CBSE Class 12-science Answered

We know that colloid particles are charged.This is because of selective adsorption on the surface of a colloidal particles. This layer attracts the counter ion fromthe medium forming a second layer. The combination of the two layers of opposite charges around the colloidal particle is called Helmholtz electrical double layer.

According to modern views, the first layer of ions is firmly held and is termed fixed layer while the second layer is mobile which is termed diffused layer.

Due to separation of charges  i.e. the charges of opposite signs on the fixed and diffused parts of the double layer, a potential difference is developed between these layers.

This potential difference between the fixed layer and the diffused layer of opposite charges is called the electrokinetic potential or zeta potential.