Air is a mixture

Is air a mixture? From Grade 6 (Nature's Treasures): air is a uniform mixture of mainly nitrogen, oxygen, argon, carbon dioxide and water vapour.

Oxygen is what most living things need to stay alive, and it helps things burn. Nitrogen makes up about 78% of air and doesn't take part in burning. The water vapour in air turns into tiny drops when warm air touches something cold, like a cold glass.

Is there carbon dioxide? (Activity 8.1) Half-fill a glass with water and slowly add a little calcium oxide (quick lime). Careful: it reacts strongly with water, gets hot, and forms calcium hydroxide. Stir to make a solution of calcium hydroxide, called lime water. Filter it: it is clear and colourless.

Leave it in an open dish for a few hours, stirring now and then. It turns milky. Carbon dioxide reacts with calcium hydroxide to make calcium carbonate, tiny white particles that don't dissolve, plus water.

Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

Only air reached the dish, so the carbon dioxide must have come from the air.

Is there anything in air besides gases? You may have seen tiny shiny specks floating in a sunbeam in a dark room. Activity 8.2: leave a clean black sheet of paper near an open window or in the garden for a few hours. Tiny particles settle on it; look at them with a magnifying glass. (Black paper makes the pale dust easy to see.)

So dust floats in the air. But it is not a real part of air: it is a pollutant, and how much there is changes from place to place and day to day. The main air pollutants are particulate matter (dust, soot) and gases such as carbon monoxide, ozone, nitrogen dioxide and sulfur dioxide. The Air Quality Index (AQI) tells us how clean or dirty the air is.