Litmus is made from plant pigment. it is red when placed in an acidic solution.
it is blue when placed in an alkaline solution.
it is purple when neutral.

what do we call substance that change colour like this?

For (a): The substances are known as acids and bases.

For (b): The color of the litmus for pH = 10 will be blue.

For (c): The color of the litmus for pure water will be purple.  

Explanation:

pH determines the acidity or basicity of a solution. There are three types of solution classified based on the pH:

Litmus paper is used as an indicator to know the type of solution. There are 2 types of litmus paper:

(a):

An acid is defined as the chemical species that produces hydrogen ions in the solution while a base is defined as the chemical species that produces hydroxide ions in the solution.

(b):

Given pH value = 10

The solution is basic in nature, thus the color of litmus will be blue.

(c):

Pure water is neutral in nature and has a pH of 7.

Thus, the solution of litmus would be purple.

it's c. elements are placed in rows based on how may valencies electrons have.