Q.1: A solution turns red litmus blue; its pH is likely to bea) 1 b) 4 c) 5 d) 10
Ans: (d)
Q.2: A solution reacts with crushed egg-cells to give a gas that turns lime water milky. The solution contains -
a) NaCl b) HCl c) LiCl d) KCl
Ans: (b)
Q.3: 10 ml of a solution of NaOH is found to be completely neutralized by 8 ml of a given solution of HCl. If we take 20 ml of the same solution of NaOH, the amount of HCL (the same solution as before) required to neutralize it will be -
a) 4 ml b) 8 ml c) 12 ml d) 16 ml
Ans: (d)
Q.6: Compounds such as alcohols and glucose also contain hydrogen but are not categorized as acids. Describe an activity to prove it.
Ans: The nails are fitted in a cork and are kept in a 100 ml beaker. the nails are then connected to the two terminals of a 6 volt battery through a bulb and a switch. some dilute HCl is poured in the beaker and the current is switched on as shown in the figure. The same experiment is repeated with glucose and alcohol solutions separately.
Observations
It will be observed that the bulb glows in HCl solution and does not glow in glucose and alcohol solutions.
Reasons
HCl dissociates into H+ and Cl– ions. These ions conduct electricity in the solution resulting in the glowing of the bulb. On the other hand, glucose and alcohol solutions do not dissociate into ions and so does not conduct electricity.
Conclusions
The above activity proves that all acids contain hydrogen but not all compounds containing hydrogen are acids. That is why, though compounds such as alcohols and glucose contain hydrogen but they are not categorized as acids.
Q.7: Why does distilled water not conduct electricity, whereas rain water does?
Ans: Distilled water is a pure form of water which is neither acidic nor basic in nature and is devoid of any ionic species. Since, conduction of electricity requires free ions so, distilled water does not conduct electricity.
Rain water being an impure form of water, contains many ionic species such as of acids etc. and therefore, conducts electricity.
Q.8: Why do acids not show acidic behavior in the absence of water?
Ans: Acids do not show acidic behaviour in absence of water because the dissociation of hydrogen ions from an acid occurs in presence of water only. It is hydrogen ions that are responsible for the acidic behaviour.
Q.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.
Ans: (a) D (b) C (c) B (d) A (e) E.
pH in increasing order of H+ ions concentration:
C (11) < E (9) < D (7) < A (4) < B (1).
Q.14: What is a neutralization reaction? Give two examples.
Ans: A reaction in which an acid and a base react to give a salt and water is known as a ‘neutralization reaction’. For example,
NaOH (aq) + HCl (aq) --> NaCl (aq) + H2O (liq)
Ca(OH)2 (aq) + H2SO4 (aq) --> CaSO4 (s) + H2O
Q.15: Give two important uses of washing soda and baking soda.
Ans:
Uses of Washing Soda:
1. Sodium carbonate (washing soda) is used in glass, soap, and paper industries.
2. It is used in the manufacture of sodium compounds such as Borax.
Uses of Baking Soda:
1. Sodium hydrogen carbonate is also ingredient in antacids. Being alkaline, it neutralizes excess acid in the stomach and provides relief.
2. It is also used in soda-acid fire extinguishers.
3. It is used as baking powder in confectionery items like cake, pastry etc.
(Page 28)
Q.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?
Q.2: What effect does the concentration of H+ (aq) ions have on the nature of the solution?
Q.3: Do basic solutions also have H+ (aq) ions? If yes, then why are these basic?
Q.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)?
(Page 33)
Q.1: What is the common name of the compound CaOCl2?
Q.2: Name the substance which on treatment with chlorine yields bleaching powder.
Q.3: Name the sodium compound which is used for softening hard water.
Q.4: What will happen if a solution of sodium hydrocarbonate is heated? Give the equation of the reaction involved.
Q.5: Write an equation to show the reaction between Plaster of Paris (POP) and water.
