What Is Filtration?
Filtration is a method used to separate an insoluble solid from a liquid (or gas). It works because the filter contains tiny holes (pores) that are large enough for liquid particles to pass through, but too small for solid particles.
The solid that remains on the filter is called the residue. The liquid that passes through is called the filtrate.
When Do You Use Filtration?
Filtration is used when:
- The solid does not dissolve in the liquid (it is insoluble).
- Examples: sand in water, chalk in water, soil in water.
It does not work for dissolved substances (like salt in water), because dissolved particles are too small to be caught by a filter. For those, you need evaporation or distillation.
Laboratory Filtration Technique
In a school laboratory, filtration is performed using:
- Filter paper folded into a cone and placed in a glass funnel.
- The funnel is placed in a clamp stand over a beaker.
- The mixture is carefully poured into the funnel.
- The liquid (filtrate) drips through the filter paper into the beaker below.
- The solid (residue) is left behind on the filter paper.
Filter paper has pores (tiny holes) of 1–30 micrometres. Dissolved particles (like ions or small molecules) are much smaller than these pores and pass straight through. Solid particles (like sand grains) are larger and are caught by the filter. This size difference is the basis of filtration.
Real-World Applications of Filtration
| Application | What is being filtered |
|---|---|
| Water treatment plants | Removing sand and debris from river water |
| Coffee maker | Coffee grounds (solid) separated from liquid coffee |
| Pool filters | Dirt particles removed from swimming pool water |
| Car engine air filter | Dust and particles removed from air before entering engine |
| Kidneys in your body | Blood is filtered to remove waste products |
After filtration, if you want to recover pure solid from the residue, you can wash the residue on the filter paper with a small amount of distilled water to remove any contaminating liquid, then dry it in an oven.
Limitations of Filtration
- It cannot separate dissolved substances from a solution.
- It is slow for very fine particles, which tend to clog the filter.
- If the solid particles are similar in size to the pores, filtration is inefficient.
For dissolved substances, other methods like evaporation or distillation are needed.
Real world: Kidneys — biological filtration 24/7
Your kidneys filter about 180 litres of blood every day. Tiny filtering units (nephrons) work like filter paper — small molecules like water, glucose, salts, and urea pass through, while larger proteins and blood cells are retained. The filtered small molecules are then selectively reabsorbed, and waste (urea) is excreted as urine. Kidney disease means the filtration fails, which is why patients need dialysis machines that perform filtration externally using a semi-permeable membrane.
Real world: Coffee machines and filtration
A drip coffee maker is a laboratory filtration in your kitchen. Ground coffee beans (the insoluble solid) are held in a paper filter. Hot water passes through, dissolving flavour compounds and caffeine — these dissolved substances pass through the filter as the coffee liquid (filtrate). The coffee grounds (residue) remain in the filter. The same principle applies to tea bags. Paper coffee filters have pore sizes of ~20 micrometres — large enough for dissolved compounds but too small for the solid coffee particles.
Real world: Water treatment plants filter river water
Before tap water reaches your home, river or lake water passes through several filtration stages in a water treatment plant. First, large debris is screened out. Then the water passes through sand and gravel beds (physical filtration) that remove suspended particles, soil, and some microbes. Charcoal filtration removes dissolved organic compounds. Final chlorination kills bacteria. Without filtration, dissolved minerals remain but suspended solids are removed — then chemicals are added to settle colloidal particles.
Where do you see this in real life? Coffee machine filtering grounds from coffee (residue vs filtrate), kidneys filtering waste from blood (biological filtration), water treatment removing mud from river water, swimming pool filters removing particles, vacuum cleaner bags trapping dust — filtration separates insoluble solids from liquids wherever particles are too big to pass through a barrier.
Practice Problems
A student mixes sand and salt water. Which separation technique should she use first to get the sand out, and why?
Show answer
Would filtration separate copper sulfate solution from water? Explain.