Investigating how fast different solids dissolve
Chemistry · Key Stage 2 · The School
Speed is a different question from amount
Earlier you found out how MUCH of a solid can dissolve. That amount is its limit, called saturation. This lesson asks a different question. How FAST does a solid dissolve? A solid can dissolve quickly or slowly. This is true whatever its eventual limit turns out to be.
Planning the test
Use the same volume of water each time. Keep the temperature the same too. Use the same amount of each solid. Time it with a stopwatch. Start when the solid goes in. Stop when it fully disappears. Do this without stirring. Changing anything except the solid would spoil the comparison.
Running it: three kinds of sugar
Granulated sugar, caster sugar and a whole sugar cube are all the same substance. The cube takes the longest to disappear. It has the least surface touching the water at any moment. Caster sugar has much smaller grains than the other two. It dissolves the fastest of the three.
What the timing tells you
Smaller pieces expose more surface to the water. This is true for the same total amount of solid. Water particles can then reach more of it at once. That is why crushing a solid speeds up dissolving. Heating and stirring the water are not the only ways to do it.