Acids, Bases & Salts

1. How is the concentration of hydronium ion (H3O+) affected when a solution of an
acid is diluted? (1 Mark)

Ans. The concentration of hydronium ions, decreases when a solution of an acid is
Diluted.

2. Fresh milk has a PH of 6. How do you think the PH will change as it turns into curd? (1 Mark)

Ans. Its PH will decrease due to the production of lactic acid which is acidic in nature.

3. What is the PH of acid rains? ( 1 Mark )

Ans. Less than 5.6

4. Tooth enamel is one of the hardest substances in our body. How does it undergo
damage due to eating chocolates & sweets. What should we do to prevent it ?. ( 2 marks )

Ans. The bacteria present in mouth produce acids by the degradation of sugar & food
particles present in the mouth after eating. The acid so produced corrodes tooth
Enamel (Ca3(PO4)2). In order to prevent tooth decay we should clean the mouth
after eating food & we should use alkaline tooth paste.

5. Explain why for diluting an acid concentrated acid is added to water & not water to concentrated acid ?

Ans. This is because the process of dissolution of an acid in water is highly exothermic
process. For dilution the acid must be added slowly to water with constant stirring.
If water is added to acid, the heat generated may cause the mixture to splash out and cause burns. The glass container may also break due to excessive local heating.

6. A house wife found that the cake prepared by her is hard and small in size. Which
ingredients has she forgotten to add that would have made the cake fluffy? Give
reason.

Ans. She has forgotten to add baking powder. When baking powder is added [Sodium bicarbonate and tartaric acid mixture] during the preparation of cake and heated then
sodium bicarbonate reacts with tartaric acid to form CO2 gas. This CO2 evolved
makes the cake to rise and become light and fluffy. Since the cake prepared by the
house wife is small and hard, it means that she had forgotten to add baking
powder.

Q.7. 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, have will you identify the contents of each test
tube?

Ans. Dip red litmus paper in solution repeatedly in each tube.
(a) The tube in which the red litmus paper turns purple contains distilled water.
(b) The tube in which red litmus paper turns blue contains basic solution.
(c) The tube in which red litmus paper remains red contains acidic solution.

Q.8. Do basic solutions also have H+ (aq) ions? If yes, then why are these basic? What is an alkali ? (3 Marks)

Ans: Basic solutions also contain H+ (aq) ions. But in basic solutions:

[H+] 10-7 M
Since (OH-aq) is more than [H+aq]hence these are basic solutions.
Bases that are soluble in water are called alkalis. For example NaOH, KOH, Ca(OH)2

Q. 9. What is neutralization reaction? Write 3 applications of neutralization reaction.

Ans. Neutralization it is an interaction of an acid with a base to form salt and water.
e.g. HCI + NaOH NaCl + H2 O
Acid Base Salt Water

Q. 10. Application :(i) Antacids neutralize the excess acids in our stomach and relieve us
from pain Ex-magnesium hydroxide (milk of magnesia).
(ii) If the pH of the sail is less than 7 i.e. it is acidic, the farmer will treat the soil
with quick time, slaked time, chalk etc to neutralize the acidity.
(iii) Bee sting contains methanoic acid which causes pain and irritation. Use of
mild base. Ex baking soda neutralizes it.
Q 10. a) How do metal carbonates and metal hydrogen carbonates react with acids?
Explain with the help of an activity.
(b) What happens when sulphuric acid in rain reacts with calcium carbonate in buildings
Made from limestone? (5marks)
Ans. (a) All metal carbonates and hydrogen carbonates react with acids to form a
corresponding salt, carbon dioxide & water.

Metal Carbonate + Acid -> Salt + Carbon dioxide + water
Metal hydrogencarbonate + Acid -> Salt + Carbon dioxide + Water.

For ex, Sodium carbonate reacts with dilute hydrochloric acid as follows:-
Na2CO3 (S) + 2 HCI (aq) -> 2 NaCI (aq + H20 (l) + CO2 (g)

Sodium hydrogen carbonate reacts with dilute hydrochloric acid as follows:-
NaHCO3 (S) + HCI (aq) -> NaCI (aq) + H2O(l) + C02 (g)

Activity :- Text book page number 20 Activity 2.5

Observation:- CO2 is produced with an effervescence. When this gas is allowed to
pass through lime water it becomes cloudy. It is a test for CO2.
b) When sulphuric acid in rain reacts with calcium carbonate in buildings made
from limestone calcium sulphate and Carbon dioxide is produced.
CaCO3 + H2SO4 -> CaSO4 + CO2 + H2O.
http://ping.fm/OF2LC