Required practical - Analysis and purification of water (AQA GCSE Chemistry Combined Science): Revision Notes
Required practical - Analysis and purification of water
What is this practical about?
This practical shows you how to get pure water from salt water using a process called distillation. You'll learn how to separate water from dissolved salt by heating and cooling.
Aim of the practical
The aim is to obtain pure water from a solution of salt in water (salt water).
Equipment you need
- Eye protection - always wear safety goggles
- Distillation apparatus including:
- Round-bottomed flask
- Liebig condenser
- Bunsen burner
- Salt water sample
- Anti-bumping granules
- Beaker to collect pure water
Safety First! Always wear eye protection throughout this practical. The apparatus will get very hot, and steam will be produced. Handle all equipment carefully and ensure the Bunsen burner is used safely.
How distillation works
The Science Behind Distillation
When you heat salt water, the water boils and turns into water vapour, but the salt does not evaporate. The water vapour travels through the apparatus and gets cooled down by cold water flowing through the condenser. This causes the water vapour to condense back into liquid water, which you collect as pure water.
Method
Follow these steps carefully:
- Pour the salt water into the round-bottomed flask
- Add anti-bumping granules to the flask (these stop the apparatus from rattling and help the water boil smoothly)
- Set up the apparatus and turn on the cold water to flow through the Liebig condenser
- Heat gently using the Bunsen burner
- Collect the pure water in the beaker
Critical Safety Point: Never forget to add anti-bumping granules! Without them, the salt water can boil violently and cause dangerous splashing or apparatus damage.
Key Points to Remember:
- Water boils but salt does not evaporate - this is why the method works
- Cold water flows through the condenser to cool down the water vapour
- Anti-bumping granules prevent violent boiling and keep the apparatus stable
- The pure water collects in the beaker at the end of the condenser
Why does this work?
Water has a much lower boiling point than salt. When heated, water molecules escape as vapour while salt remains as a solid in the flask. The condenser cools the water vapour back into liquid form, giving you pure water.
The Scientific Principle This separation works because of the different physical properties of water and salt. Water has a boiling point of 100°C at standard pressure, while sodium chloride (salt) has a much higher boiling point of over 1400°C.
Common exam questions
Worked Example: Exam Questions
Question 1: What reading would show on a thermometer during distillation? Answer: 100°C - the boiling point of water
Question 2: Where would you find water and sodium chloride after distillation? Answer: Pure water in the collection beaker, sodium chloride left in the original flask
Remember!
- Distillation separates pure water from salt water by heating and cooling
- Water evaporates at 100°C but salt stays behind in the flask
- Anti-bumping granules are essential for smooth, safe boiling
- Cold water through the condenser turns water vapour back into liquid
- This method produces pure water that can be collected and tested