(Page 18)
Q.1: You have been provided with three test tubes. One of them contains distilled water and the other two contain an acidic and basic solution, respectively. If you are given only red litmus paper, how will you identify the contents of each test tube?
Ans: First of all, we put one strip of red litmus paper in each test tube. In one of the test tubes, the litmus paper will turn blue which indicates the solution in that test tube is basic. Out of the remaining two test tubes any one can be acid or neutral (distilled water). Now a drop of basic solution is mixed with a drop of each of the remaining two solutions separately. Then we check the nature of the drops of the mixture again with red litmus. There can be two results:
1. If the colour of red litmus turns blue, then the second solution is neutral (distilled water), and
2. If there is no change in colour and the litmus remains red, then the second solution is acidic. This is because acidic and basic solutions neutralize each other.
Thus, we can identify the contents of each test tube.
(Page 22)
Q.1: Why should curd and sour substances not be kept in brass and copper vessels?
Ans: Curd and sour substances contain acids. Therefore, if we keep such food substances in brass and copper vessels, they will react with the metal to form poisonous metal compounds and liberate hydrogen gas thereby causing food poisoning and corrosion of the vessels.
Q.2: Which gas is usually liberated when an acid reacts with a metal? Illustrate with an example. How will you test for the presence of this gas?
Ans: Usually, hydrogen gas is liberated when an acid reacts with a metal. For example, let us take the reaction between zinc and sulphuric acid, as per the apparatus set-up shown in the adjacent figure.
A few pieces of zinc granules are taken in a test tube and about 5 mL of dilute H2SO4 is added. The gas produced is passed into a soap solution. We find that bubbles of soap solutions are formed. When a burning candle is brought near the soap bubbles, it burns with a pop sound which confirms the presence of hydrogen gas.
Zinc granules react with dilute sulphuric acid to for zinc sulphate and hydrogen gas according to the chemical; equation:
Zn + H2SO4 ---> ZnSO4 + H2
Q.3: A metal compound 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.
Ans: CaCO3 + 2HCl ---> CaCl2 + H2O + CO2
Sample Questions with solution
Q1: Define (i) Acid (ii) Bases (iii) Salt
Ans: Acid: A substance which liberates hydrogen ions in aqueous solution, sour in taste, and turns blue litmus to red. Metals react with acids to form salts and liberate hydrogen gas. Many acids are corrosive such as, HNO3, H2SO4, HF, etc.
Base: A substance which reacts with an acid to form a salt and water only. If dissolved in water, they give hydroxyl ions (OH–) ions. Bases are bitter in taste and change the colour of red litmus to blue.
Salt: a chemical compound formed when the hydrogen of an acid has been replaced by a metal. a salt is produced together with water, when an acid reacts with a base. Salts are named according to the acid and the metal from which the salt is derived. For example, ‘sodium sulphate’ is a salt derived from sodium metal and sulphuric acid.
Q2: What is Universal Indicator?
Ans: Universal Indicator is a mixture of several indicators. The universal indicator shows different colours at different concentration of hydrogen ions in a solution. Thus, with the help of a universal indicator we can quantitatively judge how strong a given acid or base is.
Q3: Why is it advised to clean mouth after consuming food? or pH change is a cause for toot decay. Explain.
Ans: Acids are produced in the mouth due to the degradation of sugar and food particles which remain in mouth after eating, by certain bacteria. As a result the pH of mouth is lowered. Tooth enamel made up of calcium phosphate is corroded when the pH in the mouth is lower than 5.5 So, tooth decay starts when the pH of the decreases below 5.5
Therefore, it is advised to clean the mouth after eating food with toothpastes which are generally basic.
Q4: How sodium hydroxide is produced? or What is ‘Chlor-alkali’ process? or What do we obtain by electrolysis of brine?
Ans: Sodium hydroxide is produced by electrolysis of aqueous solution of sodium chloride (NaCl) or Brine. electrolysis of brine results in the decomposition of NaCl and formation of NaOH. This process is called ‘chlor-alkali’ process because the products formed - chlor for chlorine and alkali forsodium hydroxide.
