Why fresh water & salt water don't mix

• A large, clear plastic container filled with room-temperature water
• Warm water
• Cold water
• Table salt
• Food colouring (red, blue, green and yellow)
• A funnel
• A sponge
• Cups
• A spoon for mixing

How to test water density

Begin with a large, clear plastic container filled with room-temperature water.

Step 1: Test temperature effects

1. Mix warm water with red food colouring.
2. Place the sponge beneath the funnel to reduce mixing as you pour.
3. Slowly pour the warm red water into the container.
4. Observe what happens to the coloured water.

Next:

1. Mix cold water with blue food colouring.
2. Slowly pour the blue water into the container through the funnel.
3. Observe where the blue water moves compared to the warm red water.

Step 2: Test salinity effects

1. Fill a cup with room-temperature water and add yellow food colouring.
2. Stir in 5 teaspoons of table salt until dissolved.
3. Slowly pour the yellow salt water into the container through the funnel.
4. Observe where the yellow water settles.

Step 3: Combine temperature and salinity

1. Mix warm water with green food colouring.
2. Add a small pinch of salt and stir until dissolved.
3. Slowly pour the mixture into the container.
4. Watch what happens over time as the water cools and moves through the container.

Look carefully for thin streams of descending water. These structures are sometimes called salt fingers.

Why do fresh water and salt water behave differently?

This experiment demonstrates how density drives the movement of water in oceans, lakes and rivers.

Temperature affects density:

• Warm water is less dense than cold water.
• Because it is less dense, warm water tends to float above colder water.
• Cold water is denser and tends to sink.

Salinity also affects density:

• Dissolved salt increases the mass of water.
• Salt water is denser than fresh water.
• Dense salt water tends to sink beneath less dense fresh water.

In Earth's oceans, differences in temperature and salinity help drive large-scale ocean circulation. These movements transport heat, nutrients and gases around the planet.

When warm, slightly salty water sits above fresher water, it can lose heat faster than it loses salt. As it cools, it becomes denser and begins to sink, creating narrow streams called salt fingers.

Scientists study these processes to understand ocean circulation, climate systems and the movement of heat around the Earth.

Science fair project: Take it further

Once you have explored the effects of temperature and salinity, investigate how changing different variables affects water movement.

• Create a three-layer density column using different amounts of salt and different food colouring colours. Can you keep the layers separate?
• Float a blue ice cube on top of warm yellow salt water. As the ice melts, does the fresh water mix immediately or stay near the surface?
• Test different salt concentrations. How much salt is needed before coloured water consistently sinks to the bottom?
• Compare very warm water, slightly warm water and room-temperature water. How does temperature affect the speed at which the coloured water moves through the container?
• Create a table to record your observations. Which variable had the greatest effect on water movement? Was temperature or salinity more important in determining whether the water floated or sank?