the textile and paper industries.
– Disinfection of drinking water to eliminate harmful microorganisms.
Question: 3. Name the sodium compound which is used for softening hard water.
Answer: The sodium compound that is commonly used for softening hard water is sodium carbonate, which has the chemical formula Na₂CO₃. It is also known as washing soda.
– Additional Information:
1. Function: Sodium carbonate helps to remove calcium and magnesium ions from hard water, which are responsible for hardness. This process is known as ion exchange.
2. Usage: When added to hard water, sodium carbonate reacts with the calcium and magnesium ions to form insoluble precipitates, thus softening the water.
Question: 4. What will happen if a solution of sodium hydrocarbonate is heated? Give the equation of the reaction involved.
Answer: When a solution of sodium hydrogencarbonate (commonly known as baking soda, with the chemical formula NaHCO₃) is heated, it undergoes thermal decomposition. The reaction produces sodium carbonate (Na₂CO₃), water (H₂O), and carbon dioxide (CO₂) gas.
– Reaction Equation
The balanced chemical equation for this reaction is as follows:
[ 2 \text{NaHCO}_3 (s) \rightarrow \text{Na}_2\text{CO}_3 (s) + \text{H}_2\text{O} (g) + \text{CO}_2 (g) ]
– Additional Information:
1. Products:
– Sodium Carbonate (Na₂CO₃): This is a white solid that remains after the reaction.
– Water (H₂O): Released as a gas during the reaction.
– Carbon Dioxide (CO₂): This gas is released and can be observed as bubbles during the heating process.
2. Applications: This reaction is utilized in baking, where the carbon dioxide produced helps dough rise, making baked goods light and fluffy.
Question: 5. Write an equation to show the reaction between Plaster of Paris and water.
Answer: The reaction between Plaster of Paris (calcium sulfate hemihydrate, ( \text{CaSO}_4 \cdot \frac{1}{2} \text{H}_2\text{O} )) and water results in the formation of gypsum (calcium sulfate dihydrate, ( \text{CaSO}_4 \cdot 2\text{H}_2\text{O} )).
1. Reaction Equation
– The balanced chemical equation for this reaction is: [ \text{CaSO}_4 \cdot \frac{1}{2} \text{H}_2\text{O} + \frac{3}{2} \text{H}_2\text{O} \rightarrow \text{CaSO}_4 \cdot 2\text{H}_2\text{O} ]
2. Additional Information:
– Process: When Plaster of Paris is mixed with water, it rehydrates and hardens to form a solid mass of gypsum. This process is exothermic, meaning it releases heat.
– Uses: The hardened gypsum is commonly used in construction, for making casts, and in decorative elements.
EXERCISES Page no: 34
Question: 1. A solution turns red litmus blue, its pH is likely to be
(a) 1
(b) 4
(c) 5
(d) 10
Answer: (d) 10.
Question: 2. A solution reacts with crushed egg-shells to give a gas that turns lime-water milky.
The solution contains
(a) NaCl
(b) HCl
(c) LiCl
(d) KCl
Answer: (b) HCl.
Question: 3. 10 mL of a solution of NaOH is found to be completely neutralised by 8 mL of a given solution of HCl. If we take 20 mL of the same solution of NaOH, the amount HCl solution (the same solution as before) required to neutralise it will be
(a) 4 mL
(b) 8 mL
(c) 12 mL
(d) 16 mL
Answer: (d) 16 mL
Explanation: The neutralisation reaction is proportional. If 10 mL of NaOH requires 8 mL of HCl, then 20 mL of NaOH will require double the amount of HCl, which is 16 mL.
Question: 4. Which one of the following types of medicines is used for treating indigestion?
(a) Antibiotic
(b) Analgesic
(c) Antacid
(d) Antiseptic
Answer: (c) Antacid
Explanation: Antacids are basic substances that neutralize excess stomach acid, providing relief from indigestion.
Question: 5. Write word equations and then balanced equations for the reaction taking place when
(a) dilute sulphuric acid reacts with zinc granules.
