Water (AQA GCSE Chemistry Combined Science): Revision Notes
Water
What is potable water?
Water is essential for all life on Earth. Potable water means water that is safe to drink. It must not contain high levels of dissolved salts or harmful microbes that could make us ill.
The UK is lucky because it has plenty of rain. This gives us all the fresh water we need. However, many countries around the world don't have enough fresh water. They have to use salty seawater and treat it to make it safe to drink.
Treating fresh water
Fresh water comes from rain. Rainwater contains small amounts of dissolved substances, but it's still much cleaner than seawater.
Here's how we turn fresh water into potable water:
Step 1: Collection
- Rain falls and collects in rivers and lakes
- Some water also collects underground
Step 2: Filtration
- The fresh water passes through a filter bed
- This contains layers of sand and pebbles
- The philtre removes solid particles and makes the water clearer
Step 3: Sterilisation
The filtered water must be sterilised to kill harmful microbes. This can be done in three ways:
- Chlorine - a chemical that kills bacteria
- Ozone - another chemical that destroys microbes
- UV light - ultraviolet light that kills germs
After these steps, the water is safe to drink.
Making potable water from seawater
Countries without enough fresh water must treat seawater. Seawater contains lots of dissolved salt, which makes it unsafe to drink.
The process of removing salt from seawater is called desalination. There are two main methods:
Distillation
- Seawater is heated until it turns into steam
- The steam is then cooled back into liquid water
- The salt is left behind
Reverse osmosis
- Seawater is pushed through a special membrane (like a very fine philtre)
- Water molecules can pass through, but salt molecules cannot
- This separates the clean water from the salt
Both methods need lots of energy, which makes them expensive. This is why desalination is only used when there's not enough fresh water available.
Treating waste water
Households, factories and farms produce large amounts of dirty water called wastewater. This must be cleaned before it can be returned to rivers or the sea.
Waste water treatment happens in several stages:
Stage 1: Screening and grit removal
- Large solid pieces are removed using screens
- Heavy particles like sand and grit settle out
Stage 2: Sedimentation
- Smaller solid particles are allowed to settle at the bottom
- This creates a clearer liquid on top
Stage 3: Anaerobic digestion
- Microbes break down solid organic materials
- This happens without air (anaerobic means "without oxygen")
Stage 4: Aerobic biological treatment
- Different microbes break down dissolved organic materials
- This happens with air (aerobic means "with oxygen")
- The water becomes much cleaner
After all these stages, the water is clean enough to be safely released back into the environment.
Worked Example: Water Treatment Process
A water treatment plant receives 1000 litres of fresh water from a reservoir:
Step 1: Filtration - Water passes through sand and pebble philtre beds, removing solid particles
Step 2: Sterilisation - Chlorine is added at 2 parts per million to kill harmful microbes
Step 3: Quality testing - The treated water is tested to ensure it meets safety standards
Result: 1000 litres of potable water ready for distribution to homes
Key Points to Remember:
- Potable water is water that's safe to drink - it has low levels of dissolved salts and no harmful microbes
- Fresh water treatment involves filtration and sterilisation using chlorine, ozone, or UV light
- Desalination removes salt from seawater using distillation or reverse osmosis, but needs lots of energy
- Waste water treatment uses physical processes and microbes to clean dirty water in four main stages
- The UK uses mainly fresh water, but countries with limited fresh water supplies must treat seawater instead