Science 7–10 · Year 10

Cold, salty water sinks: a tank model of the ocean's deep currents

Science understanding: Earth and space sciences (NSW Stage 5 focus area: Environmental sustainability)

PracticalLow risk

The idea

Water that is colder or saltier is denser and sinks beneath lighter water, which drives the deep ocean circulation that carries heat around the globe.

What you need

1 clear plastic tank about 30 cm long (or a 2 L rectangular container) filled with tap water at room temperature and left to settle
100 mL of salt water (35 g of salt made up to 1 L) dyed blue and chilled with ice to about 5 degrees C
100 mL of fresh water dyed red and warmed to about 40 degrees C
100 mL each of cold fresh water and warm salt water, for separating the two effects
a thermometer, a stop clock, a 100 mL measuring cylinder and a balance

How to do it

Pour the cold blue salt water gently down the inside wall at one end of the tank and watch where it goes; time it across 30 cm of the floor.
Pour the warm red fresh water gently onto the surface at the other end and watch where it spreads.
Empty and refill the tank, then repeat with cold fresh water and with warm salt water to separate the effect of temperature from the effect of salt.
Weigh 100 mL of the salt water and 100 mL of the fresh water, both at room temperature, to measure the salt's effect on density; take the temperature effects from the NIST table, because they are smaller than a 100 mL cylinder can resolve.
Draw the layers seen after 5 minutes and label the densest and least dense water.
Link the result to polar seas, where sea water freezing into sea ice pushes most of its salt out into the water below.

What you should see

The cold salt water sinks at once and runs along the floor as a blue layer; the warm water spreads across the top; the layers stay distinct for many minutes. Cold fresh water also sinks, and warm salt water still sinks if its salt outweighs its warmth. From the NIST table, fresh water at 5 degrees C (999.97 kg/m^3) is 0.78 percent denser than at 40 degrees C (992.22 kg/m^3); the salt water's extra density is what the learner measures (adding 35 g of salt to a 1 L flask and topping up to the mark adds mass at the same volume). NOAA describes the same mechanism: temperature and salinity set seawater density, and sea ice forming in polar regions keeps little of the salt, so the water left below becomes saltier and denser and sinks. Against room-temperature fresh water (998.21 kg/m^3 at 20 degrees C) the cold water is only 0.18 percent denser, below the reading error of about 0.5 mL in 100 mL on the cylinder, and the warm water 0.60 percent less dense, no better than that error, and both are blurred further as the samples warm or cool while they are weighed, so those differences come from the table and the sinking itself shows them. The learner knows it worked when the salt water weighs more than the same volume of fresh water and the order of sinking matches the order of densities, measured for salt and taken from the NIST table for temperature.

What changes

What you change: temperature and salt content of the added water
What you measure: whether it sinks or spreads at the surface, and its speed along the floor
What you keep the same: tank water temperature
volume added and pouring height
amount of dye

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Ocean currents are driven only by wind; deep currents are driven by differences in density.
Salt water and fresh water mix at once; without stirring they stay layered for a long time.
Warm water rises because heat rises; it rises because it is less dense than the colder water around it.

Safety card

Low riskLearners carry it out

Hazards

warm water at 40 degrees C
spills

Controls

water no hotter than 40 degrees C
wipe spills at once

Note

No hazardous chemical and no flame or heating apparatus: a generic classroom risk assessment (CSIS 1.7 or RiskAssess) covers trips, spills, warm lamps and sharp edges.

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), NSW Education Standards Authority (implemented from 2026; codes read at curriculum.nsw.edu.au on 22 and 23 September 2026)SC5-ENV-01 SC5-WS-03 SC5-WS-04
Australian Curriculum v9AC9S10U04AC9S10I01AC9S10I03

Sources

The pages the author read to write this activity.

All Concept Studio activities