Determine the density of seawater and tap water by measuring their relative mass and volume

Students will explore the relative mass and volume of seawater, and tap water, and in doing so will be able to understand more about the density of both liquids. Graphing their results to show the differences by up to 0.001 g.

This experiment should take 20 minutes.

## Equipment

### Apparatus

• Student worksheet
• Measuring cylinder, 5 cm3
• Sheet of graph paper

• Tap water
• Seawater

## Health, safety and technical notes

• There are no significant hazards associated with this experiment.

## Procedure

1. Place the measuring cylinder on the balance pan and tare the balance.
2. Carefully add 0.5 cm3 of tap water dropwise to the measuring cylinder. Record in a table the volume of water added and the mass.
3. Add drops of tap water until the volume is 1.0 cm3. Record the new mass.
4. Add water until the volume is 1.5 cm3 and record the mass.
5. Continue in this manner at 0.5 cm3 intervals up to 5.0 cm3 (If you cannot tare the balance, subtract the mass of the measuring cylinder each time).
6. Repeat the whole process using seawater.

## Observations

Specimen results are given below. From these results, it is possible to distinguish the density of tap water and seawater using this method.

The experiment is simple to do, although students must work carefully to get good results. A wealth of data are produced, which when examined and interpreted show the application of mathematics to experimental science. In particular, this shows the value of treating data graphically.

Students could be told that it was as a result of appreciating the significance of small differences in mass that important scientific discoveries have been made. For example, in 1894 William Ramsay and Lord Raleigh discovered the element argon in air by investigating the small but consistent discrepancy between the (higher) density of nitrogen obtained from air by removing the oxygen (atmospheric nitrogen) and nitrogen prepared from its compounds (eg heating ammonium nitrite solution).

Tap water
Vol (cm3)  Mass (g)  Density (g cm–3)
0.5 0.533 1.066
1.0  1.052 1.052
1.5  1.540 1.027
2.0  2.041 1.021
2.5  2.516 1.006
3.0  3.031 1.010
3.5  3.505 1.001
4.0  4.004 1.001
4.5  4.503 1.001
5.0  4.999 1.000
mean density 1.018
Seawater
Vol (cm3)  Mass (g)  Density (g cm–3)
0.5 0.563 1.126
1.0  1.089  1.089
1.5  1.597 1.065
2.0  2.093 1.047
2.5  2.594  1.038
3.0 3.118 1.039
3.5  3.609 1.031
4.0  4.123 1.031
4.5  4.644 1.032
5.0   5.144 1.029
mean density 1.053

## Student questions

1. What do you notice on measuring the slope of the lines on your graph?
2. What are the advantages of showing your results graphically rather than just in a table?
3. On a piece of graph paper, plot the volume against the mass for both tap water and seawater and draw a line of best fit through each set of points. Measure the slope of these lines.

(You may wish to use a spreadsheet package to do the calculations.)

## Extension

It may be interesting to extend this experiment to measure the relative densities of, for example, Coke® and Diet Coke®.