NCERT 7th Class (CBSE) Science: Acids, Bases and Salts

Question: Acids are not stored in metal containers. Why?

Answer: Acids can corrode metals like aluminium and iron due to their corrosive nature. That is why acids are stored in glass containers and not in metal containers.

Question: Why do different substances have different tastes?

Answer: Different substances have different tastes because each food item has unique chemical composition.

Question: Is the distilled water acidic / basic / neutral? How would you verify it?

Answer: Distilled water is neutral by nature. We can verify by performing litmus paper test. It neither turns blue litmus into red. Nor it turns red litmus into blue.

Question: Describe the process of neutralization with the help of an example.

Answer:The reaction between an acid and a base is known as neutralisation. Salt and water are produced in this process with the evolution of heat.

Acid +  Base  ̶ ̶ ̶ ̶ ̶ ̶ ̶ ̶ ̶➝ Salt + Water
For example, when  Hydrochloric acid (HCl) reacts with a base Sodium hydroxide (NaOH), it forms a salt (Sodium Chloride) and Water (H₂O).

Hydrochloric acid  + Sodium hydroxide  ̶ ̶ ̶ ̶ ̶ ̶ ̶ ̶ ̶➝ Sodium chloride  + Water
HCl  + NaOH  ̶ ̶ ̶ ̶ ̶ ̶ ̶➝ NaCl + H₂O

Question: Explain why:

(a) An antacid tablet is taken when you suffer from acidity.
(b) Calamine solution is applied on the skin when an ant bites.
(c) Factory waste is neutralized before disposing it into the water bodies.

Answer: An antacid tablet is taken when you suffer from acidity: Due to spicy food, our stomach releases excess of hydrochloric acid which causes acidity or indigestion. An antacid tablet consists of a base like Milk of Magnesia (magnesium hydroxide). It neutralizes the effect of excessive acid and brings relief.

Calamine solution is applied on the skin when an ant bites: The sting of an ant contains formic acid which causes irritation on the skin. Calamine solution contains zinc carbonate which is a base. Calamine solution neutralizes the acid effect of the ant bite when applied on the skin.

Factory waste is neutralized before disposing it into the water bodies: The wastes of many factories contain acids. If they are allowed to flow into the water bodies, the acids will kill fish and other organisms. The factory wastes are, therefore, neutralised by adding basic
substances.

Question: Three liquids are given to you. One is hydrochloric acid, another is sodium hydroxide and third is a sugar solution. How will you identify them? You have only turmeric indicator.

Answer: Turmeric is yellow in colour. When a base is added to it, the solution turns into pink colour.
However turmeric remains yellow when an acid or neutral solution is added to it. We shall perform the following steps to identify the solution is base, acid or neutral.

1. Take few drops from each solution and test it with turmeric solution. If the solution turns into pink colour that solution is base i.e. Sodium Hydroxide. Mark that beaker (containing solution) as BASE.
2. Take a test tube and add few drops of BASE solution and second solution. Check if test tube becomes warm and then add turmeric solution to it.
3. If the colour does not change, it means the solution added is acidic (HCl). The test tube becomes warm due to neutralization.If the test tube of the above solution does not warm and it does show pink colour when turmeric is added, the second solution is neutral.
4. Repeat step 2 and 3 for the third solution and identify if it is acid or neutral.