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Particle Theory

Topics covered

Dissolving

Changes of state

Dissolving

Dissolving can be thought of as a particular type of mixing. Mixing materials can result in a number of outcomes. Sometimes the components have little effect on each other, remain visible and can be easily separated. In other cases mixing can result in dramatic irreversible changes to the materials. Dissolving lies somewhere between the two. In the simplest case it involves mixing two materials together. The most common examples of dissolving involve a solid and a liquid, usually water. When a solid dissolves the solid (solute) and the liquid (solvent) form a very close intimate mixture called a solution. Unless the solid is coloured it will not be visible and the solution may just look like the starting liquid. However the presence of the solid can be confirmed by a number of strategies. In some cases, eg salt or sugar, taste will indicate the presence of the solid. Alternatively evaporation of the liquid should leave a solid residue.The mixing is so complete the particles in a solution will pass through most filter paper so it is not possible to separate the mixture by filtering. The particle model can provide a simple representation of dissolving in terms of mixing to show what happens when a solid dissolves, (Fig3). If a solid dissolves on mixing its particles break apart and form a loose association with the liquid (solvent) particles. A solid will not dissolve in a liquid if its particles are unable to form links to the liquid particles. The simple particle model is less helpful in explaining why there is a limit to the solubility of any material or why the solubility varies at different temperature. Although we most commonly think of dissolving solids in water other liquids can act as solvents. Nail varnish remover is used to dissolve nail varnish and, white spirit to dissolve some kinds of paint and petrol is a good solvent for grease. Many non water solvents are not really suitable for use with primary children.
Dissolving is a reversible process and the solute can be recovered from a solution by evaporation though it will not always be in the exactly the same form as at the start.

Fig 3 Mixing a solid and a liquid

Dissolving can be difficult for children to understand as they observe, and often, describe the solid "disappearing". The most common materials used for dissolving with primary children are sugar and salt. These produce colourless solutions which can reinforce the idea of a solid disappearing. Use of a variety of materials, including coloured ones such as coffee to demonstrate dissolving may help to overcome this.
Children also frequently observe that when sugar or salt is added to water it "melts" away. So they often confuse the processes of melting and dissolving. Dissolving requires two materials to be mixed together unlike melting which is the result of one material being heated.


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Website maintainer: R. Jones Updated: November 13, 2000