# NCERT Exemplar Problems and their Solutions

Reviewed By:
Krishna Kant Majee
M.Sc., B.Ed.

### Chapter 2-Chemical Reactions and Equations

Q.1. What happens when a solution of an acid is mixed with a solution of a base in a test tube?
(i) The temperature of the solution increases.
(ii) The temperature of the solution decreases.
(iii) The temperature of the solution remains the same.
(iv) Salt formation takes place.
(a) Only (i)
(b) (i) and (iii)
(c) (ii) and (iii)
(d) (i) and (iv)

Explanation: The reaction of acid and base is an exothermic reaction and results the formation of salt and water.

Q.2. An aqueous solution turns red litmus solution blue. Excess addition of which of the following solution would reverse the change?
(a) Baking powder
(b) Lime
(c) Ammonium hydroxide solution
(d) Hydrochloric acid

Explanation: Alkaline solutions turn red litmus paper to blue. Excess of acid will reverse the changes so we have to add excess of hydrochloric acid which turns the solution acidic.

Q.3. During the preparation of hydrogen chloride gas on a humid day, the gas is usually passed through the guard tube containing calcium chloride. The role of calcium chloride taken in the guard tube is to
(a) Absorb the evolved gas
(b) Moisten the gas
(c) Absorb moisture from the gas
(d) Absorb Cl- ions from the evolved gas

Explanation: Calcium chloride is a good dehydrating agent so it is used to absorb moisture from the hydrogen chloride gas.

Q.4. Which of the following salts does not contain water of crystallisation?
(a) Blue vitriol
(b) Baking soda
(c) Washing soda
(d) Gypsum

Explanation:
(a) Blue vitriol =$CuSO_{4}•5_H{2}O$
(b) Baking soda = $NaHCO_{3}$
(c) Washing soda=$Na_{2}CO_{3}•10H_{2}O$
(d) Gypsum = $CaSO_{4}• 2H_{2}O$

Q.5. Sodium carbonate is a basic salt because it is a salt of
(a) Strong acid and strong base
(b) Weak acid and weak base
(c) Strong acid and weak base
(d) Weak acid and strong base

Explanation: Sodium carbonate $(Na_{2}CO_{3})$ is the salt of carbonic acid (H2CO3) and sodium hydroxide (NaOH). Here NaOH is a strong base and carbonic acid is a weak acid.

Q.6. Calcium phosphate is present in tooth enamel. Its nature is
(a) Basic
(b) Acidic
(c) Neutral
(d) Amphoteric

Explanation: Calcium phosphate $Ca_{3}(PO_{4})_{2}$ is salt of weak acid and strong base so it is a basic salt.
$3Ca(OH)_{2}+2H_{3}PO_{4}\rightarrow Ca_{3}(PO_{4}) + 6H_{2}O$

Q.7. A sample of soil is mixed with water and allowed to settle. The clear supernatant solution turns the pH paper yellowish-orange. Which of the following would change the colour of this pH paper to greenish-blue?
(a) Lemon juice
(b) Vinegar
(c) Common salt
(d) An antacid

Explanation: pH paper gives greenish blue colour in weak alkaline medium so antacid $(Mg(OH)_{2})$ which is an alkaline compound will show greenish blue colour on pH paper.

Question. 8 Which of the following gives the correct increasing order of acid strength?
(a) Water < acetic acid < hydrochloric acid
(b) Water < hydrochloric acid < acetic acid
(c) Acetic acid < water < hydrochloric acid
(d) Hydrochloric acid < water < acetic acid

Explanation: Water is a natural substance, hydrochloric acid is a strong acid as it ionizes completely in water, whereas acetic acid ionizes only partially in water; hence, it is a weak acid.

Q.9. If a few drops of a concentrated acid accidentally spills over the hand of a student, what should be done? (a) Wash the hand with saline solution (b) Wash the hand immediately with plenty of water and apply a paste of sodium hydrogen carbonate (c) After washing with plenty of water applies solution of sodium hydroxide on the hand (d) Neutralise the acid with a strong alkali

Explanation: Use of sodium hydrogen carbonate neutralizes the effect of acid completely.

Q.10. Sodium hydrogen carbonate when added to acetic acid evolves a gas. Which of the following statements are true about the gas evolved? (i) It turns lime water milky. (ii) It extinguishes a burning splinter. (iii) It dissolves in a solution of sodium hydroxide. (iv) It has a pungent odour. (a) (i) and (ii) (b) (i), (ii) and (iii) (c) (ii), (iii) and (iv) (d) (i) and (iv)

Explanation: Reaction of sodium hydrogen carbonate with acetic acid forms sodium acetate and water with carbon dioxide $(CO_{2})$ gas. $NaHCO_{3}-> CH_{3}COONa + CO_{2} + H_{2}O$

Q.11 Common salt besides being used in kitchen can also be used as the raw material for making
(i) Washing soda
(ii) Bleaching powder
(iii) Baking soda
(iv) Slaked lime
(a) (i) and (ii)
(b) (i), (ii) and (iv)
(c) (i), (ii) and (iii)
(d) (i), (iii) and (iv)

Explanation: Common salt (sodium chloride) is used as a raw material for manufacturing of sodium hydroxide, washing soda, bleaching powder, baking soda, hydrochloric acid, chlorine, etc.

