Practical 11
Water Purification for Human Consumption
When you are on Mars, you need access to clean drinking water. You have obtained water from the polar ice-caps, but you suspect that this water is not pure, and not safe to drink. You can see that it is slightly muddy, containing small stones and other particles, and you also believe that there is salt (sodium chloride) dissolved in the water. You need to find ways to remove the impurities from the water.
Planning
You need to plan two activities:
- To remove the mud, stones, and other small particles from the water.
- To remove the salt from the water.
Activity 1: How could you remove small stones from water? What about very small particles? Write down what you will you do (Method), and what will you need (Apparatus). What do you expect to see, and how will you record your results?
Activity 2: Your water may now look clean, but you can’t see if there is salt dissolved in it. How could you find out, and how could you separate the salt and water? (Think about coastal areas of southern Europe and the Middle East where salt is produced). You will need apparatus that will help you collect water: ask your teacher for advice. What do you expect to see, and how will you record your results?
Implementing
Activity 1: Removing small stones and particles from water
A fairly easy way to remove any particles and debris from water is to filter it. There are various types of filters that could be used:
- Gravel: traps stones and large pieces of debris
- Sand: traps small stones and other debris
- Charcoal: absorbs a range of chemicals dissolved in water
- Cheesecloth: traps small particles
- Filter paper: available in a range of pore sizes, and can trap particles which are very tiny
Apparatus and Materials Needed
(per group, class, or students)
- Clear plastic soda bottle (2-liter)
- Gravel (aquarium)
- Sand
- Aquarium charcoal (activated)
- Cheesecloth (a nylon stocking can be used instead)
- Muddy water
- Rubber bands
- pH water testing kit (optional)
Safety
These safety points mainly tell you about hazard. You will need to think about the risk(s) this might pose, and how you can minimize the risk.
- The water produced from these activities will not be completely pure.
- Do not consume any water used in this activity, even after filtering.
Method (see diagram):
Step 1: Cut the bottom off the soda bottle. Cover the mouth with several layers of cheesecloth and secure them with a rubber band. Suspend the bottle upside down with its mouth over a glass to catch the filtered water.
Step 2: Fill the bottle with charcoal to a depth of 5-8 cm. Place 8-10 cm of sand on top of the charcoal. Place 5-8 cm of gravel on top of the sand.
Step 3: Stir the muddy water and pour it into the filter. Watch closely as the water seeps down through the three filtering layers of gravel, sand, and charcoal.
- gravel
- sand
- charcoal
- cheesecloth
Discussion
- What happened to the water while it passed through the different layers of the filter?
- Compare the muddy water to the filtered water. Is there a difference?
- Would it make a difference if one of the layers had been left out?
Activity 2: Removing salt from water
If you took a drop of very salty water and left it on a watch glass for a day or two, what would you see? You would be able to see a faint, white mark: this is salt (sodium chloride). What has happened to the water? It has evaporated.
It is easy to separate salt and water to leave salt. Water can be heated to make it evaporate, and this doesn’t happen to salt. However, it is not so easy to separate salt and water in a way that allows you to keep the water. You can do this in the process of ‘distillation’, and your teacher may be able to demonstrate a distillation apparatus. You can carry out simple distillation as follows:
Safety:
These safety points mainly tell you about hazard. You will need to think about the risk(s) this might pose, and how you can minimize the risk.
- Take care when using Bunsen burners, and do not move Tripod stands until cool,
- Do not consume any water used in this activity, even after distillation.
Apparatus
- 2 medium size Pyrex beakers
- One large watch glass or equivalent
- Bunsen burner
- Two tripod stands with gauze
Method
- Set up the two tripods close together.
- Add around 50cm3 of salt water to one beaker and nothing to the other.
- Place each beaker on their adjacent tripods stands close together so that they are almost touching.
- Place the large watch glass upside down above the two beakers with most over the beaker with the salt water, but a part of it over the beaker with nothing in it.
- Set up the Bunsen burner beneath the beaker with salt water, and heat for around 15-20 minutes.
Results
What do you observe happening in each of the beakers?
Discussion
- You should have noticed drops of water collecting in the beaker that was originally empty. Where did this water come from?
- What happened inside the beaker which contained the salt water?