Grade 9 · 🧪 Chemistry · Mixtures and separation
Get clean salt back from salty sand
Mix salt and sand, then separate them again using dissolving, filtering and evaporation — the same steps used to make salt from Afar brine.
Why this matters
In the Danakil Depression, workers cut salt from the ground, and salt producers elsewhere evaporate brine in the sun to get clean crystals. Water treatment plants in Addis filter river water through sand before it reaches your tap. Separating mixtures is one of the oldest and most useful skills in chemistry.
Watch the demo
Video coming soon
We're recording a demo for this experiment. Until then, follow the step-by-step guide below.
The science
Mixtures vs compounds
A mixture is two or more substances mixed together but not chemically joined, so they can be separated by physical methods. Salt + sand is a mixture.
Three separation methods
- Dissolving: salt dissolves in water, sand does not.
- Filtration: a filter lets the liquid pass but traps solid particles.
- Evaporation: heating or sun-drying removes the water and leaves the dissolved solid behind.
You'll need
- 2 spoons of table salt
- 2 spoons of sand or fine soil
- 2 cups and warm water
- A coffee filter or a clean, thin cotton cloth
- A shallow plate or tray
- A sunny window, or a stove with an adult's help
Safety first
- If you heat the salt water on a stove, ask an adult to help and never leave it unattended.
- Let the plate cool before touching it.
Step by step
0/5 doneWhat you should see
Wet sand stays in the filter. The liquid that passes through looks clear. After the water evaporates, white salt crystals are left on the plate.
Why it happens
Salt dissolves into water as ions too small to be stopped by the filter, so it passes through with the water. Sand grains are too big and do not dissolve, so they are trapped. Evaporation removes the water but not the salt, so the salt crystallises again.
Didn't work? Try this
The filtered water is still cloudy.
Fine soil particles got through. Filter it a second time, or let it stand for 10 minutes and pour off the clear top part.
No crystals appeared.
The water hasn't fully evaporated yet. Leave it longer in a warm, sunny spot, using a wide shallow plate.
Still stuck? Ask the lab assistant
Check your understanding
1. Why does the salt pass through the filter but the sand doesn't?
2. Which method removes water and leaves dissolved salt behind?
Your lab notebook
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