Answer: – Word Equation: Zinc + Dilute Sulphuric Acid → Zinc Sulphate + Hydrogen Gas
– Balanced Equation: [ \text{Zn} (s) + \text{H}_2\text{SO}_4 (aq) \rightarrow \text{ZnSO}_4 (aq) + \text{H}_2 (g) ]
(b) dilute hydrochloric acid reacts with magnesium ribbon.
Answer: – Word Equation: Magnesium + Dilute Hydrochloric Acid → Magnesium Chloride + Hydrogen Gas
– Balanced Equation: [ \text{Mg} (s) + 2 \text{HCl} (aq) \rightarrow \text{MgCl}_2 (aq) + \text{H}_2 (g) ]
(c) dilute sulphuric acid reacts with aluminium powder.
Answer: – Word Equation: Aluminium + Dilute Sulphuric Acid → Aluminium Sulphate + Hydrogen Gas
– Balanced Equation: [ 2 \text{Al} (s) + 3 \text{H}_2\text{SO}_4 (aq) \rightarrow \text{Al}_2(\text{SO}_4)_3 (aq) + 3 \text{H}_2 (g) ]
(d) dilute hydrochloric acid reacts with iron filings.
Answer: – Word Equation: Iron + Dilute Hydrochloric Acid → Iron Chloride + Hydrogen Gas
– Balanced Equation: [ \text{Fe} (s) + 2 \text{HCl} (aq) \rightarrow \text{FeCl}_2 (aq) + \text{H}_2 (g) ]
Question: 6. Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an Activity to prove it.
Answer: Activity to Prove Alcohols and Glucose are Not Acids :-
To demonstrate that compounds like alcohols and glucose contain hydrogen but do not exhibit acidic properties, you can perform the following activity:
– Materials Needed:
1. Dilute Hydrochloric Acid (HCl)
2. Dilute Sodium Hydroxide (NaOH)
3. Glucose Solution
4. Alcohol Solution (e.g., Ethanol)
5. Two nails
6. Cork
7. Battery (6 volts)
8. Bulb and Switch
9. Beaker
– Procedure:
1. Set Up the Apparatus :-
– Fix two nails on a cork and place the cork in a 100 mL beaker.
– Connect the nails to the two terminals of a 6-volt battery through a bulb and switch.
2. Test with Hydrochloric Acid :-
– Pour some dilute hydrochloric acid (HCl) into the beaker.
– Switch on the current and observe if the bulb glows.
– Observation: The bulb should glow, indicating that the solution conducts electricity due to the presence of H⁺ ions.
3. Test with Glucose Solution :-
– Remove the HCl solution and rinse the beaker.
– Pour some glucose solution into the beaker.
– Switch on the current again and observe if the bulb glows.
– Observation: The bulb will not glow, indicating that glucose does not produce H⁺ ions in solution.
4. Test with Alcohol Solution :-
– Clean the beaker and pour some dilute alcohol solution into it.
– Switch on the current and observe if the bulb glows.
– Observation: The bulb will not glow, indicating that alcohol does not produce H⁺ ions in solution.
Conclusion :-
– The bulb glows when dilute hydrochloric acid is present because it dissociates in water to produce H⁺ ions, demonstrating its acidic nature.
– In contrast, neither glucose nor alcohol solutions conduct electricity, as they do not produce H⁺ ions in solution, confirming that they are not classified as acids despite containing hydrogen.
This activity effectively illustrates the distinction between acids and other compounds that contain hydrogen but do not exhibit acidic properties.
Question: 7. Why does distilled water not conduct electricity, whereas rain water does?
Answer: To understand why distilled water does not conduct electricity, whereas rainwater does, we need to look at the composition of both types of water.
1. Distilled Water :-
– Definition: Distilled water is purified water that has been boiled to produce steam and then condensed back into liquid. This process removes impurities, including dissolved salts and minerals.
– Conductivity: Since distilled water lacks ions (charged particles), it does not conduct electricity. Pure water has a very low concentration of hydrogen ions (H⁺) and hydroxide ions (OH⁻), which are necessary for electrical conductivity.
