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Subject knowledge: Dissolving

When asking children to explain what has happened when stirring sugar into water, many often say that the sugar has disappeared.

The solid sugar has not disappeared, it has dissolved in a liquid and formed a sugar solution. The sugar is still there, but the confusion is that the resulting solution is a colourless mixture of sugar and water.

The sugar particles collide with the water particles and the water particles surround them. They are gradually moved around in the water until they are spread evenly around it. The particles of sugar are now in an aqueous form. We call this mixture of a solid dissolved in a liquid, a solution.

The total mass of the sugar and the water should be the same as the mass of the sugar and the mass of the water before mixing them together. In fact if the water is evaporated, perhaps by heating the solution by leaving it in a warm place for a few days, the sugar should come out of the solution forming sugar crystals.

A nice way of showing that the solid has not disappeared is by using a substance that is not colourless such as instant coffee. This makes it more obvious that the coffee is still in the liquid. This can also be used to show that substances dissolve more quickly in warmer water than in cold water.

This additional video from Royal Society of Chemistry provides a demonstration.

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This article is from the free online course:

Teaching Primary Science: Chemistry

National STEM Learning Centre