Potable Water and Distillation of Sea Water
Welcome to this guide on one of the most important parts of Chemistry: making water safe to drink! While our planet is covered in water, most of it isn't ready for us to gulp down straight away. In this chapter, we will explore how we turn "raw" water from rivers or the sea into potable water.
1. "Pure" vs. "Potable" – What's the Difference?
In everyday life, we might say a bottle of water is "pure," but in a Chemistry exam, you need to be more careful with that word! Don't worry if this seems a bit pedantic at first; it's a very common distinction in GCSE Chemistry.
- Pure Water: In the chemical sense, a "pure" substance contains only one type of particle. Therefore, pure water contains only \(H_{2}O\) molecules and nothing else. You can test if water is pure by checking its boiling point—pure water boils at exactly \(100^{\circ}C\).
- Potable Water: This is water that is safe for humans to drink. It is not "pure" because it contains low levels of dissolved salts and minerals (like calcium or magnesium), but it is free from harmful microbes and toxic chemicals.
Quick Tip:
If an exam question asks why tap water isn't "pure" in the chemical sense, the answer is usually: "Because it contains dissolved substances/minerals."
2. Making Water Potable
Water collected from sources like rivers and lakes (surface water) or underground rocks (groundwater) needs to be treated before it reaches your tap. The process usually involves three main steps:
Step 1: Sedimentation
Water is pumped into large tanks. Here, it is allowed to sit so that large, heavy insoluble particles (like grit and dirt) can sink to the bottom. This creates a layer of "sludge" at the base, leaving clearer water at the top.
Step 2: Filtration
The water is then passed through special filter beds made of sand and gravel. This catches the smaller solid bits that didn't sink during sedimentation. By the end of this stage, the water looks clear, but it still isn't safe because it contains invisible bacteria.
Step 3: Chlorination
This is the "cleaning" stage. Chlorine gas is bubbled through the water. Chlorine is a disinfectant that kills harmful microbes, such as bacteria and viruses, making the water safe to drink.
Key Takeaway: Sedimentation removes large bits, filtration removes small bits, and chlorination kills the germs!
3. Distillation of Sea Water
In some parts of the world, there isn't enough fresh water in rivers or lakes, so they have to use sea water. However, sea water is extremely salty and would actually dehydrate you if you drank it!
To make sea water potable, we use a process called distillation (sometimes called desalination). Here is how it works step-by-step:
- Heating: The salty sea water is heated until it boils.
- Evaporation: The liquid water turns into water vapour (steam), leaving the solid salt behind in the flask. (Remember: Salt has a much higher boiling point than water, so it doesn't evaporate!)
- Cooling/Condensing: The water vapour is passed through a condenser (like a Liebig condenser), where it is cooled down and turns back into liquid water \(H_{2}O (l)\).
- Collection: The resulting liquid is pure water that is safe to drink once minerals are adjusted.
Did you know?
While distillation produces very high-quality water, it is very expensive. This is because it requires a huge amount of energy to heat the water to its boiling point. Because of this, it is usually only used in countries where there is no other choice or where energy is cheap.
4. Comparing the Methods
It is important to understand when to use which method. Let's look at a quick comparison:
Surface Water Treatment (Filtration/Chlorination):
Pros: Relatively cheap and easy to do on a large scale.
Cons: Only works if you have a source of fresh water (rivers/lakes) to start with.
Distillation of Sea Water:
Pros: Can provide fresh water even in desert regions near the ocean; produces very pure water.
Cons: Very high energy costs; salt waste must be disposed of carefully.
5. Common Mistakes to Avoid
- Mistake: Thinking that filtration kills bacteria. Fact: Filtration only removes solid particles. You need chlorination or boiling to kill microbes.
- Mistake: Saying distillation is "easy." Fact: While the concept is simple, doing it on a large scale is energy-intensive and expensive.
- Mistake: Confusing "pure" and "potable." Fact: Potable means "safe to drink," while pure means "only one substance present."
Chapter Summary
1. Potable water is safe for human consumption but contains dissolved minerals.
2. Surface water is treated via sedimentation (settling), filtration (sand/gravel), and chlorination (killing bacteria).
3. Sea water can be made potable through distillation, which involves boiling water to separate it from salt and then condensing the steam.
4. Distillation is effective but requires a lot of energy, making it expensive.