# Weighing liquids

##### Class practical

Comparing the masses of equal volumes of liquids, to develop a general feeling for density.

#### Apparatus and materials

For each student group

Perspex containers (rectangular boxes, open at top), 2

Balance, lever-arm or top-pan

Ruler

Marker pen

Sand, wheat, rice or dried peas

Liquids (e.g. water, paraffin, brine, cooking oil and syrup)

#### Health & Safety

Choose liquids that are low risk.

#### Procedure

a The transparent containers will allow you to measure and weigh standard quantities of liquids and other 'pourable' substances. Mark a line on the outside of each container at a height of, say, 8 cm.

b Fill a container to the line with sand. Weigh the container to find its mass.

c Repeat with water, and then another liquid. How do the different substances compare?

#### Teaching notes

1 The object of this experiment is not to make systematic measurements of liquid densities but rather to extend a general developing feeling for density. It can be used as an extension activity to follow Measuring and weighing solid blocks. The liquids are placed in rectangular containers so that the 'counting cubes' approach to volume measurement is relatively easy

2 Those students who raise the question of the weight of the box itself will weigh it of their own accord, but the teacher should not give a general instruction to do this.

3 Weighing sand might lead to a discussion of an 'average' density for the sand and spaces combined. Start such a discussion by trying wheat or dried peas. Considering water after this may lead to a discussion once again of atoms.

4 When the box is filled with sand or rice then there are spaces between the grains. A discussion about the average density of air and sand is necessary. However when the box is filled with water, what is it they are measuring? What is between the atoms? [Empty space.]

This experiment was safety-checked in July 2007