Earth and Environmental Science 11–12 · Year 11
Density-driven ocean currents: cold, salty water sinking under warm, fresh water
Module 3: Energy Transformations
The idea
Temperature and salinity set seawater density, and density differences drive the deep circulation that moves heat between the poles and the tropics.
What you need
- clear tank or large plastic box 30 cm long, room-temperature tap water
- two 250 mL beakers: cold salty water (35 g salt per litre, chilled with ice, blue colouring) and warm fresh water (about 40 °C, red colouring)
- a card divider or a length of tube to introduce each water at one end near the base or surface
- thermometer, salinity or conductivity meter if available, stopwatch
How to do it
- Pour the cold salty water gently through the tube to the base at one end of the tank and record its path every 15 s.
- Pour the warm fresh water on the surface at the other end and record.
- Repeat with cold fresh water and warm salty water to separate the two effects.
- Measure temperature and, if available, conductivity of each water and rank them by density.
- Relate the sinking layer to the North Atlantic and Antarctic bottom-water formation regions and the surface layer to tropical warm pools.
What you should see
The cold salty water flows along the base as a distinct blue layer and the warm fresh water spreads across the top as a red layer. Seawater at a salinity of 35 and 20 °C has a density of 1024.76 kg/m³ against 998.21 kg/m³ for fresh water at the same temperature (computed with the UNESCO one-atmosphere equation of state that TEOS-10 supersedes), 2.7 % denser, and cooling adds to that, so cold salty water sinks under warm fresh water. Cold fresh against warm salty shows the two effects competing.
What changes
- What you change
- temperature and salinity of the introduced water
- What you measure
- path of the coloured water (sinks, spreads on top, or mixes)
- What you keep the same
- tank water temperature
- volume introduced
- rate of pouring
- colour intensity
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- Salt water and fresh water mix instantly.
- Ocean currents are driven only by wind.
Safety card
Hazards
- warm water
- spills
Controls
- water below 45 °C
- tray under the tank
Note
No hazardous chemicals or heat sources: record the activity in the school's RiskAssess risk assessment, following the NSW Department of Education Science safety and compliance page; the Chemical Safety in Schools package is not triggered.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Earth and Environmental Science Stage 6 Syllabus (2017), NESA; current, replaced by the 11–12 Syllabus (2025) from 2028EES11-10EES11/12-5EES11/12-6EES11/12-7
- Earth and Environmental Science 11–12 Syllabus (2025), NESA; to be implemented from 2028, not yet taughtEES-11-03
- Investigating Science Stage 6 Syllabus (2017), NESA; currentINS11-9
- Australian Curriculum v9No Australian Curriculum v9 code is listed.
Sources
The pages the author read to write this activity.