What it shows
When sugar disappears into water, its mass does not disappear: the sugar water weighs exactly as much as the sugar and the water did separately. Weighing before and after shows that dissolving spreads the sugar out but destroys nothing.
The science
Dissolving is a physical change. The sugar crystals break apart into molecules too small to see, and these spread evenly between the water molecules, but every molecule is still there. Because nothing leaves and nothing is created, the total mass stays the same. This is called conservation of mass. The volume, however, rises by a little less than the volume of the sugar, because the sugar molecules fit into gaps between the water molecules.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A kitchen balance that reads grams
- A glass of water
- Sugar, about 20 g
- A small dry cup for the sugar
- A spoon
- Salt for a repeat test
Safety
For this project: No hazards beyond a glass that could break; use a plastic cup if a young child is doing it.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Put the glass of water and the cup of sugar on the balance together and write down the total mass.
Mark the water level on the glass with a pen or tape.
Pour the sugar into the water and stir until no grains are left; notice the sugar seems to vanish.
Put the glass and the empty cup back on the balance and write down the new total.
Compare the two numbers; they should be the same to within a gram.
Check the water level; it rises a little, but less than you might expect.
Repeat with salt and compare the results.
Taste a drop from the top and a drop from the bottom of the glass to confirm the sugar is spread everywhere.
What you should see
The balance reads the same before and after dissolving, within a gram, for both sugar and salt, and the water level rises only slightly, proving that the sugar is still present even though it cannot be seen.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Did the sugar disappear?
No, it broke into tiny pieces that spread through the water; the mass stayed the same.
2What is conservation of mass?
In a physical change, nothing is lost or gained, so the total mass does not change.
3Why did the water level rise so little?
Sugar molecules fit into gaps between water molecules.
4How could you get the sugar back?
By evaporating the water.
Take it further
Weigh a glass of water with a fizzing tablet beside it, drop the tablet in, and explain why the mass goes down this time.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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