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Chemistry · Chapter 2: Separation and Purification
Filtration
Separating an insoluble solid from a liquid by filtration: residue, filtrate, steps, and uses in water treatment.
Key termRememberExampleWatch out
01Concept
Most things around us are mixtures. Because the parts of a mixture are not chemically joined, they can be separated by physical methods. The method we choose depends on the properties of the substances: whether they dissolve, their boiling points, and so on.
Filtration is the method used to separate an insoluble solid from a liquid. Sand from water, tea leaves from tea, chalk powder from water.
How it works
Filter paper has tiny holes (pores). Water molecules and dissolved particles are small enough to pass through, but the larger grains of the insoluble solid are not.
| Term | Meaning | Example |
|---|---|---|
| Residue | The solid left on the filter paper | Sand |
| Filtrate | The liquid that passes through the filter paper | Salt solution |
Steps
- Fold a round filter paper in half and then in half again, and open it into a cone.
- Place it in a funnel and wet it with a little water so that it sticks to the glass.
- Pour the mixture slowly down a glass rod, keeping the level below the top of the paper.
- Collect the filtrate in a beaker or conical flask below.
- Wash the residue with a little distilled water to remove any dissolved substance still on it.
02Virtual Lab
Try this topic for yourself in the virtual lab. It opens on its own page with steps, tasks and questions.
03Real World Examples
Water filtration plants
Karachi's water is pumped from Keenjhar Lake and passed through filter beds of sand and gravel at treatment plants such as the one at Pipri, which remove mud and suspended particles.
Home water filters
Many homes use a filter with a ceramic or carbon cartridge. The cartridge has very fine pores that trap particles, a form of filtration.
Car and air-conditioner filters
Air filters trap dust from the air entering a car engine or an AC unit, the same idea applied to a gas.
04Practice Questions
Try to answer in your own words first, then open the model answer.
Short questions
Q1.What is filtration? Give two examples.Show answer
Answer
Filtration is the separation of an insoluble solid from a liquid by passing the mixture through filter paper.
Examples: sand from water; tea leaves from tea.
Q2.Define residue and filtrate.Show answer
Answer
- Residue: the insoluble solid left on the filter paper.
- Filtrate: the liquid that passes through the filter paper.
Q3.Why can filtration not separate salt from salt water?Show answer
Answer
Because salt is dissolved. Its particles are spread between the water molecules and are small enough to pass through the pores of the filter paper.
Q4.Why is a glass rod used when pouring a mixture into a filter funnel?Show answer
Answer
To guide the liquid gently onto the filter paper, so it does not splash, overflow or tear the paper.
Q5.Give two everyday uses of filtration.Show answer
Answer
- Water purification: water is passed through beds of sand and gravel to remove suspended particles.
- Making tea: a strainer separates tea leaves from the tea.
Long questions
Q6.Describe how you would obtain pure, dry sand from a mixture of sand and salt.Show answer
Answer
- Add the mixture to water and stir. The salt dissolves; the sand does not.
- Filter the mixture. The sand remains on the filter paper as the residue; the salt solution passes through as the filtrate.
- Wash the sand on the filter paper with a little distilled water to remove any salt solution.
- Dry the sand in a warm oven or between filter papers.