Q.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?
Ans. If the colour of red litmus does not change then, it is acid. If the colour of red litmus changes to blue, then it is base. If there is a slight change in the colour of red litmus (such as purple), then it is distilled water.
Q.1. Why should curd and sour substances not be kept in brass and copper vessels?
Ans. Curd and the sour substances if kept in brass or copper vessels react with them and form hydrogen and other harmful substances due to the presence of acid in them. These toxic substances can cause food poisoning or other damage to health. Due to this reason, curd and sour substances should not be kept in brass and copper 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.
The reaction of zinc granules with dil. H2SO4 to liberate hydrogen gas which burns with a ‘pop’ sound
(i) Take 5 g of zinc granules in a test tube.
(ii) Set the apparatus, as shown in the diagram.
(iii) Add 20 mL of dil. H2SO4 with the help of a thistle funnel.
(iv) Collect the gas evolved in a gas jar, as shown in the figure.
(v) Observe the colour and odour of the gas.
(vi) Bring a burning matchstick near the gas jar and record your observations.
Observation: A colourless, odourless gas is evolved. It burns explosively with a ‘pop’ sound when a burning matchstick is brought near it, indicating the presence of hydrogen gas.
Chemical Reaction: Zn(s) + H2SO4(dil) → ZnSO4(aq) + H2(g)
Q.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.
Ans. Calcium carbonate (A), when reacts with hydrochloric acid, produces carbon dioxide gas with effervescence. Carbon dioxide gas is used as a fire extinguisher. Therefore, it extinguishes a burning candle. Hence, the metal compound A is calcium carbonate.
Chemical Reaction: CaCO3(s) + 2HCl(aq) → CaCl2(s)+ CO2 (g) + H2O(l)
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. HCI, HNO3, etc. dissociate into their ions in the presence of water. They 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+
Similarly, HNO3 → H+ + NO-3
► H+ + H2O → H3O+
Q.2. Why does an aqueous solution of acid conduct electricity?
Ans. An aqueous solution of acid conduct electricity because of the presence of charged particles called ions in it. When dissolved in water, acids dissociate to form ions.
► HCl + H2O → Cl- + H3O-
These ions are responsible for conducting electricity.
Q.3. Why does dry HCl gas not change the colour of the dry litmus paper?
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 in water is highly exothermic. 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 severe burns. The glass container may also break due to excessive heating.
Q.5. How is the concentration of hydronium ions (H3O+) affected when a solution of an 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?
Q.1. You have two solutions, A and B. The pH of solution A is 6, and the 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 the acidic nature, while greater than 7 indicates the 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. The concentration of H+(aq) can have a varied effect on the nature of the solution. With an increase in H+ ion concentration, 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, the basic solution also has H+(aq) ions. However, their concentration is less as compared to the concentration of OH- ions that makes the solution basic.
Q.4. Under what soil conditions 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 to be more acidic, then to increase the basicity of the soil, he should treat the soil of his field with quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate).
Q.1. What is the common name of the compound Ca(OCl)2?
Ans. Bleaching Powder
Bleaching powder structural formula
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 (Na2CO3.10H2O)
Q.4. What will happen if a solution of sodium hydro carbonate is heated? Give the equation of the reaction involved.
Ans. When sodium hydrogen carbonate is heated, then sodium carbonate and water are formed along with carbon dioxide gas evolution.
Q.5. Write an equation to show the reaction between Plaster of Paris and water.
Q.1. A solution turns red litmus blue, its pH is likely to be
Solution. pH = 10, bases turn red litmus blue, and their pH is more than 7.
Q.2. A solution reacts with crushed egg-shells to give a gas that turns lime-water milky. The solution contains
Eggshells are made up of CaCO3 which reacts with HCl to form CO2, and this CO2 turns lime water milky.
► CaCO3(s) + 2HCl(dil)→ CaCl2(aq) + H2O(l) + CO2(g)
► Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)
Q.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 of HCl solution (the same solution as before) required to neutralise it will be
(a) 4 mL
(c) 12 mL
(d) 16 mL
∴ 10 mL of NaOH will neutralise
= 8 mL of HCI.
= 20 mL of NaOH will neutralise
= 8/10 x 20 = 16 mL.
Q.4. Which one of the following types of medicines are used for treating indigestion?
Q.5. Write word equations and then balanced equations for the reaction taking place when:
(a) Dilute sulphuric acid reacts with zinc granules.
(b) Dilute hydrochloric acid reacts with magnesium ribbon.
(c) Dilute sulphuric acid reacts with aluminium powder.
(d) Dilute hydrochloric acid reacts with iron filings.
Q.6. Compounds such as alcohols and glucose also contain hydrogen but are not categorized as acids. Describe an activity to prove it.
Observation: You will observe that in the case of dilute hydrochloric acid bulb glows, but when glucose or alcohol solution is taken in the beaker, the bulb does not glow.
Conclusion: The aqueous solution of hydrochloric acid conducts electricity due to the presence of types of charged particles: Hydrogen ions and chloride ions. Unlike acids glucose and ethanol do not ionise in aqueous solution, i.e. they do not give H+ ions; therefore, they cannot conduct electricity. Thus, glucose and ethanol are not categorized as acids.
Q.7. Why do acids not show acidic behaviour in the absence of water?
Ans. It is because acids do not dissociate into ions in the absence of water. But when an acid is dissolved in water, it forms hydrogen ions and hence, shows acidic behaviour.
Q.8. 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
(b) Strongly alkaline?
(c) Strongly acidic?
(d) Weakly acidic?
(e) Weakly alkaline?
Arrange the pH in increasing order of hydrogen-ion concentration.
Ans.pH level from acidic to basic
Q.13. What is a neutralization reaction? Give two examples.
Ans. A reaction in which an acid and base react with each other to give a salt and water is termed as neutralization reaction.
► NaOH(aq) + HCl(aq) → NaCl(aq) +H2O(l)
► H2SO4(aq) + 2NH4OH(aq) → (NH4)2SO4(aq) + 2H2O
Q.14. Give two important uses of washing soda and baking soda.
► Uses of Washing Soda
(i) It is used in the manufacture of glass, soap, paper and other sodium compounds like borax, etc.
(ii) It is used in softening of hard water.
► Uses of Baking Soda
(i) It is used as an antacid to neutralise the excess of acidity (hyper-acidity) in the stomach.
(ii) It is an ingredient of baking powder which contains NaHCO3 and tartaric acid.
(iii) It is used in making soda acid fire extinguisher.