Science – Acids, Bases and Salts
Long Answer Type Questions
Q.1) What is the chemical name of Bleaching powder? How is it prepared? List its uses.
Ans) Chemical name of bleaching powder is calcium oxychloride with chemical formula CaOCl₂.
Preparation – It is prepared by passing chlorine gas through dry slaked lime [Ca(OH)₂]
Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O
Slaked lime + Chlorine → Bleaching powder + Water
Uses:-
- As a bleaching agent in the textile industry for bleaching cotton and linen.
- As an oxidising agent in many chemical industries.
- To make drinking water free from germs.
- In paper industry for bleaching wood pulp.
Q.2) Tooth enamel is one of the hardest substances in our body. How does it undergo damage due to eating of chocolates and sweets? What should we do to prevent it?
Ans) Tooth decay starts when the pH of mouth is lower than 5-5. Tooth enamel made of calcium phosphate is the hardest substance in the body. It does not dissolve in water, but is corroded when the pH of mouth is below 5-5. Bacteria present in the mouth produce acids by degradation of sugar, due to chocolates and sweets, and food particles remaining in the mouth after eating. The best way to prevent this is to clean the mouth after eating food. Using toothpastes, which are generally basic, for cleaning the teeth, can neutralise the excess acid and prevent tooth decay.
Q.3) 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 liberated when an acid reacts with a metal.
Take few pieces of zinc (Zn) granules in a delivery tube. Add 5 ml of hydrochloric acid (HCI). White fumes will evolve. Pass the gas to a soap solution or collect it in a balloon. The gas collected is hydrogen. Since hydrogen gas has low density in comparison to air, the balloon will float in air.
Zn(s) + 2HCl(aq) → ZnCl₂(s) + H₂(g)
The evolved gas is hydrogen, this can be tested by bringing a burning candle near it. The gas burns with a pop sound.
Q.4). Differentiate between acids and bases. Name a strong acid and a base. Or
Write any four differences between acid and base.
Ans) Difference between Acids and Bases are: –
| Acids | Bases |
|---|---|
| Acids are sour in taste. | Bases are bitter in taste. |
| They change the colour of blue litmus red. | They change the colour of red litmus blue. |
| Acids dissolve in water to form hydronium (H₃O⁺) ions. | Bases dissolve in water to form hydroxyl (OH⁻) ions. |
| They react with metals to form hydrogen gas. | They react with metals to form salt and hydrogen. |
| They react with carbonates to form CO₂.. | They do not form CO₂ with carbonates. |
| Their solution contains H+ ions. | Their solution contains OH⁻ ions. |
| They react with bases to form salt and water. H2SO4 is a strong acid. | They react with acids to form salt and water. NaOH is a strong base. |
Q.5) Write any four uses of Plaster of Paris.
Ans) Uses of Plaster of Paris-
- In making of plaster to mend bones.
- In making casts of teeth in dentistry.
- In making fire proof substances and air proof laboratory apparatus.
- In making of toys, statues, jewellery, chalk and decorative designs.
Q.6) How does tooth decay occur due to pH change? How to prevent this?
Ans) Tooth decay due to pH change – When the pH level in the mouth falls below 5-5 the tooth decay occurs. The neutral pH level is 7-0. So this will drop when acidic food and drinks are consumed.
To prevent tooth decay – We should avoide acidic food and drinks, we should clean our teeth after taking meal. We should brush our teeth at bed time and in morning.
Q.7) Write two uses of each of the following-
Ans) Use of Baking soda
- As an antacid.
- In bakery production.
Ans) Use of Bleaching powder
- For bleaching clothes and papers.
- As an insecticide.
Ans) Use of Plaster of Paris
- In making of plaster to mend bones.
- In making of toys, statues, jewelry, chalk, and decorative designs.
Ans) Use of washing soda
- For washing and cleaning.
- IN glass industries.