Some heavy preparation may need to go into this lesson from a technician point of view if a class set of apparatus is required. Five or more plastic tubes containing different liquids may need to be made for each pupil for the investigation to be effective. The tubes do not have to be very big but all in the set need to be identical. Plastic tubes in which long 30cm thermometers are sometimes packaged are perfect for this practical. Simply fill each tube with one of the chosen liquids, leaving a small air bubble in the top of each and seal with super glue. Try to make the air bubbles in each tube the same size (leaving a gap of around 1cm at the top is usually sufficient)
Pupils also need a stopclock with each set.
One at a time the tubes are inverted. Pupils measure the time it takes for the air bubble to travel the length of the tube and reach the top end. Because the viscosity of each liquid is different, each bubble should take a different time. These times can then be compared and the liquids rated to determine their differing viscosity.
Recommended liquids to be used are:
- Washing-up liquid
- Ethanol (highly flammable)
- Glycerol (glycerine)
- Clean unused engine oil
- Cooking oils
Making these tubes up takes a long time depending on the type of tubes and method for keeping the liquid inside. Glass tubing may be used and small bungs can be placed in each end to keep the liquid inside although they may also need to be glued in place.
Some teachers may also require the use of a water bath set at a specific temperature. Pupils would record the time taken for a bubble to travel the length of the tube at room temperature and again once the tube has been warmed in a water bath for a few minutes. As the liquid is warmed, it should become less viscous and the bubble should travel faster.
Because of the increase in temperature, the liquid expands in the tube which means the ends need to withstand this increased pressure.
Another way viscosity is sometimes investigated in schools is by using a pipette and angled board. A sample of liquid is sucked up in the pipette and dropped onto a solid board at a 45° angle. The liquid then flows down the board. The time it takes the liquid to reach the bottom is recorded. This approach is both messy and wasteful and so the above method is recommended. Also once the tubes are made, it is an easier practical for the technician to repeat.