The thermal properties of water

In this series of experiments, students observe illustrations of the thermal properties of water using a Bunsen burner, a paper cup and a balloon.

The fact that water has a boiling point of 100 °C means that paper will not reach its ignition temperature if water is heated in a paper cup. The high specific heat capacity of water means the temperature rise in a heated balloon is limited if it contains water. The good thermal conductivity of water conducts the heat away from the hot spot above the Bunsen flame in each case.

These are quite ‘showy’ demonstrations, but with a definite theoretical background.

The experiments will take about ten minutes, unless you have a lot of ‘patter’.

Equipment

Apparatus

• Eye protection
• Two disposable, waxed-paper cups (as used for cold drinks at parties) – alternatively, paper cake cases can be used (see note 3 below)
• Two party balloons
• Thermocouple-type thermometer with a large display or interfaced to a computer and monitor (see note 4)
• Tongs
• Stand, boss and clamp, x2
• Bunsen burner
• Pipeclay triangle
• Tripod
• Heat resistant mats, x2
• Wooden splints

Chemicals

• Water

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection throughout.
• A black background to the paper cup enables steam to be seen more easily.
• A display thermometer is not essential. The probe can be placed in the cup to monitor the temperature.
• Please remember that you are dealing with boiling water (in a paper cup) or hot water (in a balloon). These unconventional containers may be less stable than normal ones so care is needed to ensure they are not knocked over or splashed.

Procedure

For the paper cup experiment

1. Place a Bunsen burner on a heat resistant mat. Light the Bunsen and adjust it to a small, yellow-tipped flame.
2. Using tongs, hold a paper cup over the flame. The cup will catch fire within a few seconds. Allow it to burn out or extinguish it by placing it on the heat resistant mat and covering with either another heat resistant mat or a damp cloth.
3. Place a pipeclay triangle on top of a tripod and set it over the Bunsen burner. Half fill the second paper cup with tap water and carefully rest it on top of the pipeclay triangle, over the same flame as before. Take care to centre the flame on the base of the cup and ensure that the flame does not play on the sides of the cup above the water level. After a few minutes the water will boil and the cup will remain undamaged except for a little charring around the rim on the base. Measure the temperature of the water with the thermometer.

For the balloon experiment

1. Inflate one of the balloons to the usual size and knot the end. Put about 100 cm³ of water from a tap in the second balloon and then inflate it to the same size as the first balloon and tie the neck.
2. Hold or clamp the first balloon and apply the flame of a lit splint to its base. It will burst almost instantly.
3. Hold or clamp the second balloon similarly and apply the flame of the lit splint to the base where the water is. The balloon will not burst and the flame can be held in place for some time. Be careful not to heat the balloon above the level of the water.
4. If desired, show that this is caused by the presence of the water. Work over a sink or a tray and move the flame to a part of the balloon not filled with water. It will burst instantly.

Teaching notes

Paper will not ignite below about 230 °C. When it reaches 100 °C, the water in the cup will boil and stay at that temperature while it boils away. Water is a relatively good conductor of heat and this, together with convection effects, transfers heat away from the hot spot above the Bunsen flame.

In the absence of water, the rubber of the balloon soon heats up, softens, and the balloon bursts. The specific heat capacity (SHC) of water is high (4.2 J g^–1 K^–1) and so it takes a lot of heat to produce a relatively small temperature rise in water. Again, the water conducts the heat away from the hot spot.

The high SHC of water is due to the strong intermolecular hydrogen bonding – it takes a lot of energy to separate water molecules.