# Measuring the density of liquids

##### Class practical

A simple method for comparing the density of liquids.

#### Apparatus and materials

*For each group*

Measuring cylinders, 100 ml or 250 ml, clean and dry, 2 or more

Chemical balance

Access to water and vegetable or olive oil

Any other liquids that are safe to handle (OPTIONAL)

#### Health & Safety

Take care with any spillages, particularly with the oil, which can create a slip hazard.

#### Procedure

**a** Take the measuring cylinder and measure its mass, in grams, as accurately as possible.

**b** Take the measuring cylinder off the balance and add the water carefully, either by careful pouring or with a pipette until the level is as close to the 10 ml mark as possible. Put the measuring cylinder back on the balance. Measure and record the new mass (cylinder plus water), in grams.

**c** Repeat the procedure, adding 10 ml at a time as accurately as possible and recording the volume and total mass, until the measuring cylinder is full. Then, for each volume calculate the mass of the liquid alone.

*NOTE: If a 250 ml measuring cylinder is being used you may wish to use 20 ml or 25 ml intervals.*

**d** Repeat steps **a** to **c** for the oil (and any other liquids being tested).

**e** Draw a graph of mass of liquid (y-axis) against volume (x-axis). Try to scale the graph so that you can plot all your data sets on a single graph.

**f** For each set of data try and draw a straight ‘best fit’ line passing through the origin. Calculate the density of each liquid from the gradient of its graph line.

#### Teaching notes

**1** Students will need to have studied density previously and be familiar with the density equation. Examples may have used cm^{3} as the unit of volume and g/cm^{3} as the unit of density, or m^{3} and kg/m^{3}. Either sets of units are generally acceptable, but all length measurements must use the same unit. Students may need to be told that with a measuring cylinder 1 ml =1 cm^{3}.

**2** The density of water is measured before the oil because water can be easily and quickly rinsed out of the measuring cylinder and oil cannot. When adding the oil to the measuring cylinder, instruct students to try and avoid pouring it down the side otherwise it will form a coating on the sides which will increase the mass without raising the level from which the volume is read, so dry the measuring cylinder before weighing.

**3** If there are limitations to the number of balances available then it is still possible to carry this out with students sharing a balance, although care needs to be taken that there are no spillages. If students are not familiar with the meniscus that is formed, show them how to take volume readings correctly.

**4 How Science Works extension:** If asked to find the density of a liquid, students may take only a single set of readings. The ease with which water and other liquids can be poured allows the refinement of this method to collect multiple results and use a graphical method to minimize the effect of any systematic error in the measurements.

Finding densities of liquids and their behaviour is important to food scientists. You could illustrate this by having students measure the density of vinegar, making and measuring the density of a vinaigrette, and then predicting which of these will sit on top when they are poured into a single container.

*This experiment was safety-checked in January 2007*