A solution is defined as a homogeneous mixture of two or more chemically non–reacting substances whose composition can be varied within limits. If a mixture is to be called a solution, it must satisfy the following two conditions: (i) The components of a mixture should be non–reacting. (ii) Mixture should be homogeneous.

A colloidal solution is a heterogeneous mixture in which the size of the particle is in the range of 1-100 nm in diameter. Due to the small size of particles, these mixtures appear to be homogeneous, but actually they are heterogeneous. Moreover, the colloidal particles pass through ordinary filter paper. Hence, they cannot be separated by filtration. The colloidal particles are not visible to the naked eye. However, in some cases, they are visible under ultramicroscope. So, only option (3), i.e. a colloid is a homogeneous mixture, is an incorrect statement about colloids. 

Cheese is a colloid in which dispersing medium is solid while dispersed phase is liquid. Rubber is a colloid in which dispersion medium is solid and dispersed phase is gas. In face cream, both dispersed phase and dispersion medium are liquids. Moreover, in mist, dispersion medium is gas and dispersed phase is liquid. Thus, option (2) is correct.

True solutions are homogeneous mixtures in which the size of the particles is less than 1 nm in diameter. Because of the small size, the solute particles do not scatter a beam of light passing through the solution. Therefore, the path of light is not visible in a true solution.

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