CBSE Class 7 Solved Objective Questions Chapter - Acids, Bases and Salts (MCQs)

CBSE Class 7 Solved Objective Questions  Chapter - Acids, Bases and Salts MCQs

1. The word acid comes from the Latin word ------ which means sour.( acere)

2. The substances used to test whether a substance is acidic or basic are known as ----(indicators)

3. The most commonly used natural indicator is litmus paper . It is extracted from ----(lichens)

4. In acidic solution, litmus paper turns into ------(red)

5. In basic solution, litmus paper turns into ------(blue)

6. In distilled water , litmus paper turns into ------(purple)

7. China rose indicator turns acidic solutions ------and basic solutions to ------ [dark pink (magenta) , green]

8. The reaction between an acid and a base is known as ----- [neutralization]

9. Salt and water are produced in neutralization process with the evolution of heat.[ water]

10. H C l + N a O H ---->      ------- + H2O

11. Our stomach contains -------. It helps us to digest food[hydrochloric acid]

12. indigestion is painful so to relieve indigestion, we take an antacid such as milk of magnesia , which contains ------- [magnesium hydroxide]

13. The sting of an ant contains ---------[ formic acid]

14. The effect of the sting can be neutralized by rubbing moist baking soda (sodium hydrogen carbonate) or calamine solution, which contains ------- [zinc carbonate]

15. When the soil is too acidic, it is treated with ---------------[ bases like quick lime (calcium oxide) or slaked lime (calcium hydroxide)]

16. Ammonia is found in many household products, such as window cleaners. It turns red litmus blue. its nature ---------------------- [Basic]

17. The wastes of many factories contain-------- [ acids]

18. Blue litmus paper is dipped in a solution. It remains blue. -------- is the nature of the solution. [basic]

19. Proteins that build part of our cells are made up of------ [amino acids]

20. The fats in our body contain ------ [fatty acids]

21. Each cell in our body contains an acid called ---------- [the deoxyribonucleic acid]

22. ----------- is common to all acids?[hydrogen ion]

23. ------ gas evolved when dilute sulphuric acid acts on sodium carbonate[CO2]

24. Acids do not show acidic behavior in absence of water because ------ dissociates from an acid only in in presence of water .[ hydrogen ions]

25. ---- base is also used in soda-acid fire extinguishers.[ Baking Soda]

26. -------- gas is usually liberated when an acid reacts with a metal [ H2]

27. The hydrogen ions combine with H2O to form ------------- [ hydronium ions (H3O+)]

28. Aqueous solution of acid conduct electricity due to -------- present in it. [ ion]

29. ------------ is the common name of the compound CaOCl2 [Bleaching powder]

30. -------------- is Universal Indicator [ pH Scale]

31. pH is the measure of Acidity or Alkalinity of a solution. The term pH stands for ----------[“Potential of hydrogen”]

32. If the pH rain water drops below ----------------. it is called acid rain[5.6]

33. The bases that dissolve in water are known as -----------[ alkalies]

34. Aqueous solution of sodium chloride (NaCl) is known as -------------[ Brine]

35. Salts containing water molecules in their crystal form are called --------- [hydrated salts]

36. water molecules present in salt s known as ------------ [‘water of crystallization’]

37. Phenolphthalein become colourless in ---- and pink in ------[ Acid, base]

38. --------- is the colour of methyl orange in acidic solution.[ pink]

39. ------------ and ------- acid are present in vinegar and lemon.[ acetic acid, citric acid]

chemical reaction and equation: Why do fire flies glow at night?

1. A solution of potassium chloride when mixed with silver nitrate solution, an insoluble white substance is formed. Write the chemical reaction involved and also mention the type of the chemical reaction?

Answer:  KCl (aq) + AgNO3 (aq) → AgCl (s) + KNO3 (aq)

It is a double displacement and precipitation reaction.

2. Ferrous sulphate decomposes with the evolution of a gas having a characteristic odour of burning sulphur. Write the chemical reaction involved and identify the type of reaction.

Answer:  2FeSO4(s) --H---e---at→ Fe2O3(s) + SO2(g) + SO3(g)

It is a thermal decomposition reaction

3. Why do fire flies glow at night?

Answer: Fire flies have a protein which in the presence of an enzyme undergoes aerial oxidation. This is a chemical reaction which involves emission of visible light. Therefore, fire flies glow at night.

4. Grapes hanging on the plant do not ferment but after being plucked from the plant can be fermented. Under what conditions do these grapes ferment? Is it a chemical or a physical change?

Answer: Grapes when attached to the plants are living and therefore their own immune system prevents fermentation. The microbes can grow in the plucked grapes and under anaerobic conditions these can be fermented. This is a chemical change.

5. Which among the following are physical or chemical changes?

(a) Evaporation of petrol 

(b) Burning of Liquefied Petroleum Gas (LPG)

(c) Heating of an iron rod to red hot.

(d) Curdling of milk

(e) Sublimation of solid ammonium chloride

Answer:  (a), (c) and (e) — are physical changes. 

