What is rancidity? Mention any two ways by which rancidity can be prevented?

The following diagram displays a chemical reaction. Observe it carefully and answer the following questions:
$(a)$ Identify the type of chemical reaction that will take place and define it. How will the colour of the salt change?
$(b)$ Write the chemical equation of the reaction that takes place.
$(c)$ Mention one commercial use of this salt.

Write two observations for the following chemical reaction:
$(a)$ Lead nitrate is strongly heated in a hard glass test tube.

Observe the following diagram and answer the questions that follow:
$(i)$ Identify the gases evolved at anode and cathode.
$(ii)$ Why are the amounts of gases collected in the two test tubes not of the same volume?
$(iii)$ What type of reaction is this?
$(iv)$ Why should we use salt water?

Write two observations for the following chemical reaction:
$(a)$ Potassium iodide solution is added to lead nitrate solution.

Can oxidation or reduction take place alone? Why or why not? What are such reactions called?