2NaCl (aq) + 2H2O(liq) --> 2NaOH (aq) + Cl2(g) + H2(g)
Chlorine is given off at the anode while hydrogen at cathode.
Q5: Give some uses of sodium hydroxide.
Ans: Sodium hydroxide is a very useful substance. It is used in the manufacturing of paper, soaps, detergents, and synthetic fibers. It is also useful in the manufacturing of house-hold bleaches and dyes.
Q6: How bleaching powder is formed? Give some uses of bleaching powder.
Ans: Chlorine gas obtained by the electrolysis of brine is used for the manufacturing of bleaching powder. Bleaching powder is produced by the action of chlorine on dry slaked lime [Ca(OH)2] as per the following equation -
Ca(OH)2 + Cl2 --> CaOCl2 + H2O
Uses of Bleaching Powder
1. Disinfecting drinking water to make it free from germs.
2. As an oxidizing agent in many chemical industries.
3. As a bleaching agent in textile industry, paper industry, etc.
Q8: Give the chemical equation for the production of baking soda.
Ans: The chemical name of baking soda is Sodium Hydrogen Carbonate (NaHCO3). It is produced using sodium chloride as one of the raw materials as per the given equation -
NaCl + H2O + CO2 + NH3 --> NH4Cl + NaHCO3
Q9: What is baking powder?
Ans: Baking Powder is a mixture of baking soda and a mild edible acid such as tartaric acid. When baking powder is heated or mixed in water, the following reaction takes place -
NaHCO3 + H+ --> CO2 + H2O + sodium salt of acid.
Carbon dioxide produced during the reaction causes bread or cake to rise making them soft and spongy.
Q10: What happens when we heat sodium hydrogen carbonate (baking soda)?
Ans: By heating sodium hydrogen carbonate we get sodium carbonate (Na2CO3).
The sodium carbonate after recrystallization produces washing soda.
Na2CO3 + 10H2O --> Na2CO3.10H2O
Q11: What is ‘water of crystallization’?
Ans: Water of recrystallization is a fixed number of water molecules present in one formula unit of salt. For example, in washing soda (Na2CO3.10H2O) there are 10 water molecules present as water of crystallization.
The water of crystallization remain in chemical combination with crystal. It is necessary for the maintenance of crystalline properties of the crystal. It can be removed by sufficient heat.
Q12: What are hydrated salts? Give an example.
Ans: Salts containing a fixed number of water molecules in their crystal structure are called hydrated salts.
A molecule of sodium carbonate (Na2CO3.10H2O) contains ten molecules of water. This is known as hydrated salt of sodium carbonate.
Q13: Why does an indicator change its colour in an acidic or basic solution?
Ans: An indicator is itself a weak acid or base. When added to an acidic or basic solution, it accepts or loses protons de[pending on the conditions, and this rearranges the distribution of electrons of its molecules. The colour of the indicator changes because of this redistribution.
Q14: Name some chemicals obtained by using sodium chloride (common salt) as a raw material.
Ans: Sodium hydroxide (NaOH), Baking soda (NaHCO3), Washing soda (Na2CO3.10H2O), Bleaching powder (CaOCl2), etc.
Q15: What are strong and weak acids?
Ans: acids that give rise to more H+ ions are said to be strong acids and acids that give less H+ ions are said to be weak acids. a strong acid has pH value closer to zero while acids with higher pH values or closer to 7 are weak acids.
Q16: What is ‘Plaster of Paris’?
Ans: On heating gypsum at 3730 K, it loses water molecules and becomes calcium sulphate hemihydrates (CaSO4.1/2 H2O). This is called ‘Plaster of Paris’.
