Mixtures of substances
A pure substance has only one kind of particle (sugar, salt). A mixture consists of two or more substances (air, juice, tea, soil). Pure substances are rare in nature; even tap water is a mixture because salts and minerals are dissolved in it. A liquid mixture in which the particles cannot be seen is a solution: when we put sugar into tea, the sugar dissolves in the water. A solid mixture is an alloy. Steel and cast iron are made from a mixture of iron and carbon; knives and tools are made of steel, cooking pots of cast iron. Glass is made by melting sand and other substances. In some mixtures the particles can be seen: sand put into water does not dissolve and sinks to the bottom. Such a mixture can be separated by letting it settle or by filtering.
Three glasses: 1) water + salt, 2) water + sand, 3) water + sugar. Stir and wait 10 minutes. In which can you see particles and in which not? Record in a table (dissolved / not dissolved). Do not drink the salty water.