Which pot of water contains a secret ingredient NaCl? In this experiment, we can find out by using the CLUE temperature sensor display.

Boiling Point Elevation

Water boils at 212° Fahrenheit (100° Celsius). Or does it?!

When we say water boils at 212° F, we are talking about "typical" drinking water that comes from a tap, filter, or bottle. However, there's a common ingredient you can add to your drinking water to significantly raise its boiling point -- salt.

By dissolving enough salt in the water, you will observe a phenomenon called "boiling point elevation". You are essentially making the solution use an increased amount of heat energy to make the phase transition from liquid to gas begin.

How it Works

It takes heat energy to transition water from its liquid phase state to gas. When salt (a.k.a., sodium chloride or NaCl) is dissolved in water it splits into sodium and chlorine ions.

Water molecules (a.k.a., dihydrogen monoxide or H20) are a bit like magnets for each other. They have a negative side and a positive side. Breaking these bonds between molecules with heat causes the water to phase change from liquid to gas.

However, the dissolved salt has just provided some extra strength to these bonds. The sodium ions are positively charged and align with the negative oxygen side of the water molecules. The chlorine ions are negatively charged, thus align with the positively charged hydrogen side of the water molecules.

These convenient alignments strengthen the interactions between the water molecules, thus requiring more heat energy to boil.

Let's put this theory into practice and see how much we can raise the boiling point.

Experiment Materials

Here's what we'll need to run the experiment:

  • Two small pots (I used 1-1/4 Qt. pots)
  • Clothes pins
  • Rubber bands
  • Measuring cups
  • Salt
  • Water
  • Stove

Probe Prep

First, we'll create a rig to hold the probes with the metal tips near the center of the pot without touching. (We need to measure the water temperature, not the temperature of the pot.)

Wrap a rubber band a few times around the probe end and slip it over one leg of the clothespin.

Clip the clothespin to the side of the pot.

Water

Pour a quart of water into each pot. I used filtered tap water.

Add Salt to Pot B

Pot A will be the control pot, with regular water in it.

To pot B, add 1/2 C. of table salt. Stir to dissolve.

Turn on the Heat

With the pots positioned on two equal sized burners, turn up the heat to high. If you are using a gas range, make sure there is no danger of the probe, clothespin, or wire being in direct contact with the open flame.

You will see the temperatures rise on both pots on the CLUE's display. Note how the pot B is slower to raise its temperature due to the added thermal mass of the salt.

When the water is at a full boil on pot A, the temperature will read right about 212° F (100° C) and stay at that temperature. It can go no higher!

However, pot B with all of that salt will continue on well beyond -- mine went all the way up to 217.4° F (103° C)!

 

You have now solved the mystery of the hotter than usual boiling water!

Further Experiments

What happens if you increase or decrease the ratio of salt to water? Do other solutes behave similarly or differently?

This guide was first published on Apr 10, 2020. It was last updated on Apr 10, 2020.
This page (Kitchen Science Experiment) was last updated on Sep 07, 2020.