Practical Activity for 11-14
- Activity time 15 mins
In this demonstration students observe oil floating on water and ice floating on oil. You can use it to test understanding of density.
- Two 50 ml measuring cylinders
- 250ml beaker
- Two electronic balances (capable of measuring to nearest g or better)
- 400 ml vegetable oil
- Water in a glass or clear plastic jug (at least 100 ml)
- Blue food colouring
- Ice cube tray
- Tissues (to mop up any spillages)
The night before the activity prepare blue ice cubes by adding a few drops of food colouring to water in an ice-cube tray and freezing. The blue food dye will make it easier for the students to distinguish the ice and resulting meltwater from the oil.
When carrying out the activity avoid getting oil on the bench or floor where it may cause a slipping hazard. Afterwards, dispose of the oil in the non-recycling waste by putting an absorbent material (e.g. newspaper or cat litter) into a strong bin bag and pouring the top layer of oil from the cylinders and beaker into the bag. The remaining coloured water can be washed down the sink with the tap running.
- Add a few drops of blue colouring into the water in the jug.
- Pour 40 ml of the blue water into a measuring cylinder and add 10 ml of oil.
- Pour 40 ml of oil into the other measuring cylinder and add 10 ml of blue water.
- Put the beaker on a balance and zero it.
- Pour in the remainder of the oil into the beaker and add the ice cube.
- The total volumes of liquid in each cylinder are the same. Are the masses?
- When the ice in the beaker melts what will happen?
Students may talk about ice and water being lighter or heavier than oil. Encourage them to think in terms of the density of these materials. Some may think that the mass or volume of an individual ice cube is important, show them this isn’t the case by floating both large and small ones in the beaker.
In both measuring cylinders the water settles at the bottom because it has a higher density than oil. The oil-water mixtures have the same volume, but the one with a greater percentage of water will have the larger mass because water contributes more mass per volume than oil. Confirm this by putting each measuring cylinder in turn on a balance (the mass difference should be about 3g).
The ice floats on the oil because it’s less dense than oil. When it melts, it turns to water and so we would expect it to sink. If they look at the beaker, they can confirm this. Blue water droplets are detaching from the bottom of the ice cube, dropping through the oil and collecting at the bottom of the beaker (if the ice cubes haven’t started melting use a ruler to submerge them to speed up the process).
During the change of state the mass doesn’t change (the reading on the balance under the beaker remains constant). The increase in density must be due to a decrease in volume. The molecules must pack more closely together when the ice melts (water is very unusual in this regard as most solid substances are denser than when they’re liquid).
Students describe density changes during a change of state in terms of a rearrangement of the molecules.
This experiment was safety-checked in March 2020.