Filtering
This Topic is About...
- I will find out how to separate a mixture of a solid and a liquid.
- I will draw clear diagrams.
- I will record observations in tables and diagrams.
- I will use our results to say whether our predictions were correct or not.
- I will do practical work safely.
You’re going to be a science explorer!
Key Words
- filter
- filter paper
- funnel
- layer
Tap to Learn the Meanings!
- filter: A tool or process used to separate solids from liquids.
- filter paper: Special paper used in a filter to trap tiny solid pieces.
- funnel: A cone-shaped tool that helps pour liquids into containers without spilling.
- layer: A thin sheet or level of material lying on top of something else.
Great job! You're learning important vocabulary about separating mixtures.
How does a filter work?
Dirty water is a mixture of solid and liquid materials. We can separate the insoluble solids from the liquid by pouring the mixture through a filter. A filter has holes that let some of the mixture pass through. Solids in the mixture stay in the filter because the pieces are too big to pass through the holes. A sieve is a kind of filter but most filters have much smaller holes.
Scientists use filter paper to separate mixtures. Filter paper has very tiny holes and stays strong when it gets wet so it does not tear easily. A funnel is a cone-shaped piece of equipment that can be used to support the filter paper.
FUN QUESTIONS
1. What are the two poles of a magnet called?
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2. Which pole is blue, and which is red?
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3. Do all magnets have poles?
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Let’s Investigate!
Make a simple filter
You will need
- two cups
- a paper towel
- some sand
- some water
Arun and Zara are using things they find in their classroom to make a filter. They want to use their simple filter to separate a mixture of sand and water.
Method
- Make a simple filter using one cup as a container and the paper towel as the filter paper.
- Place the filter cup above the second cup so the clean water can drip through.
- Mix sand with water and pour it slowly into the filter.
- Watch what happens as the water passes through the paper towel.
- Check whether your prediction was correct.
What other mixtures could you separate using a simple filter like this? Talk with your friends about your ideas.
How am I doing?
Did your filter separate the sand and water?
Was the water that passed through clearer than before?
Did you pour the mixture slowly and carefully?
Can you suggest other mixtures that could be filtered?
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What should happen?
- The sand should stay on top of the paper towel.
- The water should drip through into the second cup.
- The filtered water will look cleaner than the mixture.
Why it works
The holes in the paper towel are very small. Water can pass through them, but larger particles like sand cannot.
Examples of mixtures you can filter
- soil and water
- leaves and water
- rice grains and water
- chalk dust and water (partially)
Filters work best when one part of the mixture is a liquid and the other part is made of larger solid pieces.
Filtering waste water
Large filters like these are used to make waste water clean.
The water drips through different layers including rock, gravel and sand. The different layers filter out different-sized pieces of waste.
Let’s Investigate!
Make a layered filter
You will need
- some dirty water in a cup
- some sand
- some gravel
- some stones
- some cotton wool or fabric
- a large plastic bottle that has been cut in half
This is a layered filter. Each layer covers the material below. Look carefully at the dirty water. What solid pieces can you see? What size holes will you need to stop the solids going through the filter?
Be careful with any sharp edges where the plastic has been cut. Do not drink the water.
Make a filter with different layers that you think will make the water clean.
Method
- Draw and label a diagram of your layered filter.
- Record a prediction about how clean you think the water will be when it comes out.
- Use your filter to filter the dirty water.
- Draw and label a diagram of the water that comes out.
- Record whether your prediction was correct.
- Think about how to make a better filter. Would it work better with different fabrics, more sand, or more gravel?
- Predict what would make your filter better.
- Change your filter design and test it again.
- Draw more diagrams to show your new filter and the water that comes out.
How am I doing?
Talk with another group about how you changed your filters.
Did the changes make the filters better?
What do they think would make your filter even better? Do you agree or disagree?
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What should happen?
- The cotton wool catches the smallest pieces.
- The sand catches medium-sized particles.
- The gravel and stones catch the biggest pieces.
- The water that drips out should be clearer than before.
Why it works
Each layer has different-sized gaps. Large solids get trapped at the top, smaller ones get trapped lower down. The water passes through all layers.
How to improve your filter
- Use more cotton wool for finer filtering.
- Add an extra sand layer for medium-sized particles.
- Use smaller gravel pieces to catch more solids.
- Pour the water more slowly to avoid mixing the layers.
A good filter removes most—but not all—impurities. Very tiny particles may still pass through, which is why real water filters use even finer materials.