(b) and (d) are chemical changes

6. During the reaction of some metals with dilute hydrochloric acid, following observations were made.

(a) Silver metal does not show any change

(b) The temperature of the reaction mixture rises when aluminium (Al) is added.

(c) The reaction of sodium metal is found to be highly explosive

(d) Some bubbles of a gas are seen when lead (Pb) is reacted with the acid.  

Explain these observations giving suitable reasons.

Answer:(a) Silver metal does not react with dilute HCl

(b) The temperature of the reaction mixture rises when aluminium is added because it is an exothermic reaction.

(c) Reaction of sodium metal is found to be highly explosive because it is an exothermic reaction

(d) When lead is treated with hydrochloric acid, bubbles of hydrogen gas are evolved

                  Pb + 2HCl → PbCl2 + H2

6. A substance X, which is an oxide of a group 2 element, is used intensively in the cement industry. This element is present in bones also. On treatment with water it forms a solution which turns red litmus blue. Identify X and also write the chemical reactions involved.

Answer: Calcium oxide CaO(s) + H2O(l) → Ca(OH)2(aq)

7. Write a balanced chemical equation for each of the following reactions and also classify them.

(a) Lead acetate solution is treated with dilute hydrochloric acid to form lead chloride and acetic acid solution.

(b) A piece of sodium metal is added to absolute ethanol to form sodium ethoxide and hydrogen gas.

(c) Iron (III) oxide on heating with carbon monoxide gas reacts to form solid iron and liberates carbon dioxide gas.

(d) Hydrogen sulphide gas reacts with oxygen gas to form solid sulphur and liquid water.

Answer:(a) Pb(CH3COO)2 + 2HCl → PbCl2 + CH3COOH

 Double displacement reaction

(b) 2Na + 2C2H5OH → 2C2H5ONa + H2

 Displacement reaction

(c) Fe2O3 + 3CO → 2Fe + 3CO2

 Redox reaction

(d) 2H2S + O2 → 2S + 2H2O

 Redox reaction

8. Why do we store silver chloride in dark coloured bottles?

Answer:  Silver chloride on exposure to sunlight may decompose as per the following reaction.

2AgCl → 2Ag + Cl2 Therefore, it is stored in dark coloured bottles.

9.Take about 2 g barium hydroxide in a test tube. Add 1 g of ammonium chloride and mix with the help of a glass rod. Touch the bottom of the test tube with your palm. What do you feel? Is this an exothermic or endothermic reaction?

Answer:  Ba(OH)₂ +2NH₄ Cl ----------------à BaCl2 + 2NH₄(OH)      Endothermic reaction

10. What is the difference between the displacement and double displacement reactions? Write equations for these reactions.  

Answer: In displacement reaction, one element from its salt is displaced by a more reactive element e.g. in following example, Cu is displaced by Zn from CuSO₄ because Zn is more reactive.

CuSO₄ + Zn ---> ZnSO₄ + Cu  

Also, Zn(s) + CuSO4 (aq) → ZnSO4 (aq) + Cu(s) ;   Pb(s) + CuCl2(aq) → PbCl2(aq) + Cu(s)

Zinc and lead are more reactive elements than copper. They displace copper from its compounds

In double displacement reaction, exchange of ions takes place between two reactants to form new products.

Na₂SO₄ + BaCl₂ ---> BaSO₄ + 2NaCl

Here, the white precipitate of BaSO4 is formed by the reaction of SO4^-2 and Ba²⁺               

Why do some elements have dual valency?

Why do some elements have dual valency?


Iron exhibits two different oxidation numbers in compounds, +2 and +3, which are seen in two different chemical compounds, FeCl2 and FeCl3, for instance. Stable compounds form because the products are at a lower total energy than the reactants. That is the case for both FeCl2 and FeCl3. But it is possible to cause the further oxidation of iron in FeCl2 to the +3 oxidation state, as in this reaction.2FeCl2 + Cl2 --> 2FeCl3                                                                                                                              Transition metals show variable valency and these elements have d-orbital as penultimate orbital and the outermost orbital is the s-orbital. 
Now, the atomic number of Fe, Iron is 26 
The electronic configuration is 
(1s)2 (2s)2 (2p)6 (3s)2 (3p)6 (4s)2 (3d)6 (aufbau's principle). 


But the exact arrangement is also possible with (1s)2 (2s)2 (2p)6 (3s)2 (3p)6 (3d)6 (4s)2. 


Therefore, because of the different electronic configurations which the element can have it shows variable valency. 


Another reason:  An atom has to complete 2 or 8 electrons in its outermost shell therefore, when Fe or any other transition element reacts with the other element then these transition atoms share the electrons according to their requirements

An atom of an element can sometimes lose more electrons than are present in its valence shell, i.e. Loss from penultimate shell, and hence exhibit more than one or variable valency. 


Eg: 1. Cu 1+ & Cu 2+
      2. Ag 1+ & Ag 2+
      3. Hg 1+ & Hg 2+
      4. Au 1+ & Au 3+
      5. Fe 2+ & Fe 3+
      6. Pb 2+ & Pb 4+
      7. Sn 2+ & Sn 4+
      8. Pt 2+ & Pt 4+
      9.Mn 2+ & Mn 4+