Water (AQA GCSE Chemistry): Revision Notes
Water
What is potable water?
Potable water is water that is safe for humans to drink. This water must be clean and free from harmful substances. It cannot contain too many dissolved salts or dangerous microbes that could make us ill.
Water is essential for all life on Earth. Without clean drinking water, people cannot survive. The human body needs a constant supply of safe water to function properly.
The World Health Organisation estimates that over 2 billion people worldwide lack access to safely managed drinking water at home, highlighting the critical importance of water treatment processes.
Water availability around the world
The availability of clean water varies greatly between different countries:
- The UK has plenty of rain throughout the year
- This rain provides all the fresh water we need
- Other countries have very little fresh water available
- These countries must use salty seawater to make potable water
Countries with limited fresh water must spend significantly more money and energy to create safe drinking water for their populations. This creates economic challenges and affects development in water-scarce regions.
How we treat water to make it safe
Step 1: Collecting fresh water
Fresh water starts as rain, which contains small amounts of dissolved substances. This rainwater flows into rivers and lakes or soaks underground. We collect this fresh water from these natural sources.
Step 2: Filtering the water
Water Filtration Process
The collected water passes through a filter bed containing different layers:
Step 1: Water enters at the top
Step 2: Sand removes medium-sized particles
Step 3: Pebbles at the bottom catch larger debris
This filtration removes solid particles and makes the water much cleaner.
Step 3: Killing harmful microbes
The final step is sterilisation - killing any dangerous microbes in the water. We can use three different methods:
- Chlorine gas - a chemical that destroys bacteria and viruses
- Ozone - another chemical that kills microbes
- UV light - special light that damages microbes so they cannot harm us
After these three steps, the water becomes safe potable water ready for drinking.
Making seawater drinkable
Some countries must remove salt from seawater to create fresh water. This process is called desalination.
Desalination means removing sodium chloride (salt) from salty seawater. There are two main ways to do this:
- Distillation - heating seawater to create steam, then cooling it back to pure water
- Reverse osmosis - using special membranes to philtre out the salt
Both desalination methods require lots of energy, making desalination expensive. This is why countries with plenty of fresh water don't usually use these methods. The high energy costs can make desalinated water up to 10 times more expensive than treated fresh water.
Treating dirty water from homes and factories
When we use water in our homes and workplaces, it becomes wastewater containing organic matter, microbes, and harmful chemicals. This dirty water needs cleaning before we can safely return it to rivers and lakes.
Sewage treatment process
Sewage Treatment Process - Four Key Stages
Stage 1: Screening and removing grit
- Large solid particles are removed from the wastewater
- This includes things like plastic, paper, and other debris
Stage 2: Sedimentation
- Smaller solid particles settle to the bottom
- The cleaner water floats on top
Stage 3: Anaerobic digestion
- Microbes break down solid organic materials
- This happens without air present
- Creates a thick mixture called sewage sludge
Stage 4: Aerobic biological treatment
- The remaining water gets treated with air present
- Special microbes break down dissolved organic materials
- Produces clean effluent that can be safely released
This treatment process ensures that water returned to the environment is clean and safe.
Key Points to Remember:
- Potable water is safe drinking water with low levels of salts and no harmful microbes
- Water treatment involves three steps: collection, filtration, and sterilisation
- Sterilisation uses chlorine, ozone, or UV light to kill dangerous microbes
- Desalination removes salt from seawater but requires lots of energy
- Sewage treatment cleans wastewater through screening, settling, and biological breakdown