Write the essential condition for the following reaction to take place

$2\mathrm{AgBr}\to 2\mathrm{Ag}+{\mathrm{Br}}_2$

Write one application of this reaction.

When the solution of substance X is added to a solution of potassium iodide, then a yellow solid separates out from the solution.

a) What do you think substance X is likely to be?

b) Name the substance which the yellow solid consist of?

c) Which characteristic of the chemical reaction is illustrated by this example?

d) Write a balanced chemical equation for the reaction which takes place. 

In the electrolysis of water :

Name the gas collected at cathode and anode.

In the electrolysis of water:

Why is the volume of gas collected at one electrode double than the other?

In the electrolysis of water :

What would happen if dilute ${\mathrm{H}}_2{\mathrm{SO}}_4$ is not added to water? 

Solid calcium oxide was taken in a container and water was added slowly to it.

State two observations made in the experiment.