2. Rain Water :-
– Definition: Rainwater is naturally occurring water that falls from the atmosphere. It can pick up various substances as it falls through the air and when it comes into contact with surfaces.
– Composition: Rainwater often contains dissolved gases (like carbon dioxide), minerals, and pollutants from the atmosphere. For example, when carbon dioxide dissolves in rainwater, it forms carbonic acid (H₂CO₃), which dissociates into H⁺ and bicarbonate ions (HCO₃⁻).
– Conductivity: The presence of these ions in rainwater allows it to conduct electricity. The more ions present, the better the conductivity.
Summary :-
1. Distilled Water: Pure, lacks ions, does not conduct electricity.
2. Rain Water: Contains dissolved substances and ions, conducts electricity.
This difference in ionic content is the key reason why distilled water is a poor conductor of electricity, while rainwater can conduct electricity due to the presence of various dissolved ions.
Page no: 35
Question: 8. Why does dry HCL gas not show acidic behaviour in the absence of water?
Answer: Answer: Dry hydrochloric acid (HCl) gas does not exhibit acidic properties when it is not in contact with water because:
1. Absence of Ionization: Acidity is primarily due to the presence of hydrogen ions (H⁺) in solution.
In the absence of water, HCl gas remains in its molecular form and does not dissociate into ions. The dissociation of HCl into H⁺ and Cl⁻ ions occurs only when it is dissolved in water:
HCl (g)+H2O (l)→H3O+(aq)+Cl−(aq)
2. Hydration of Hydrogen Ions: Hydrogen ions cannot exist freely; they associate with water molecules to form hydronium ions (H₃O⁺). Therefore, without water, there are no H⁺ ions available to impart acidity.
In summary, the acidic behavior of HCl is dependent on its ability to ionize in water, which is not possible in its dry gaseous state.
Question: 9. Five solutions A,B,C,D and E when tested with universal indicator showed pH as 4,1,11,7 and 9, respectively. Which solution is
(a) neutral?
(b) strongly alkaline?
(c) strongly acidic?
(d) weakly acidic?
(e) weakly alkaline?
Arrange the pH in increasing order of hydrogen-ion concentration.
Answer: To analyze the pH values of the solutions A, B, C, D, and E, we can categorize them based on their pH levels:
Solution A: pH 4
Solution B: pH 1
Solution C: pH 11
Solution D: pH 7
Solution E: pH 9
(a) Neutral Solution: Solution D (pH 7) is neutral.
(b) Strongly Alkaline Solution: Solution C (pH 11) is strongly alkaline.
(c) Strongly Acidic Solution: Solution B (pH 1) is strongly acidic.
(d) Weakly Acidic Solution: Solution A (pH 4) is weakly acidic.
(e) Weakly Alkaline Solution: Solution E (pH 9) is weakly alkaline.
– Arrangement in Increasing Order of Hydrogen-Ion Concentration
To arrange the solutions in increasing order of hydrogen-ion concentration, we consider that lower pH values correspond to higher concentrations of hydrogen ions (H⁺):
1. Solution C (pH 11) – Weakly alkaline (lowest H⁺ concentration)
2. Solution E (pH 9) – Weakly alkaline
3. Solution D (pH 7) – Neutral
4. Solution A (pH 4) – Weakly acidic
5. Solution B (pH 1) – Strongly acidic (highest H⁺ concentration)
– Summary
(a) Neutral: Solution D (pH 7)
(b) Strongly Alkaline: Solution C (pH 11)
(c) Strongly Acidic: Solution B (pH 1)
(d) Weakly Acidic: Solution A (pH 4)
(e) Weakly Alkaline: Solution E (pH 9)
– Order of H⁺ Concentration: C < E < D < A < B
Question: 10. Equal lengths of magnesium ribbons are taken in test tubes A and B. Hydrochloric acid (HCl) is added to test tube A, while acetic acid (CH3 COOH) is added to test tube B. Amount and concentration taken for both the acids are same. In which test tube will the fizzing occur more vigorously and why?