Ans: (d)
Q.2: A solution reacts with crushed egg-cells to give a gas that turns lime water milky. The solution contains -
a) NaCl b) HCl c) LiCl d) KCl
Ans: (b)
Q.3: 10 ml of a solution of NaOH is found to be completely neutralized by 8 ml of a given solution of HCl. If we take 20 ml of the same solution of NaOH, the amount of HCL (the same solution as before) required to neutralize it will be -
a) 4 ml b) 8 ml c) 12 ml d) 16 ml
Ans: (d)
Q.6: Compounds such as alcohols and glucose also contain hydrogen but are not categorized as acids. Describe an activity to prove it.
Ans: The nails are fitted in a cork and are kept in a 100 ml beaker. the nails are then connected to the two terminals of a 6 volt battery through a bulb and a switch. some dilute HCl is poured in the beaker and the current is switched on as shown in the figure. The same experiment is repeated with glucose and alcohol solutions separately.
Observations
It will be observed that the bulb glows in HCl solution and does not glow in glucose and alcohol solutions.
Reasons
HCl dissociates into H+ and Cl– ions. These ions conduct electricity in the solution resulting in the glowing of the bulb. On the other hand, glucose and alcohol solutions do not dissociate into ions and so does not conduct electricity.
Conclusions
The above activity proves that all acids contain hydrogen but not all compounds containing hydrogen are acids. That is why, though compounds such as alcohols and glucose contain hydrogen but they are not categorized as acids.
Q.7: Why does distilled water not conduct electricity, whereas rain water does?
Ans: Distilled water is a pure form of water which is neither acidic nor basic in nature and is devoid of any ionic species. Since, conduction of electricity requires free ions so, distilled water does not conduct electricity.
Rain water being an impure form of water, contains many ionic species such as of acids etc. and therefore, conducts electricity.
Q.8: Why do acids not show acidic behavior in the absence of water?
Ans: Acids do not show acidic behaviour in absence of water because the dissociation of hydrogen ions from an acid occurs in presence of water only. It is hydrogen ions that are responsible for the acidic behaviour.
Q.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.
Ans: (a) D (b) C (c) B (d) A (e) E.
pH in increasing order of H+ ions concentration:
C (11) < E (9) < D (7) < A (4) < B (1).
Q.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 (CH3COOH) is added to test tube B. In which test tube will the fizzing occur more vigorously and why?
Ans: In test tube A, fizzing occur more vigorously. It is because HCl is a strong acid than acetic acid and therefore, produces H2 gas at a faster rate due to which fizzing occurs.
Q.11: Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.
Ans: Curd is acidic in nature which means its pH will decrease from 6.
Q.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? (b) Why does this milk take a long time to set as curd?
Ans:
Ans: In test tube A, fizzing occur more vigorously. It is because HCl is a strong acid than acetic acid and therefore, produces H2 gas at a faster rate due to which fizzing occurs.
Q.11: Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.
Ans: Curd is acidic in nature which means its pH will decrease from 6.
Q.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? (b) Why does this milk take a long time to set as curd?
Ans:
(a) By adding small amount of baking soda, the milkman shifts the pH of the fresh milk from 6 to slightly alkaline so that he can keep it for longer time as the milk in alkaline condition, does not set curd easily.
(b) Since, this milk is slightly basic than the usual milk, acids produced due to bacterial actions in it are neutralized by the base. Therefore, it takes longer time to set as curd.
Q.13: Plaster of Paris should be stored ion a moisture-proof container. Explain why.
Ans: Plaster of Paris easily absorbs water and forms hard gypsum. Thus, if Plaster of Paris is not kept in a moisture-proof container, then all Plaster of Paris will get converted into gypsum.
CaSO4.½H2O + 1½H2O --> CaSO4.2H2O
(b) Since, this milk is slightly basic than the usual milk, acids produced due to bacterial actions in it are neutralized by the base. Therefore, it takes longer time to set as curd.
Q.13: Plaster of Paris should be stored ion a moisture-proof container. Explain why.
Ans: Plaster of Paris easily absorbs water and forms hard gypsum. Thus, if Plaster of Paris is not kept in a moisture-proof container, then all Plaster of Paris will get converted into gypsum.
