NCERT Solutions for Class 10th Science
Chapter 2 – ACIDS, BASES AND SALTS
(Complete Downloadable Chapter Solution PDF file is at the bottom of the page)
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 1. Take all the solutions in different beakers or test tubes.
2. We will put drops of all the solutions on red litmus paper. The solution which shows colour change of red litmus paper to blue is the basic solution whereas the acidic and neutral solution will not show any colour change.
3. Now we put drops of the remaining two solutions on blue litmus paper (which changed colour due to basic solution). The solution which shows colour change of blue litmus paper to red is the acidic solution whereas neutral solution will not show any colour change.
4. In this way all the three are identified.
The other method after step 2 can also be:
3. Now we will mix basic solution with the other two solutions in two different beakers.
4. The beaker which becomes hot due to neutralization reaction is the one with the mixture of basic and acidic liquid whereas the second one is with basic and neutral solution.
Q.2 Why should curd and sour substances not be kept in brass and copper vessels?
Ans Curd and sour substances contain acids. When they are kept in brass and copper vessels the metal reacts with the acid to liberate hydrogen gas and other harmful products which may spoil the substances.
Metal + Acid → Salt + Hydrogen gas
Q.3 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 Hydrogen (H2) gas is usually liberated when an acid reacts with a metal.
Metal + Acid → Salt + Hydrogen gas
Q.4 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.
Q.5 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, HNO3 etc. show acidic characters in aqueous solutions because they dissociate into hydrogen ions H+ (aq) or hydronium ion (H3O+) in the presence of water.
HCl → H+ + Cl-
H+ + H2O → H3O+
Compounds like alcohol and glucose which are having hydrogen do not show acidic character because they do not dissociate into hydrogen ions in aqueous solution.
Q.6 Why does an aqueous solution of an acid conduct electricity?
Ans Acid dissociates into ions in aqueous solution, which conduct electricity.
Q.7 Why does dry HCl gas not change the colour of the dry litmus paper?
Q.8 While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?
Q.9 How is the concentration of hydronium ions (H3O+) affected when a solution of an acid is diluted?
Ans Decreases. Mixing an acid with water results in decrease in the concentration of ions (H3O+) per unit volume.
Q.10 How is the concentration of hydroxide ions (OH–) affected when excess base is dissolved in a solution of sodium hydroxide?
Ans Increases
Q.11 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.12 What effect does the concentration of H+(aq) ions have on the nature of the solution?
Q.13 Do basic solutions also have H+(aq) ions? If yes, then why are these basic?
Q.14 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)?
Q.15 What is the common name of the compound CaOCl2?
Ans Bleaching powder
Q.16 Name the substance which on treatment with chlorine yields bleaching powder.
Q.17 Name the sodium compound which is used for softening hard water.
Q.18 What will happen if a solution of sodium hydrocarbonate is heated? Give the equation of the reaction involved.
Q.19 Write an equation to show the reaction between Plaster of Paris and water.
EXERCISES
Q.1 A solution turns red litmus blue, its pH is likely to be
(a) 1 (b) 4 (c) 5 (d) 10
Ans (d) 10
A solution which turns red litmus blue is basic with pH > 7.
Q.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
Ans (b) HCl
Egg-shells contain calcium carbonate (CaCO3) which on reaction with HCl liberates carbon dioxide (CO2) which turns lime-water milky.
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 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
Ans (d) 16 mL
Since the quantity of NaOH solution is doubled, the amount of HCl required to neutralise it will also be doubled.
Q.4 Which one of the following types of medicines is used for treating indigestion?
(a) Antibiotic
(b) Analgesic
(c) Antacid
(d) Antiseptic
Ans (c) Antacid
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 categorised as acids. Describe an Activity to prove it.
Q.7 Why does distilled water not conduct electricity, whereas rain water does?
Ans The electric current is carried by ions in solutions. Distilled water does not conduct electricity because it does not have ionic compounds which can dissociate and give hydronium H3O+ ions. On the other hand, rain water contains acids such as carbonic acid(H2CO3) and sulphurous acid (H2SO3) due to dissolution of CO2, SO2 etc. from the atmosphere and dissociates to give H3O+ ions and thus conducts electricity.
Q.8 Why do acids not show acidic behaviour in the absence of water?
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.
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. Amount and concentration taken for both the acids are same. In which test tube will the fizzing occur more vigorously and why?
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.
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?
Q.13 Plaster of Paris should be stored in a moisture-proof container. Explain why?
Q.14 What is a neutralisation reaction? Give two examples.
Q.15 Give two important uses of washing soda and baking soda.