Answer: In this experiment, we have two test tubes:
– Test Tube A: Contains hydrochloric acid (HCl).
– Test Tube B: Contains acetic acid (CH₃COOH).
Both test tubes have equal lengths of magnesium ribbons and the same amount and concentration of acids.
– Observation
Fizzing Reaction: When magnesium reacts with an acid, it produces hydrogen gas, which is observed as fizzing or bubbling.
– Comparison of Acids
1. Hydrochloric Acid (HCl):
– HCl is a strong acid that completely dissociates in solution to produce a high concentration of hydrogen ions (H⁺).
– The reaction with magnesium can be represented as: [ \text{Mg} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2(g) ]
– This reaction occurs vigorously, producing a significant amount of hydrogen gas quickly.
2. Acetic Acid (CH₃COOH):
– Acetic acid is a weak acid that does not fully dissociate in solution, resulting in a lower concentration of hydrogen ions compared to hydrochloric acid.
– The reaction with magnesium can be represented as: [ \text{Mg} + 2\text{CH}_3\text{COOH} \rightarrow \text{Mg(CH}_3\text{COO)}_2 + \text{H}_2(g) ]
– The fizzing will occur, but it will be less vigorous due to the lower concentration of available hydrogen ions.
– Conclusion
More Vigorous Fizzing: The fizzing will occur more vigorously in Test Tube A (HCl) because hydrochloric acid is a strong acid that provides a higher concentration of hydrogen ions, leading to a faster and more vigorous reaction with magnesium.
– Summary
Test Tube A (HCl): More vigorous fizzing due to being a strong acid.
Test Tube B (CH₃COOH): Less vigorous fizzing due to being a weak acid.
Question: 11. Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.
Answer: Fresh milk has a pH of approximately 6, which indicates that it is slightly acidic. As milk undergoes the process of fermentation and turns into curd, the pH changes due to the production of lactic acid by the bacteria present in the milk.
– Change in pH During Fermentation :-
1. Lactic Acid Production: When milk is converted into curd, lactic acid bacteria (such as Lactobacillus) ferment lactose (the sugar in milk) and produce lactic acid. This process lowers the pH of the milk.
2. Resulting pH: As lactic acid accumulates, the pH of the milk decreases, making it more acidic. The pH of curd typically ranges from 4.5 to 5.0, indicating a more acidic nature compared to fresh milk.
– Explanation
1. Acidic Environment: The increase in acidity is essential for the curd formation, as it helps in coagulating the proteins (casein) in milk, leading to the thickening of milk into curd.
2. Taste and Texture Changes: The change in pH not only affects the acidity but also influences the taste (making it tangy) and the texture of the curd.
– Summary
As fresh milk turns into curd, the pH decreases from 6 to around 4.5 to 5.0 due to the production of lactic acid during fermentation, resulting in a more acidic environment that is crucial for curd formation.
Question: 12. A milkman adds a very small amount of baking soda to fresh milk.
(a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline?
Answer: When a milkman adds a small amount of baking soda (sodium bicarbonate, NaHCO₃) to fresh milk, it increases the pH of the milk, making it slightly alkaline.
– Reason for pH Shift :-
1. Baking Soda as a Base: Baking soda is a basic compound. When it dissolves in water (or milk), it dissociates to release hydroxide ions (OH⁻), which increases the pH of the solution.
2. Neutralization of Acidity: The alkaline nature of baking soda counteracts the natural acidity of the milk, which has a pH of about 6. This results in a shift towards a more neutral or slightly alkaline pH.
(b) Why does this milk take a long time to set as curd?
Answer: When the pH of the milk is shifted to slightly alkaline, it affects the process of curd formation.
– Impact on Curd Formation :-
1. Lactic Acid Bacteria Activity: Curd formation relies on the activity of lactic acid bacteria, which ferment lactose to produce lactic acid. This acidification is crucial for coagulating the proteins in milk.
