Question

In colloidal systems, which particles have size of $1$ nm - $1000$ nm?

Answer 1

We know that the colloids are heterogeneous mixtures of two substances in which minute particles of one substance are dispersed in another substance. The substance whose minute particles are suspended in another substance is called the dispersed phase while the substance in which it is suspended is called dispersion medium.

Complete answer: Colloids are used in foods and food industries at large levels. Many foods which we consume are actually colloidal in nature. Such as milk, cheese etc. Colloids have various applications in the medicinal field as well. Many medicines which we use are in the form of emulsions. Antibiotics such as penicillin and streptomycin are given in the form of colloidal solutions so that they can be absorbed by the human body easily. Colloids are used in water purification. There are various systems, where the particles are considerably larger and may in fact range from upper limit for ordinary solutions up to several micrometers in size. Systems which consist of dissolved or dispersed particles ranging approximately between $1$ to $1000$ nm are called colloids. Colloidal particles are smaller in size, and cannot be normally seen with naked eye. The particles of colloids can be easily passed through normal filter paper.
The phase which is dispersed or scattered through the dispersion medium is called Dispersed phase or discontinuous phase. The phase in which the scattering is done is called the dispersion medium or continuous medium. Each of the two phases constituting a colloidal system may be a gas, a liquid or a solid. In milk, for example, the fat globules are dispersed in water. Hence fat globules form a dispersed phase and water is the dispersed medium.
Therefore, the dispersion medium is the substance in which the colloidal particles are distributed in the particle range of $1$ nm to $1000$ nm.

Note:
Remember that the colloids are used in the form of smoke in smoke screens to hide things in the military. Blue color of the sky is due to a colloidal property shown by the sky. Dust particles dispersed in the air scatter sunlight. Many nanomaterials are prepared by colloids. These are used in metallurgy during froth floatation.