CaSO4.½H2O + 1½H2O --> CaSO4.2H2O
Q.14: What is a neutralization reaction? Give two examples.
Ans: A reaction in which an acid and a base react to give a salt and water is known as a ‘neutralization reaction’. For example,
NaOH (aq) + HCl (aq) --> NaCl (aq) + H2O (liq)
Ca(OH)2 (aq) + H2SO4 (aq) --> CaSO4 (s) + H2O
Q.15: Give two important uses of washing soda and baking soda.
Ans:
Uses of Washing Soda:
1. Sodium carbonate (washing soda) is used in glass, soap, and paper industries.
2. It is used in the manufacture of sodium compounds such as Borax.
Uses of Baking Soda:
1. Sodium hydrogen carbonate is also ingredient in antacids. Being alkaline, it neutralizes excess acid in the stomach and provides relief.
2. It is also used in soda-acid fire extinguishers.
3. It is used as baking powder in confectionery items like cake, pastry etc.
(Page 28)
Q.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?
Q.2: What effect does the concentration of H+ (aq) ions have on the nature of the solution?
Q.3: Do basic solutions also have H+ (aq) ions? If yes, then why are these basic?
Q.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)?
(Page 33)
Q.1: What is the common name of the compound CaOCl2?
Q.2: Name the substance which on treatment with chlorine yields bleaching powder.
Q.3: Name the sodium compound which is used for softening hard water.
Q.4: What will happen if a solution of sodium hydrocarbonate is heated? Give the equation of the reaction involved.
Q.5: Write an equation to show the reaction between Plaster of Paris (POP) and water.
(Page 18)
Q.1: You have been provided with three test tubes. One of them contains distilled water and the other two contain an acidic and basic solution, respectively. If you are given only red litmus paper, how will you identify the contents of each test tube?
Ans: First of all, we put one strip of red litmus paper in each test tube. In one of the test tubes, the litmus paper will turn blue which indicates the solution in that test tube is basic. Out of the remaining two test tubes any one can be acid or neutral (distilled water). Now a drop of basic solution is mixed with a drop of each of the remaining two solutions separately. Then we check the nature of the drops of the mixture again with red litmus. There can be two results:
1. If the colour of red litmus turns blue, then the second solution is neutral (distilled water), and
2. If there is no change in colour and the litmus remains red, then the second solution is acidic. This is because acidic and basic solutions neutralize each other.
Thus, we can identify the contents of each test tube.
(Page 22)
Q.1: Why should curd and sour substances not be kept in brass and copper vessels?
Ans: Curd and sour substances contain acids. Therefore, if we keep such food substances in brass and copper vessels, they will react with the metal to form poisonous metal compounds and liberate hydrogen gas thereby causing food poisoning and corrosion of the vessels.
Q.2: Which gas is usually liberated when an acid reacts with a metal? Illustrate with an example. How will you test for the presence of this gas?
Ans: Usually, hydrogen gas is liberated when an acid reacts with a metal. For example, let us take the reaction between zinc and sulphuric acid, as per the apparatus set-up shown in the adjacent figure.
Zinc granules react with dilute sulphuric acid to for zinc sulphate and hydrogen gas according to the chemical; equation:
Zn + H2SO4 ---> ZnSO4 + H2
Q.3: A metal compound 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.
Ans: CaCO3 + 2HCl ---> CaCl2 + H2O + CO2
(Page 25)
Q.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?
Ans: HCl and HNO3 etc. dissociate in presence of water to form hydrogen ions (H+). These hydrogen ions combine with H2O to form hydronium ions (H3O+). The reaction can be given as follows:
HCl ---> H+ + Cl–
H+ + H2O ---> H3O+
Because of this property HCl and HNO3 show acidic character in aqueous solutions. On the other hand, alcohol and glucose can not dissociate in water to form hydrogen ions. Hence, they do not show acidic character.
Q.2: Why does an aqueous solution of acid conduct electricity?
