Class 10 Science
Chapter 2 Acids, Bases and Salts
Intext Questions
Page No: 18
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 slight change in the colour of red litmus (such as purple) then it is distilled water.
Page number 22
1. Why should curd and sour substances not be kept in brass and copper vessels
Ans:
Curd and sour food substances contain acids; these acidic substances combine with metal. This reaction turns food to poison which damage people’s health.
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:
Hydrogen gas is usually liberated when an acid reacts with metal.
Take few pieces of zinc granules and add 5 ml of dilute H2SO4. Shake it and pass the gas produced into a soap solution. The bubbles of the soap solution are formed. These soap bubbles contain hydrogen gas.
H2SO4 + Zn → ZnSO4 + H2 ↑
We can test the evolved hydrogen gas by its burning with a pop sound when a candle is brought near the soap bubbles.
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:
CaCO3 (s) + 2HCl (Aq) → CaCl2( Aq) + CO2 (g) + H2O (l)
Page number – 25
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:
H+ ions in aqueous solution are responsible for acidic character. HCl, HNO3, etc. give H+ ions in water while alcohol and glucose do not give H+ ion in water. Therefore, alcohol and glucose do not show acidic character.
2. Why does an aqueous solution of an acid conduct electricity?
Ans:
The aqueous solution of an acid conducts electricity due to the presence of charged particles called ions in it.
3. Why does dry HCl gas not change the colour of the dry litmus paper?
Ans:
Dry HCl gas does not give H+ ions and therefore does not change the colour of dry litmus paper.
4. While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?
Ans:
because if water is added to concentrated acid, it release huge amount of heat which may result in explosion and can cause acid burns on face, clothes and body parts.
5. How is the concentration of hydronium ions (H3O+) affected when a solution of an acid is diluted?
Ans:
There is a fixed number of hydronium ions per volume of the solution. On dilution, the number of hydronium ions per volume decreases and concentration decreases.
6. How is the concentration of hydroxide ions (OH–) affected when excess base is dissolved in a solution of sodium hydroxide ?
Ans:
The concentration of hydroxide ions (OH−) would increase when excess base is dissolved in a solution of sodium hydroxide.
Page number – 28
1. 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?
Ans:
A pH value of less than 7 indicates an acidic solution, while greater than 7 indicates a basic solution. Since solution A has more hydrogen ion concentration, solution A is acidic and solution B is basic.
2. What effect does the concentration of H+ (aq) ions have on the nature of the solution ?
Ans:
More the concentration of H+ ions, higher the acidic nature of the solution.
3. Do basic solutions also have H+ (aq) ions ? If yes, then why are these basic ?
Ans:
Yes, H+ ions are far less in number than OH– ions that is responsible for their basic nature.
4. 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) ?
Ans:
If the soil is too acidic (having low pH) then it is treated with materials like quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate).
Page Number: 33
1. What is the common name of the compound CaOCl2 ?
Ans: Bleaching powder.
2. Name the substance which on treatment with chlorine yields bleaching powder.
Ans: Slaked lime Ca (OH)2.
3. Name the sodium compound which is used for softening hard water.
Ans: Sodium carbonate.
4. What will happen if a solution of sodium hydrogen carbonate is heated. Give the equation of the reaction involved ?
Ans:
Solution of sodium hydrogen carbonate on heating gives sodium carbonate and carbon dioxide gas is evolved.
5. Write an equation to show the reaction between plaster of Paris and water.
Ans:
Exercise questions
1. A solution turns red litmus blue, its pH is likely to be
a) 1 (b) 4 (c) 5 (d) 10
Ans: (d) 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
Ans:
(b) HCl
3. 10 mL of a solution of NaOH is found to be completely neutralised by 8 mL of a given solution of HC1. If we take 20 mL of the same solution of NaOH, the amount of HC1 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
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
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.
Ans:
a) Zn (s) + H2SO4 (aq) → ZnSO4 (aq) + H2 (g)
b) Mg (s) + 2 HCl (aq) → MgCl2 (aq) + H2 (g)
c) 2Al (s) + 3H2SO4 (aq) → Al2 (SO4)3 (aq) + 3H2 (g)
d) 2Fe (s) + 6HCl (aq) → 2FeCl3 (aq) + 3H2 (g)
6. Compounds such as alcohol and glucose also contain hydrogen but are not categorised as acids. Describe an activity to prove it.
Ans:
> Take solutions of alcohols and glucose.
> Fix two nails on a cork, and place the cork in 100 mL beaker.
> Connect the nails to the two terminals of a 6 volt battery through a bulb and a switch, as shown in the given Figure.
> Now pour alcohol in the beaker and switch on the current.
> The bulb does not glow.
> Repeat the experiment with glucose. The bulb does not glow in this case also.
> This means no ions or H+ ions are present in the solution.
> This shows that alcohols and glucose are not acids.
Ans:
> Distilled water does not contain any ionic compounds in it.
> Rainwater has dissolved acidic gas such as carbon dioxide from the air and that forms carbonic acid. presence of acids, rainwater can conduct electricity.
8. Why do acids not show acidic behaviour in the absence of water ?
Ans:
The acid produces hydrogen ions only in the presence of water.
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.
Ans:
(a) D (b) C (c) B (d) A (e) E
Increasing order of hydrogen ion concentration
11 < 9 < 7 < 4 < 1
i. e., C < E < D < A < B
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 ?
Ans:
The fizzing will occur strongly in test tube A, in which hydrochloric acid (HCl) is added. This is because HCl is a stronger acid than CH3COOH and therefore produces hydrogen gas at a faster speed due to which fizzing occurs.
11. fresh milk has a pH of 6. How do you think the pH will change as it turns into curd ? Explain your answer.
Ans:
pH of milk falls below 6 as it turns into curd due to the formation of lactic acid during this process. Lactic acid present in it reduces its pH value.
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 ?
Ans:
a) because in alkaline condition, milk does not set as curd easily.
b) The alkaline milk takes a longer time to set into curd because the lactic acid being formed has to first neutralise the alkali present in it.
13. Plaster of Paris should be stored in a moisture proof container. Explain why?
Ans:
Because it absorbs water from moisture and turn into hard substance (Gypsum).
14. What is a neutralisation reaction ? Give two examples.
Ans:
The reaction between an acid and a base to form salt and water is called a neutralisation reaction.
Examples:
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