Some More Written Practice Questions

9). Define olfactory indicators. Name two subtances which can be used as olfactory indicator.

Ans – Those substances whose smell (odour) changes in acidic or basic solution are called olfactory indicators, e.g. onion and vanilla.

10).  What is tooth enamel chemically? State the condition when it starts corroding. What happens when food particles left in the mouth after eating degrade? Why do doctors suggest use of tooth powder/toothpaste to prevent tooth decay?  [CBSE 2011]

Ans) It is made up of calcium phosphate. It starts corroding due to acid formed in mouth. The food particles which are left in mouth form acids which cause tooth decay. Toothpaste and tooth powder are basic and neutralise acid formed in mouth which prevents tooth decay

11). (a) Mention the pH range within which our body works. Explain how antacids give relief from acidity. Write the name of one such antacid.

(b) Fresh milk has a pH of 6. How does the pH will change as it turns to curd? Explain your answer.

(c) A milkman adds a very small amount of baking soda to fresh milk. Why does this milk take a longer time to set as curd?

(d) Mention the nature of toothpastes. How do they prevent tooth decay? [CBSE 2012] [5 marks]

(a) Our stomach has pH equal to 2. Antacids neutralize excess acid in our body and give relief from hyperacidity. Sodium hydrogencarbonate is one of such antacids.

(b) pH decreases  as it turns to curd because curd is acidic due to the presence of lactic acid.

(c) It takes longer to set as a curd because bacteria do not work well in presence of sodium hydrogen carbonate, i.e. fermentation will take place slowly.

(d) Toothpastes are basic in nature. They neutralize the acid formed in the mouth which causes tooth decay.

12).  (i) Give the constituents of baking powder

(ii) Why does cake or bread swell on adding baking powder? Write chemical equation. [CBSE 2013] [3 marks]

Answer –
(i) Baking powder contains sodium hydrogen carbonate and tartaric acid.(ii)It is due to carbon dioxide
The chemical equation is as follows: 
2NaHCO3 (s) heat——–> Na2CO3 (s) + CO2(g) + H2O (l)

13). (a) Identify the acid and the base whose combination forms the common salt that you use in your food. Write its formula and chemical name of this salt. Name the source from where it is obtained.

(b) What happens when electricity is passed through brine? Write the chemical equation for it.

Ans – (a) HCl is acid and NaOH is base whose combination forms the common salt. Its formula is NaCl (Sodium chloride). It is obtained from sea water.

(b) When electricity is passed through brine, following reaction takes place:

2Nacl  + H2O electrolysis——-> 2NaOH + Cl2 + H2