1. A solution turns red litmus blue, its pH is likely to be
(a) 1 (b) 4 (c) 5 (d) 10
Answer: (d) 10
Solution turns red litmus blue suggests that solution is basic in nature. Also, pH of basic solutions is more than 7. Therefore, solution will have pH 10.
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
Answer: (b) HCl
Crushed egg shells contains calcium carbonate, which on reaction with lime water releases carbon dioxide gas, that turns lime water milky.
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
Answer: (d) 16 mL
10 ml of NaOH needs 8 ml of HCl solution. Therefore, 20 ml of NaOH will need 8 x 2 = 16 ml of HCl solution.
4. Which one of the following types of medicines is used for treating indigestion?
(a) Antibiotic (b) Analgesic (c) Antacid (d) Antiseptic
Answer: (c) Antacid
Medicines used in treating indigestion is called antacid.
5. Write word equations and then balanced equations for the reaction taking place when –
(a) dilute sulphuric acid reacts with zinc granules.
Answer: Dilute sulphuric acid + Zinc ⟶ Zinc sulphate + Hydrogen
H2SO4(aq) + Zn ⟶ ZnSO4(aq) + H2(g)
(b) dilute hydrochloric acid reacts with magnesium ribbon.
Answer: Dilute hydrochloric acid + Magnesium ⟶ Magnesium chloride + Hydrogen
2HCl(aq) + Mg ⟶ MgCl2(aq) + H2(g)
(c) dilute sulphuric acid reacts with aluminium powder.
Answer: Dilute sulphuric acid + Aluminium ⟶ Aluminium sulphate + Hydrogen
3H2SO4 + 2Al ⟶ Al2(SO4)3 + 3H2(g)
(d) dilute hydrochloric acid reacts with iron filings.
Answer: Dilute hydrochloric acid + Iron ⟶ Ferric chloride + hydrogen
6HCl(aq) + 3Fe ⟶ 3FeCl2 + 3H2(g)
6. Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an Activity to prove it.
Answer: Take two nails fitted in cork and place it in a beaker. Nails are then connected to battery terminals through bulb and switch. Pour dilute HCl in beaker and switch on the current. Repeat this experiment for alcohol and glucose.
It can be observed that bulb glows in case of HCl solutions and bulb doesn’t glows in case of glucose or alcohol solution. Bulb glows in case of HCl solution because HCl dissociates into H+ and Cl- ions, which conducts electricity causing bulb to glow. Glucose or alcohol doesn’t dissociates, hence donot conduct electricity. This activity concludes that all acids contains hydrogen but all hydrogen containing compounds are not acids. This is the reason, why alcohols and glucose are not categorised into acids.
7. Why does distilled water not conduct electricity, whereas rain water does?
Answer: Distilled water is pure form of water which do not contain ionic species and thus, it do not conduct electricity. Rain water is impure form of water containing impurities like acids, salts, etc having ionic species. Thus, rain water conducts electricity.
8. Why do acids not show acidic behaviour in the absence of water?
Answer: Hydrogen ions in acids are responsible for acidic behaviour of acids. In presence of water dissociation of hydrogen ions takes place. In absence of water dissociation do not takes place thus acids do not show acidic behaviour.
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.
Answer: (a) neutral – Solution D having pH 7
(b) strongly alkaline – Solution C having pH 11
(c) strongly acidic – Solution B having pH 1
(d) weakly acidic – Solution A having pH 4
(e) weakly alkaline – Solution E having pH 9
pH in increasing order of concentration hydrogen ion 11 < 9 < 7 < 4 < 1
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?
Answer: Fizzing will occur more vigorously in test tube A containing hydrochloric acid is added. This is because hydrochloric acid is strong acid in comparison to acetic acid, thus produce more amount of hydrogen through fizzing.
11. Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.
Answer: pH of milk is 6. Curd is acidic in nature. As milk changes to curd its pH will decrease.
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?
Answer: (a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline? – This is because in this condition milk doesn’t get converted into curd easily.
(b) Why does this milk take a long time to set as curd?- This milk is basic that actual milk, therefore acid produced for setting it into curd is neutralised by the base, thus it takes long time to set as curd.
13. Plaster of Paris should be stored in a moisture-proof container. Explain why?
Answer: Plaster of paris when comes in contact with moisture hardens to form a substance called gypsum. Therefore, plaster of paris should be stored in moisture proof container.
14. What is a neutralisation reaction? Give two examples.
Answer: Reaction in which acid and base reacts to form salt and water is called neutralisation reaction. Following example shows neutralization reaction.
Sodium hydroxide NaOH reacts with hydrochloric acid HCl to form sodium chloride and water.
NaOH + HCl ⟶ NaCl + H2O
During acidity we take antacid which a base that neutralises excess of acids in the stomach. This provides relief during acidity.
15. Give two important uses of washing soda and baking soda.
Answer:
Washing soda-
- Used in removing hardness of water
- Used in soap and detergents
Baking Soda-
- Used to make bakery products like cakes, biscuits, etc.
- Used to make idlis fluffy
- Used as fire extinguisher
