Why do cities with a lot of smoke and dust in the air often look hazy?
Cities with a lot of smoke and dust in the air often look hazy primarily due to the scattering of light.
Smoke and dust particles suspended in the air form a heterogeneous mixture with gas as one of the components. This mixture behaves like a colloid or a suspension. Scattering of light occurs when it passes through a colloid or a suspension, but not through a transparent solution. The suspended smoke and dust particles scatter the light in various directions, which makes the air appear hazy instead of clear.
Explanation
The textbook context explains that smoke and dust in the air are heterogeneous mixtures where solid or liquid particles are suspended in a gas. These mixtures act as colloids or suspensions. The context explicitly states that 'Scattering of light occurs when it passes through a colloid or a suspension'. It is this scattering of light by the suspended dust and smoke particles that causes the hazy appearance in cities.