Science 7–10 · Year 7
Density of tap water and seawater from a mass-volume graph
Solutions and mixtures
The idea
Density is the mass of each millilitre of a substance, so the gradient of a mass-against-volume graph measures it, and dissolved salt packs more mass into the same volume.
What you need
- 5 mL or 10 mL measuring cylinder, balance reading to 0.01 g, dropping pipette
- tap water and seawater (or a sea-salt solution made up by the teacher), about 20 mL of each
- thermometer, graph paper or a spreadsheet
How to do it
- Stand the empty measuring cylinder on the balance and tare it. Record the water temperature.
- Add tap water dropwise to the 0.5 mL mark, reading the bottom of the meniscus at eye level, and record the mass.
- Keep adding water in 0.5 mL steps up to 5.0 mL, recording the mass at every step.
- Empty and dry the cylinder and repeat with seawater.
- Plot mass (vertical axis) against volume for both liquids, draw a line of best fit through each set, and find each gradient in g/mL. Compare the tap-water gradient with the accepted density of water.
What you should see
Both sets of points lie on straight lines through the origin; the gradient is the density. Pure water at 20 °C has a density of 0.9982 g/mL (0.9970 g/mL at 25 °C), so 5.0 mL of tap water weighs close to 4.99 g. Seawater of salinity 35 has a density of 1.0248 g/mL at 20 °C (UNESCO 1981 equation of state), 2.7 percent more than pure water, so its line is slightly steeper and 5.0 mL weighs about 5.12 g. The RSC sample class results (1.018 and 1.053 g/mL) sit a little above these values, as small-cylinder reading errors usually do, but the difference between the two liquids survives. The learner knows it worked when the tap-water gradient is within 3 percent of the accepted value and the seawater gradient is the larger of the two.
What changes
- What you change
- volume of liquid
- What you measure
- mass of liquid
- What you keep the same
- same measuring cylinder and balance
- temperature of the liquids
- reading the meniscus at eye level
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- Heavier objects or larger volumes of a liquid have a greater density.
- Dissolved salt adds no mass because it cannot be seen.
- Density is the same as weight.
Safety card
Hazards
- glass measuring cylinder can break
- spilt water near the balance
Controls
- stand the cylinder on the balance pan, never hold it while reading
- wipe spills at once
Note
No hazardous chemicals; the RSC lists no significant hazards for this activity.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Science 7–10 Syllabus (2023)SC4-SOL-01SC4-WS-05
- Australian Curriculum v9AC9S7U05AC9S7I04
Sources
The pages the author read to write this activity.