Page 40 - NCERT Science Class 10 English Medium

8. Why do acids not show acidic behaviour in the absence 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.
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 (CH COOH) is added to test
3
tube B. Amount and concentration taken for both the acids are same. In which test
tube will the fizzing occur more vigorously and why?

11. Fresh milk has a pH of 6. How do you think the pH will change as it turns into
curd? Explain your answer.
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?
13. Plaster of Paris should be stored in a moisture-proof container. Explain why?
14. What is a neutralisation reaction? Give two examples.
15. Give two important uses of washing soda and baking soda.

Group Activity

(I) Prepare your own indicator

n Crush beetroot in a mortar.
n Add sufficient water to obtain the extract.
n Filter the extract by the procedure learnt by you in earlier classes.
n Collect the filtrate to test the substances you may have tasted earlier.
n Arrange four test tubes in a test tube stand and label them as A,B,C and D. Pour
2 mL each of lemon juice solution, soda-water, vinegar and baking soda solution
in them respectively.

n Put 2-3 drops of the beetroot extract in each test tube and note the colour change
if any. Write your observation in a Table.
n You can prepare indicators by using other natural materials like extracts of red
cabbage leaves, coloured petals of some flowers such as Petunia, Hydrangea and
Geranium.

Acids, Bases and Salts 35

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