What it shows
Dissolved salt or sugar raises the boiling point of water by a small amount that grows with the number of dissolved particles. You measure the rise with a lab thermometer and compare it with the tiny amount people imagine when they salt cooking water.
The science
Dissolved particles get in the way of water molecules leaving the surface as vapour, so the liquid must be hotter before its vapour pressure matches the air pressure. This colligative effect depends on the number of particles. Salt splits into two ions per unit, so for the same moles it raises the boiling point twice as much as sugar. One spoon of salt in a litre of water raises it by well under half a degree.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A steel pan or small vessel
- A lab thermometer reading up to 110 C (borrow from school)
- Common salt, 4 spoons
- Sugar, 4 spoons
- A measuring cup and 4 cups of water
- A gas stove or induction plate (adult only)
- Notebook and a clock
Safety
For this project: An adult must run the stove and hold the pan, because boiling water and steam cause bad burns. The thermometer must hang from a stand or clamp and never rest on the pan bottom. Keep children at arm length from the steam.
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.
Fix the thermometer in a clamp so its bulb sits in the middle of the water and does not touch the pan.
Boil 500 ml of plain water with an adult and read the steady temperature after the bubbling is vigorous; note it as the baseline.
Let the pan cool completely and then pour in fresh water for the next trial.
Dissolve one level spoon of salt in 500 ml of water, boil it and note the steady reading.
Repeat with two, three and four spoons of salt, always letting the pan cool and using fresh water.
Repeat the whole series with sugar instead of salt, and keep the same amount of water each time.
Plot a graph of spoons added against boiling temperature for both substances on one sheet.
Mark the water level before and after boiling to check that you did not lose so much water that the solution got stronger.
What you should see
Both graphs rise slowly with the amount added, and the salt line climbs faster than the sugar line for equal masses. The rise is small, usually under 1 C for a few spoons, showing that salting pasta water does not cook it noticeably faster.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does salt raise the boiling point?
Dissolved ions reduce how easily water escapes as vapour, so more heat is needed to reach boiling.
2Why does the thermometer bulb need to stay off the pan bottom?
The pan base is hotter than the water, so it would give a false high reading.
3Which raises it more per gram, salt or sugar?
Salt, because each unit gives two ions and its particles are lighter.
4Does salting water make food cook faster?
Not noticeably, the rise is under a degree, salt is added for taste.
Take it further
Compare your readings with the calculated rise using 0.5 C per mole of dissolved particles in a kilogram of water.
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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