which of the following is not a colloid ?

Colloids are mixtures where microscopically dispersed insoluble particles are suspended throughout another substance, with particle sizes typically ranging from 1 nm to 1 m [1]. Milk and butter are specifically classified as emulsions, which are liquid-liquid colloidal systems [2]. Mud is a suspension of soil particles in water; soil itself contains soil colloids, which are fine particles that hold nutrients for plants [2]. In contrast, boric acid (H3BO3) is a white, crystalline solid that dissolves in water to form a true solution rather than a colloidal dispersion. While specialized "micro-colloidal" compositions involving boric acid can be engineered for specific industrial or bioenhancing purposes, the pure substance in its standard form acts as a crystalloid that forms a homogeneous solution. Therefore, boric acid is not inherently a colloid.

Sources

1. [1] https://www.sciencedirect.com/topics/chemistry/colloid-chemistry
2. [2] Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 1: BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY > sEdimEntary cyclEs. > p. 25