All liquids possess viscosity. The arrangement of molecules in a liquid allows for them to “flow,” unlike a solid which is fixed in position. The fact that the molecules can move relative to one another but still have some degree of cohesion is one of the key properties of a liquid, and the viscosity of a liquid is how much it resists this movement. It’s like friction happening within the liquid. As an example, honey has high viscosity, whilst water’s is very low. You can test exactly how viscous various liquids are using marbles.
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Things you need
- Corn syrup
- Vegetable/sunflower/olive oil
- Marbles (same size, but multiple colours are better)
- Straight-sided, cylindrical measuring jug / tall glass
- Water-soluble marker
- Strainer / sieve
Mark the outside of your straight-sided measuring jug or tall glass 5 cm (2 inches) down from the top using your marker pen. Make another mark 2 cm (4/5 of an inch) down from your first mark. Measure 5 cm (2 inches) up from the bottom of the container and make another mark on the outside. You’ll time how long it takes a marble to sink the distance between the lowest two marks to determine the liquid’s viscosity. Measure the distance between these two points (preferably in metric measurements such as cm) and make a note of it.
Fill the measuring jug or glass up to the highest mark with water. Prepare your stopwatch, asking a friend or family member to operate it for you if possible. Drop the marble onto the surface of the water, watching it as it descends. When the marble reaches the upper mark, start the stopwatch (or signal for your helper to start timing). Stop the clock when it reaches the lowest mark. Record the time it took for the marble to travel the set distance through the water in seconds. This can be difficult, so if you have more than one marble, repeat the experiment several times (it doesn’t matter if you leave the marbles in the bottom of the container) and take an average measurement.
Empty the contents of the container into the sink through a kitchen strainer or a sieve. For water, the marbles and container will already be clean, but forother of the liquids you should clean both after the experiment.
Repeat the process in steps 2 and 3 with your other of your liquids. Make notes of the time taken for the marbles to travel the distance through each of your liquids.
Convert the distance you measured (between the lower two marks) into metres. A centimetre is 0.01 metres, so if you’ve measured 45 cm, convert the distance to 0.45 metres. If you’ve used imperial measurements, convert the distance to metres (see Resources) and make a note of it. Divide the distance in metres by the average times you calculated for each liquid in seconds to find their velocity in metres per second.
Measure the diameter of your marbles and divide the answer by 2 to find the radius. The diameter is the distance from one side of the sphere to the other, in a straight line (not around the marble).
Use a density chart (see Resources) to find the density (in kg by metres cubed) of both your marbles (glass marbles are 2800 kg/m^3) and the various liquids you used (water is 1000 kg/m^3). Note these densities down. For each one, subtract the density of the liquid from the density of the marble to find the difference in density between the two.
Square the radius of your marble to start the viscosity calculation for each liquid. Multiply the result by 9.81 (this is the acceleration due to gravity, in metres per second squared). Multiply the result of this by the difference in density between the specific liquid you calculated for and the marble, and then multiply the answer by 2. Make a note of this number.
Multiply the speed of the marble through the specific liquid by 9. Divide the answer from the previous step by this result to determine the viscosity of the liquid in Newton-seconds per metre squared (Ns/m^2). Complete these calculations for each liquid to determine its viscosity.
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