Chemical Reactions and Equations
1. How photochemical reactions have played an important role in photography?
Ans: A photography film used in black and white photography is a type of coating with silver chloride. Its working is based on the decomposition of silver chloride in the presence of sunlight.
Ans: Iron particles are shiny when new but get coated with a reddish brown powder when left for some time. This process is commonly known as rusting of iron which is an example of oxidation reaction.
3. Why do not a wall immediately acquire white colour when a coating of slaked lime is applied on it?
Ans: When a solution of slaked lime is applied on the wall, carbon di oxide gas present in air slowly reacts with thin layer of calcium hydroxide to form a thin layer of calcium carbonate, that is quite while.
4. Metal compound A reacts with dilute hydrochloric acid to produce effervescence. The gas evolved extinguishes a burning candle. Write a balanced chemical equation for the reaction, if one of the compounds formed is calcium chloride.
Ans: Carbon di oxide is the gas that extinguishers a burning candle.
Carbon di oxide is produced when metal carbonate react with acid.
Since, one of the products is Calcium chloride.
Thus, the compound A should be calcium carbonate.
The balanced chemical equation is:
CaCO3(s) + 2HCl → CaCl2(aq) + CO2(g) +H2O(l)
5. What is the common name of the compound CaOCl2?
Ans: Bleaching powder
6. Name the sodium compound which is used for softening hard water.
Ans: Sodium carbonate.
7. Why do acids not show acidic behaviour in the absence of water?
Ans: Ions are produced only in aqueous medium and presence of H+ ions are responsible for the existence of acidic properties. Hence, acids show acidic behaviour only in the presence of water and not in its absence.
8. Give two important uses of washing soda and baking soda.
Ans: Uses of washing soda are:
(i) It is used as a cleansing agent.
(ii) It is used to remove permanent hardness of water.
Uses of baking soda are:
(i) It is used in bakery.
(ii) It is used for extinguishing fire.( as Soda-acid fire extinguishers)
9. If a few drops of concentrated acid accidentally spills over the hand of a student, what should be done?
Ans: Wash the hand immediately with plenty of water and apply a paste of baking soda. Baking soda is a mild base. Here, a strong base cannot be used to neutralize the acid due to its corrosive nature.
10. What happen when nitic acid acid is added to egg shell?
Ans: Egg shells contain calcium carbonate. On reacting it with nitic acid, a brisk effervescence of carbon di oxide ga is produced.
The reaction involved is:
CaCO3(s) + 2HNO3 (aq) → Ca(NO3)2(aq) + H2O (l) + CO2(aq)
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