Why fresh water & salt water don't mix

This activity is part of our Science Lab series. Check out the ANU Science Lab for more experiments! In this activity, you will conduct a fluid dynamics experiment using coloured water, temperature variations, and salt to explore ocean currents and learn why fresh water and salt water do not easily mix. Watch the video above to see how it is done.

Water density experiment materials & ingredients

For this activity:

• Large, clear plastic container filled with room-temperature tap water
• Warm tap water and cold water
• Table salt
• Food colouring (e.g. red, blue, green, and yellow)
• Funnel 
• Sponge (to diffuse water flow and prevent mixing)
• Cups
• spoon for mixing

How to test water density: Step-by-step instructions

Start with a large, clear plastic container filled with room-temperature tap water

Step 1: Test temperature effects

1. Warm water: Mix warm water with red food colouring. Gently pour it through the funnel over the sponge. The warm red water floats across the top surface.
2. Cold water: Mix cold water with blue food colouring. Gently pour it through the funnel. The dense blue water sinks straight to the bottom.

Step 2: Test salinity effects

1. Mix room temperature water with yellow food colouring and stir in 5 teaspoons of table salt.
2. Gently pour the yellow salt water through the funnel. It rapidly sinks to the bottom because dissolved salt makes water denser than fresh water.

Step 3: Combine temperature and salinity (salt fingers)

1. Mix hot tap water with green food colouring and a small pinch of salt.
2. Gently pour it into the container. It floats at first due to its heat. As it cools to the surrounding water, the dense salt water sinks downward in delicate, root-like tendrils called salt fingers.

Why fresh water and salt water do not mix: The science explained

This demonstration illustrates thermohaline circulation (heat and salt dynamics) driving global ocean currents:

• Temperature: Heating water spreads its molecules apart, making warm water lighter so it floats on cooler water. Cold water molecules pack tightly, making cold water denser so it sinks.
• Salinity: Dissolved salt adds mass to water, making salt water denser and heavier than fresh water.

When warm, slightly salty water sits on top of fresh water, it loses heat faster than it loses salt. As it cools, it becomes denser and sinks in vertical streams called salt fingers. 

Science fair projects & taking it further: Experimenting with variables

• Salinity gradient: Try layering three distinct salt concentrations dyed different colours to build a 3-layer density column.
• Ice melt simulation: Float an ice cube dyed blue on top of warm, salty yellow water. Watch how the melting fresh water floats across the surface instead of mixing.