Ans: When dissolved in water, acids dissociate to form ions e.g.,
HCl + H2O ---> Cl– + H3O+
These ions are responsible for electrical conductivity.
Q.3: Why does dry HCl gas not change the colour of the dry litmus paper?
Ans: We know that the colour of the litmus is changed by H+ ions of an acid. Dry HCl does not dissociate to give H+ ions. It is only in the aqueous that an acid dissociate to give ions. Since in this case, neither HCL is in aqueous form nor is the litmus paper wet, so the colour of litmus paper does not change.
Q.4: While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?
Ans: The process of dissolving an acid or a base in water is a highly exothermic one. Care must be taken while mixing concentrated HNO3 acid or H2SO4 with water. The acid must always be added slowly to water with constant stirring. If water is added to a concentrated acid, the heat generated may cause the mixture to splash out and cause burns. The glass container may also break due to excessive local heating.
Q.5: How is the concentration of hydronium ions (H3O+) affected when a solution of an acid is diluted?
Ans: Concentration of hydronium ions (H3O+) decreases. As a result the acid is diluted.
Q.6: How is the concentration of hydroxide ions (OH–) affected when excess base is dissolved in a solution of sodium hydroxide?
Ans: Concentration of hydroxide ions (OH–) increases.
(Page 28)
Q.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?
Ans: Solution A has more H+ ion concentration. A pH value of less than 7 indicates an acidic nature while greater than 7 indicates basic nature of a solution. So, solution A is acidic and solution B is basic.
Q.2: What effect does the concentration of H+ (aq) ions have on the nature of the solution?
Ans: As the concentration of H+ ions increases the solution becomes more acidic while a decrease of H+ ion causes an increase in the basicity of the solution.
Q.3: Do basic solutions also have H+ (aq) ions? If yes, then why are these basic?
Ans: Yes, basic solutions also have H+ ions. However, their concentration is less as compared to the OH– ions that makes the solution basic.
Q.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)?
Ans: If the farmer finds his soil turned acidic, then to increase the Basicity of soil, he should use the above materials to neutralize it.
(Page 33)
Q.1: What is the common name of the compound CaOCl2?
Ans: Bleaching powder.
Q.2: Name the substance which on treatment with chlorine yields bleaching powder.
Ans: Dry slaked lime [Ca(OH)2].
Q.3: Name the sodium compound which is used for softening hard water.
Ans: Washing soda i.e. sodium carbonate (Na2CO3.10H2O).
Q.4: What will happen if a solution of sodium hydrocarbonate is heated? Give the equation of the reaction involved.
Ans: The following reaction takes place when it is heated:
Q.5: Write an equation to show the reaction between Plaster of Paris (POP) and water.
Ans: The chemical equation for the reaction of Plaster of Paris and water can be represented as follows:
CaSO4.½H2O + 1½H2O ---> CaSO4.2H2O
Q.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?
Ans: HCl and HNO3 etc. dissociate in presence of water to form hydrogen ions (H+). These hydrogen ions combine with H2O to form hydronium ions (H3O+). The reaction can be given as follows:
HCl ---> H+ + Cl–
H+ + H2O ---> H3O+
Because of this property HCl and HNO3 show acidic character in aqueous solutions. On the other hand, alcohol and glucose can not dissociate in water to form hydrogen ions. Hence, they do not show acidic character.
Q.2: Why does an aqueous solution of acid conduct electricity?
Ans: When dissolved in water, acids dissociate to form ions e.g.,
HCl + H2O ---> Cl– + H3O+
These ions are responsible for electrical conductivity.
Q.3: Why does dry HCl gas not change the colour of the dry litmus paper?
Ans: We know that the colour of the litmus is changed by H+ ions of an acid. Dry HCl does not dissociate to give H+ ions. It is only in the aqueous that an acid dissociate to give ions. Since in this case, neither HCL is in aqueous form nor is the litmus paper wet, so the colour of litmus paper does not change.
Q.4: While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?
