Separating by Filtering — Chemistry (Grade 7)

Define and explain Separating by Filtering in your own words
Use key terms such as filter accurately
Apply what you have learned to new examples and questions
Avoid the common mistakes learners make with this topic

This lesson focuses on Separating by Filtering: use filter paper to separate a solid from a liquid, like sand from water.

Definition: Separating by Filtering

Use filter paper to separate a solid from a liquid, like sand from water.

Key ideas

Choose the method that fits

Each separation method suits a particular mixture: sieving separates solids of different sizes, filtering separates an insoluble solid from a liquid, and evaporation recovers a dissolved solid from its solution. Scientists look at the properties of the parts — size, solubility, state — and then pick the method. Using the wrong method, like trying to sieve salt water, simply does not work.

Mixtures can be undone

In a mixture, the materials are jumbled together but not joined — a bowl of sand and water is still sand plus water. Because nothing new has been made, mixtures can usually be separated back into their parts. This is different from baking, where the ingredients change into something new.

Key term — filter: To pass a mixture through paper or cloth that traps the solid but lets the liquid through.

Worked example: Separating by Filtering

Describe how to separate a mixture of sand and water.

Pour the mixture through filter paper: the sand is trapped and the water passes through.

Answer: Pour the mixture through filter paper: the sand is trapped and the water passes through.

Common mistakes

Trying to filter a dissolved solid Filter paper traps solid lumps but dissolved salt passes straight through with the water. To get dissolved salt back, evaporate the water instead.
Thinking sieving works for liquids A sieve separates solids by size — small pieces fall through the holes. It cannot separate two liquids, and it cannot separate a dissolved solid from a liquid.

Practice

Which method would separate pebbles from sand: sieving, filtering or evaporating?

What is different about the two solids?

Sieving — the sand falls through the holes but the pebbles are too big.

After filtering muddy water, the water still tastes slightly salty. What does this tell you, and what would you do next?

Something passed through the filter.

Some salt was dissolved in the water and passed through the filter. Evaporate the water to recover the salt.

Why can you not separate salt water with a sieve?

What size are the dissolved salt pieces?

The salt is dissolved into pieces far too small to be caught — everything passes through the holes.

You stir sugar into tea until you cannot see it. Has the sugar disappeared? How could you get it back?

Think about what happens when the water is gone.

No, it has dissolved. Evaporate the water and the sugar is left behind.

Quick check

Separating by Filtering — quick check

Which of these best defines "filter"?

To pass a mixture through paper or cloth that traps the solid but lets the liquid through.

Is a fruit salad a mixture? Explain why.

Yes. The fruits are mixed but not joined, and you could pick them apart again.

Key takeaways

Separating by Filtering: use filter paper to separate a solid from a liquid, like sand from water.
Choose the method that fits: Each separation method suits a particular mixture: sieving separates solids of different sizes, filtering separates an insoluble solid from a liquid, and evaporation recovers a dissolved solid from its solution.
mixture: Two or more materials mixed together that are not joined chemically and can be separated again, such as sand and water.
Watch out for: trying to filter a dissolved solid

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