Dissolving — Chemistry, 7–10 years
When a substance dissolves, it spreads through a liquid so evenly that we can no longer see separate pieces. It has not vanished.
Where did the sugar go?
Dissolving means that tiny parts of a substance spread among the particles of a liquid. The substance is still there, even though our eyes cannot pick out its pieces. Stirring and warming often make dissolving happen faster, but they do not create or destroy the substance.
Why learn about dissolving?
People wanted to mix medicines, flavours and cleaning substances evenly, without leaving lumps at the bottom. Understanding dissolving helps us predict whether a powder will spread in water and how to make it happen. It also explains why adding more and more substance can eventually leave some undissolved.
Sugar in water
1. Pour 200 millilitres of water into a glass. 2. Add one teaspoon of sugar and stir. 3. The crystals become smaller and are no longer visible, while the water tastes sweet: the sugar has dissolved. 4. Add four more teaspoons; if crystals remain at the bottom, the water cannot dissolve all five teaspoons at that temperature.
Dissolving is not melting
It is reasonable to call sugar in water “melted” because the crystals disappear and the drink becomes liquid. But melting is a change caused by heat, from solid to liquid sugar. In dissolving, the sugar spreads through water; tasting or evaporating the water can show that it is still present.
Dissolving in daily life
Dissolving is used when salt spreads through soup, sugar mixes into tea or a powdered drink becomes even. It matters in each case because every sip or spoonful should contain a similar mixture. The idea does not mean everything dissolves in every liquid: oil, for example, forms a separate layer in water.
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