Ans: The process of dissolving an acid or a base in water is a highly exothermic one. Care must be taken while mixing concentrated HNO3 acid or H2SO4 with water. The acid must always be added slowly to water with constant stirring. If water is added to a concentrated acid, the heat generated may cause the mixture to splash out and cause burns. The glass container may also break due to excessive local heating.
Q.5: How is the concentration of hydronium ions (H3O+) affected when a solution of an acid is diluted?
Ans: Concentration of hydronium ions (H3O+) decreases. As a result the acid is diluted.
Q.6: How is the concentration of hydroxide ions (OH–) affected when excess base is dissolved in a solution of sodium hydroxide?
Ans: Concentration of hydroxide ions (OH–) increases.
(Page 28)
Q.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?
Ans: Solution A has more H+ ion concentration. A pH value of less than 7 indicates an acidic nature while greater than 7 indicates basic nature of a solution. So, solution A is acidic and solution B is basic.
Q.2: What effect does the concentration of H+ (aq) ions have on the nature of the solution?
Ans: As the concentration of H+ ions increases the solution becomes more acidic while a decrease of H+ ion causes an increase in the basicity of the solution.
Q.3: Do basic solutions also have H+ (aq) ions? If yes, then why are these basic?
Ans: Yes, basic solutions also have H+ ions. However, their concentration is less as compared to the OH– ions that makes the solution basic.
Q.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)?
Ans: If the farmer finds his soil turned acidic, then to increase the Basicity of soil, he should use the above materials to neutralize it.
(Page 33)
Q.1: What is the common name of the compound CaOCl2?
Ans: Bleaching powder.
Q.2: Name the substance which on treatment with chlorine yields bleaching powder.
Ans: Dry slaked lime [Ca(OH)2].
Q.3: Name the sodium compound which is used for softening hard water.
Ans: Washing soda i.e. sodium carbonate (Na2CO3.10H2O).
Q.4: What will happen if a solution of sodium hydrocarbonate is heated? Give the equation of the reaction involved.
Ans: The following reaction takes place when it is heated:
Q.5: Write an equation to show the reaction between Plaster of Paris (POP) and water.
Ans: The chemical equation for the reaction of Plaster of Paris and water can be represented as follows:
CaSO4.½H2O + 1½H2O ---> CaSO4.2H2O
Sample Questions with solution
Q1: Define (i) Acid (ii) Bases (iii) Salt
Ans: Acid: A substance which liberates hydrogen ions in aqueous solution, sour in taste, and turns blue litmus to red. Metals react with acids to form salts and liberate hydrogen gas. Many acids are corrosive such as, HNO3, H2SO4, HF, etc.
Base: A substance which reacts with an acid to form a salt and water only. If dissolved in water, they give hydroxyl ions (OH–) ions. Bases are bitter in taste and change the colour of red litmus to blue.
Salt: a chemical compound formed when the hydrogen of an acid has been replaced by a metal. a salt is produced together with water, when an acid reacts with a base. Salts are named according to the acid and the metal from which the salt is derived. For example, ‘sodium sulphate’ is a salt derived from sodium metal and sulphuric acid.
Q2: What is Universal Indicator?
Ans: Universal Indicator is a mixture of several indicators. The universal indicator shows different colours at different concentration of hydrogen ions in a solution. Thus, with the help of a universal indicator we can quantitatively judge how strong a given acid or base is.
Q3: Why is it advised to clean mouth after consuming food? or pH change is a cause for toot decay. Explain.
Ans: Acids are produced in the mouth due to the degradation of sugar and food particles which remain in mouth after eating, by certain bacteria. As a result the pH of mouth is lowered. Tooth enamel made up of calcium phosphate is corroded when the pH in the mouth is lower than 5.5 So, tooth decay starts when the pH of the decreases below 5.5
Therefore, it is advised to clean the mouth after eating food with toothpastes which are generally basic.
Q4: How sodium hydroxide is produced? or What is ‘Chlor-alkali’ process? or What do we obtain by electrolysis of brine?
