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Science project idea

Does salt raise the boiling point of water? A measured test

A dissolved solid raises the boiling point of water, and the rise grows with the amount of solute. This is called boiling point elevation and is a colligative property.

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What it shows

A dissolved solid raises the boiling point of water, and the rise grows with the amount of solute. This is called boiling point elevation and is a colligative property.

The science

A liquid boils when its vapour pressure equals the pressure of the air. Dissolved particles occupy part of the surface and lower the escaping tendency of the water molecules, so a higher temperature is needed. The rise is about proportional to the number of dissolved particles, and salt gives two particles per formula unit, Na+ and Cl-. For salt in water, 58.5 g in 1 kg of water raises the boiling point by only about 1 degree C, so you need strong solutions to see a change on a school thermometer.

Materials

Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.

  • A lab thermometer reading to 110 degrees C
  • A steel pan of 500 ml and a gas stove or induction plate
  • Table salt: 0, 25, 50, 100 and 150 g in separate packets
  • A kitchen scale
  • A clamp and stand, or a long spoon to hold the thermometer away from the base
  • A measuring jug
  • A notebook

Safety

For this project: An adult must do all the heating and handle the boiling water. Keep children away from the stove. Do not touch the pan, and do not let the thermometer rest on the hot base. Let the pan cool fully before pouring.

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 rules

Step 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.

  1. Measure 500 ml of tap water into the pan and bring it to a steady rolling boil. Record the temperature when it is steady as the reference for your place and day.

  2. Turn off the heat for a moment, add 25 g of salt, stir until it dissolves and bring it back to a boil.

  3. Read the thermometer when the boiling is steady for one minute. Keep the bulb in the liquid, not touching the pan.

  4. Add the next salt packet to the same pan, stir, boil again and record the reading.

  5. Repeat until all the packets are added. A common mistake is to read the temperature while the water is still heating up, which gives a low value.

  6. Calculate the molality of each solution: moles of salt, with 58.5 g per mole, divided by the mass of water in kilograms.

  7. Plot the boiling temperature against the molality and draw the best straight line.

  8. Write the slope of the line and compare it with the theoretical value 2 x 0.51 = 1.02 degrees C per mole per kilogram.

What you should see

A graph with the boiling temperature rising steadily as the salt increases, going about 3 to 6 degrees above the starting value at the highest concentration, with the slope in the right range.

Viva questions

A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.

1What is boiling point elevation?

The increase in the boiling point of a liquid when a non-volatile solute is dissolved in it.

2Why does the water not boil exactly at 100 degrees C in your town?

Air pressure is lower at height, so the boiling point is lower than at sea level.

3Why does salt give about twice the effect of sugar for the same moles?

Salt gives two ions for each unit while sugar stays as one molecule.

4Does adding salt to cooking water cook food faster?

Only slightly, because the usual amount raises the boiling point by a tiny fraction of a degree.

Free ebook: Viva Questions for Science Projects

Take it further

Repeat with sugar and compare the effect per mole to show that ions count separately.

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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