Q.12 One of the constituents of baking powder is sodium hydrogen carbonate, the other constituent is
(a) Hydrochloric acid
(b) Tartaric acid
(c) Acetic acid
(d) Sulphuric acid

Explanation: Baking powder is a mixture of sodium hydrogen carbonate and a weak edible acid named as tartaric acid.

Q.13. To protect tooth decay we are advised to brush our teeth regularly. The nature of the tooth paste commonly used is
(a) Acidic
(b) Neutral
(c) Basic
(d) Corrosive

Explanation: The tooth paste commonly used is alkaline or basic in nature. So they can neutralize the effect of extra acids present in mouth cavity which are mainly responsible for tooth decay.

Q.14. Which of the following statements is correct about an aqueous solution of an acid and a base?
(i) Higher the pH, stronger the acid
(ii) Higher the pH, weaker the acid
(iii) Lower the pH, stronger the base
(iv) Lower the pH, weaker the base
(a) (i) and (iii)
(b) (ii) and (iii)
(c) (i) and (iv)
(d) (ii) and (iv)

Explanation: The pH values lie between 0 to 14 in which 0 to 7 is acidic medium and 7 to 14 is alkaline or basic medium.

Q.15. The pH of the gastric juices released during digestion is
(a) Less than 7
(b) More than 7
(c) Equal to 7
(d) Equal to 0

Explanation: Hydrochloric acid is produced by our stomach during digestion.

Q.16. Which of the following phenomena occur, when a small amount of acid is added to water?
(i) Ionisation
(ii) Neutralisation
(iii) Dilution
(iv) Formation
(a) (i) and (ii)
(b) (i) and (iii)
(c) (ii) and (iii)
(d) (ii) and (iv)

Explanation: Ionization can be defined as the dissociation of molecule into its respective ions. Mixing of an acid with water decreases the concentration of acid and is called as dilution.

Q.17. Which one of the following can be used as an acid-base indicator by a visually impaired student?
(a) Litmus
(b) Turmeric
(c) Vanilla essence
(d) Petunia leaves

Explanation: An olfactory indicator is required for visually impaired student. Vanilla essence imparts different smell in different medium so can be used for such students.

Q.18. Which of the following substances will not give carbon dioxide on treatment with dilute acid?
(a) Marble
(b) Limestone
(c) Baking soda
(d) Lime

Explanation: Marble or lime stone (CaCO3) and baking soda (NaHCO3) release CO2 with HCl but lime (CaO) with HCl form CaCl2 and water.

Q.19. Which of the following is acidic in nature?
(a) Lime juice
(b) Human blood
(c) Lime water
(d) Antacid

Explanation: Lime juice contains citric acid so acidic in nature.

Q.20. In an attempt to demonstrate electrical conductivity through an electrolyte, the following apparatus (figure) was set up.

Which among the following statement(s) is/are correct?
(i) Bulb will not glow because electrolyte is not acidic.
(ii) Bulb will glow because NaOH is a strong base and furnishes ions for conduction.
(iii) Bulb will not glow because circuit is incomplete.
(iv) Bulb will not glow because it depends upon the type of electrolytic solution.
(a) (i) and (iii)
(b) (ii) and (iv)
(c) Only (ii)
(d) Only (iv)

Explanation: Strong acids or bases are good conductors of electricity because they ionize completely in aqueous solution to give ions.

Q.21. Which of the following is used for dissolution of gold?
(a) Hydrochloric acid
(b) Sulphuric acid
(c) Nitric acid
(d) Aqua-regia

Explanation: Aqua-regia is a very strong acid and can be used to dissolve gold. It is a 1: 3 mixture of concentrated $HNO_{3}$ and concentrated HCl.

Q.22. Which of the following is not a mineral acid?
(a) Hydrochloric acid
(b) Citric acid
(c) Sulphuric acid
(d) Nitric acid

Explanation: Mineral acids are mainly prepared from the minerals present in the earth’s crust so they are also called as inorganic acids like HCI (hydrochloric acid), H2SO4 (sulphuric acid) and HNO3 (nitric acid). Organic acids are produced from plants or animals and contain carbon atoms like acetic acid, citric acid.

Q.23. Which among the following is not a base?
(a) NaOH
(b) KOH
(c) $NH_{4}OH$
(d) $C_{2}H_{5}OH$

Explanation: $C_{2}H_{5}OH$is an organic compound with $^{-}OH functional group that is known as alcohol. It cannot give OH^{-} ions in its solution. Q.24. Which of the following statements is not correct? (a) All metal carbonates react with acid to give a salt, water and carbon dioxide. (b) All metal oxides react with water to give salt and acid. (c) Some metals react with acids to give salt and hydrogen. (d) Some non-metal oxides react with water to form an acid. Answer. (b) Explanation: Some of the metal oxides react with water to form alkali like sodium oxide reacts with water to form sodium hydroxide. Q.25. Match the chemical substances given in Column I with their appropriate application given in Column II. Answer. (c) Explanation: • Bleaching powder$(CaOCl_{2}) $– Used for bleaching or decolourisation of clothes. • Baking soda$(NaHCO_{3})$- Used as an antacid for relieving stomach acidity. • Washing soda$(Na_{2}CO_{3})$- Used in preparation of glass. • Sodium chloride$ (NaCl)^{-}$Used for production of$H_{2}$and$Cl_{2}\$ gases.