2. Reduced Acidity: By increasing the pH and making the milk slightly alkaline, the environment becomes less conducive for lactic acid bacteria to thrive and produce enough lactic acid. As a result, the milk does not coagulate as quickly.
3. Longer Setting Time: The milk takes a longer time to set into curd because the necessary acidic conditions for the coagulation of proteins are not achieved promptly.
– Summary
(a) The addition of baking soda raises the pH of fresh milk from 6 to slightly alkaline due to its basic nature.
(b) This alkaline environment slows down the activity of lactic acid bacteria, leading to a longer time for the milk to set as curd.
Question: 13. Plaster of Paris should be stored in a moisture-proof container. Explain why?
Answer: Storage of Plaster of Paris
Plaster of Paris is a chemical compound that is sensitive to moisture. It is primarily composed of calcium sulfate hemihydrate (CaSO₄·1/2H₂O). Here’s why it should be stored in a moisture-proof container:
Reasons for Moisture-Proof Storage
1. Hydration Reaction: Plaster of Paris can absorb moisture from the air. When it comes into contact with water, it undergoes a hydration reaction, converting back into gypsum (CaSO₄·2H₂O). This reaction can be represented as: CaSO4⋅21H2O+H2O→CaSO4⋅2H2O
This transformation causes the plaster to lose its powdery form and become a solid mass, making it unusable for its intended applications.
1. Loss of Workability:
Once Plaster of Paris has absorbed moisture and turned back into gypsum, it cannot be reconstituted into its original powder form. This loss of workability can lead to wastage and inefficiency in construction and artistic applications.
2. Quality Control:
Storing Plaster of Paris in a moisture-proof container helps maintain its quality and ensures that it remains effective for use in various applications, such as casting, molding, and construction.
Conclusion
To prevent the hydration of Plaster of Paris and ensure its longevity and usability, it is essential to store it in a moisture-proof container. This practice helps maintain its properties and prevents it from clumping or hardening prematurely.
Question: 14. What is a neutralisation reaction? Give two examples.
Answer: Neutralisation Reaction: A neutralisation reaction is a chemical process in which an acid reacts with a base to produce salt and water. This reaction effectively cancels out the acidic and basic properties of the reactants, resulting in a neutral solution.
General Equation: The general form of a neutralisation reaction can be represented as:
Acid+Base→Salt+Water
Example 1. Hydrochloric Acid and Sodium Hydroxide
Reaction: HCl(aq)+NaOH(aq)→NaCl(aq)+H2O(l)
Explanation: When hydrochloric acid reacts with sodium hydroxide, sodium chloride (table salt) and water are formed.
Example 2. Sulfuric Acid and Potassium Hydroxide:
Reaction: H2SO4(aq)+2KOH(aq)→K2SO4(aq)+2H2O(l)
Explanation: In this reaction, sulfuric acid reacts with potassium hydroxide to produce potassium sulfate and water.
These reactions illustrate how acids and bases interact to form neutral products, which is fundamental in various applications, including titrations and the formulation of antacids.
Question: 15. Give two important uses of washing soda and baking soda.
Answer: 1. Washing Soda (Sodium Carbonate, Na₂CO₃)
– Cleaning Agent: Washing soda is widely used in household cleaning products. It effectively removes stains and grease from fabrics and surfaces, making it a popular choice for laundry and general cleaning.
– Water Softening: Washing soda is used to soften hard water by precipitating calcium and magnesium ions, which helps in improving the effectiveness of soaps and detergents.
2. Baking Soda (Sodium Bicarbonate, NaHCO₃)
– Cooking Ingredients: Baking soda is commonly used as a leavening agent in baking. When it reacts with an acid (like vinegar or lemon juice), it produces carbon dioxide gas, which helps dough rise and makes baked goods light and fluffy.
– Antacid: Baking soda is used as an antacid to relieve heartburn and indigestion. It neutralizes excess stomach acid, providing quick relief from discomfort.
These uses highlight the versatility of washing soda and baking soda in both household and culinary applications.