Ans: Sodium hydroxide is produced by electrolysis of aqueous solution of sodium chloride (NaCl) or Brine. electrolysis of brine results in the decomposition of NaCl and formation of NaOH. This process is called ‘chlor-alkali’ process because the products formed - chlor for chlorine and alkali forsodium hydroxide.
2NaCl (aq) + 2H2O(liq) --> 2NaOH (aq) + Cl2(g) + H2(g)
Chlorine is given off at the anode while hydrogen at cathode.
Q5: Give some uses of sodium hydroxide.
Ans: Sodium hydroxide is a very useful substance. It is used in the manufacturing of paper, soaps, detergents, and synthetic fibers. It is also useful in the manufacturing of house-hold bleaches and dyes.
Q6: How bleaching powder is formed? Give some uses of bleaching powder.
Ans: Chlorine gas obtained by the electrolysis of brine is used for the manufacturing of bleaching powder. Bleaching powder is produced by the action of chlorine on dry slaked lime [Ca(OH)2] as per the following equation -
Ca(OH)2 + Cl2 --> CaOCl2 + H2O
Uses of Bleaching Powder
1. Disinfecting drinking water to make it free from germs.
2. As an oxidizing agent in many chemical industries.
3. As a bleaching agent in textile industry, paper industry, etc.
Q8: Give the chemical equation for the production of baking soda.
Ans: The chemical name of baking soda is Sodium Hydrogen Carbonate (NaHCO3). It is produced using sodium chloride as one of the raw materials as per the given equation -
NaCl + H2O + CO2 + NH3 --> NH4Cl + NaHCO3
Q9: What is baking powder?
Ans: Baking Powder is a mixture of baking soda and a mild edible acid such as tartaric acid. When baking powder is heated or mixed in water, the following reaction takes place -
NaHCO3 + H+ --> CO2 + H2O + sodium salt of acid.
Carbon dioxide produced during the reaction causes bread or cake to rise making them soft and spongy.
Q10: What happens when we heat sodium hydrogen carbonate (baking soda)?
Ans: By heating sodium hydrogen carbonate we get sodium carbonate (Na2CO3).
The sodium carbonate after recrystallization produces washing soda.
Na2CO3 + 10H2O --> Na2CO3.10H2O
Q11: What is ‘water of crystallization’?
Ans: Water of recrystallization is a fixed number of water molecules present in one formula unit of salt. For example, in washing soda (Na2CO3.10H2O) there are 10 water molecules present as water of crystallization.
The water of crystallization remain in chemical combination with crystal. It is necessary for the maintenance of crystalline properties of the crystal. It can be removed by sufficient heat.
Q12: What are hydrated salts? Give an example.
Ans: Salts containing a fixed number of water molecules in their crystal structure are called hydrated salts.
A molecule of sodium carbonate (Na2CO3.10H2O) contains ten molecules of water. This is known as hydrated salt of sodium carbonate.
Q13: Why does an indicator change its colour in an acidic or basic solution?
Ans: An indicator is itself a weak acid or base. When added to an acidic or basic solution, it accepts or loses protons de[pending on the conditions, and this rearranges the distribution of electrons of its molecules. The colour of the indicator changes because of this redistribution.
Q14: Name some chemicals obtained by using sodium chloride (common salt) as a raw material.
Ans: Sodium hydroxide (NaOH), Baking soda (NaHCO3), Washing soda (Na2CO3.10H2O), Bleaching powder (CaOCl2), etc.
Q15: What are strong and weak acids?
Ans: acids that give rise to more H+ ions are said to be strong acids and acids that give less H+ ions are said to be weak acids. a strong acid has pH value closer to zero while acids with higher pH values or closer to 7 are weak acids.
Q16: What is ‘Plaster of Paris’?
Ans: On heating gypsum at 3730 K, it loses water molecules and becomes calcium sulphate hemihydrates (CaSO4.1/2 H2O). This is called ‘Plaster of Paris’.
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