C. Matter and Energy · Properties of and Changes in Matter

Design Challenge: A Sun-Powered Water Still

A1.3 A1.4 A1.5 C2.6 C2.3About 60 minutes

Learning goals

I can use the engineering design process to build and improve a device.
I can explain how my still uses evaporation and condensation.
I can explain where thermal energy is absorbed and released in my still.
I can test my design, record results and share improvements.

Words to know

engineering design process
The steps engineers use to solve a problem: define the problem, plan, build a prototype, test, improve, and share.

prototype
A first model of a design, built to be tested and improved.

solar still
A device that uses the Sun's energy to evaporate water and condense it somewhere else, leaving salt and dirt behind.

criteria
The things a design must do to be successful.

constraints
Limits on a design, like which materials or how much time you can use.

The challenge

Imagine you are on a beach with only salty water. You can't drink salt water. Can you use the Sun and what you know about changes of state to separate fresh water from the salt?

Your challenge: design, build and test a solar still that collects as much fresh water as possible from salty water in 4 hours of sunshine.

The science behind it

Sunlight warms the salty water. The water absorbs thermal energy and evaporates into water vapour (gas). The salt does not evaporate; it stays behind in the bowl.
The water vapour rises and touches the plastic wrap, which is cooler because air outside the bowl cools it. The vapour releases thermal energy and condenses into drops of liquid water.
The drops run down the sloping wrap to its lowest point, under the stone, and drip into the cup.

This is like the water cycle you learned in Grade 2, in a bowl!

A solar still. Sunlight makes water evaporate from the salty water. The vapour condenses on the plastic wrap, runs to the low point under the stone, and drips into the empty cup. Not to scale.

Step	What you do
1. Define the problem	Collect fresh water from salty water using sunlight. Criteria: collects water; the collected water is not salty; nothing spills. Constraints: household materials only; no stove or heat source other than the Sun.
2. Plan	Sketch your design. Label the parts and materials. Predict how much water you'll collect.
3. Build a prototype	Build it in a sunny spot.
4. Test	Wait 4 hours. Measure the water in the cup with a measuring spoon or small measuring cup.
5. Improve	Change one thing and test again on a similarly sunny day.
6. Share	Present your design, results and improvements.

The engineering design process for your still.

Worked example

Let's improve a design

Kai's first still collected 2 teaspoons of water in 4 hours. His bowl was white plastic, and the plastic wrap was loose, with gaps at the edges. How could he improve it?

Look for leaks: gaps let water vapour escape. → Stretch the wrap tight and seal the edges with tape.
Absorb more thermal energy: dark colours absorb more sunlight than white ones (remember Grade 4!). → Use a dark bowl, or put a dark cloth under the bowl.
Help drops find the cup: make sure the stone sits right above the cup, so the low point is over it.
Test fairly: change one thing at a time and test on a similarly sunny day, so he knows which change helped.
Kai switched to a dark bowl (only change). He collected 5 teaspoons.

Kai's dark bowl collected 3 teaspoons more (5 − 2 = 3). His next test could try sealing the edges.

Kai's results. Changing only one thing, the bowl's colour, raised his water from 2 to 5 teaspoons: 3 teaspoons more.

Just for fun

Why didn't the salt leave the bowl with the water vapour?

It didn't want to get carried away! (Really: salt does not evaporate, so only fresh water reaches the cup.)

Hands-on activity

Build and test your solar still

You need: a large bowl (dark-coloured works best), a short, heavy cup or a small jar that is shorter than the bowl's sides, plastic wrap, tape, a small clean stone, water, 2 tablespoons of salt, a measuring cup and measuring spoons, a sunny spot outside or a sunny windowsill, paper and pencil.

Plan: sketch and label your design. Predict how much water you'll collect.
Mix 2 tablespoons of salt into about 500 mL of water. Pour it into the bowl, so it is a few centimetres deep.
Put the empty cup in the middle. The salty water should not splash into it, and the cup must be shorter than the bowl's sides.
Cover the bowl tightly with plastic wrap and tape the edges.
Put the stone in the middle of the wrap, right above the cup, so the wrap dips down.
Leave it in strong sunlight for 4 hours.
Test: measure the water in the cup. Then check it the scientist's way, without tasting: put a few drops on a dark plate, and a few drops of the salty water from the bowl on another spot. Let them evaporate. Which spot leaves salt crystals?
Improve: change one thing (dark bowl, tighter seal, sunnier spot, wider bowl) and test again.
Share: present your sketch, results and what you'd change next. Explain where thermal energy is absorbed and released.

What to look for: drops on the underside of the wrap, and a small amount of water in the cup that is not salty. A few teaspoons is a good result.

Safety: use only the Sun to warm the still; no stoves, kettles or heaters. Use a plastic or sturdy cup, not thin glass, outdoors. Don't drink the collected water: this is a science model, and the bowl and wrap may not be clean enough. Keep the salt water away from plants and pets. Wear a hat and sunscreen outside, and never look directly at the Sun.

Your turn

Which change of state happens to the salty water in the bowl?

Which change of state happens on the plastic wrap?

Why isn't the water in the cup salty?

What is a prototype?

Name the six steps of the engineering design process.

Why does the stone go right above the cup?

More practice: warm-up

What gives the still its energy?

Does water absorb or release thermal energy when it evaporates?

Does water vapour absorb or release thermal energy when it condenses?

What is a criterion for the solar still?

What is a constraint for the solar still?

More practice: on your own

Why does a dark bowl usually work better than a white bowl?

Why should you change only one thing between tests?

Kai collected 5 teaspoons in his second test and 8 teaspoons in his third. How many more in the third test than in the second?

How could you check that the collected water has no salt, without tasting it?

Why would a still collect less water on a cloudy day?

Challenge

Explain the whole still using the words absorb, release, evaporate and condense.

Why does the plastic wrap need to be cooler than the water vapour?

People in some dry places use large solar stills. Give one advantage and one disadvantage of solar stills.

Check your understanding

Quiz

In a solar still, the salty water...

Drops form on the plastic wrap because water vapour...

Why is the water in the cup not salty?

A first model built to be tested is called a...

To improve a design fairly, you should...

Which change would most likely help a solar still collect more water?

For parents and teachers

Big idea: This design challenge uses the engineering design process (A1.3), safety procedures (A1.4) and communication (A1.5) to apply C2.6 (changes of state with thermal energy absorbed or released) and C2.3 (a change of state like the water cycle).

How to help: Let your child plan and make their own decisions; ask questions rather than fixing the design. Help them choose a sunny day and a safe outdoor spot. Celebrate improvements, even small ones.

Common difficulties:

Salty water splashing into the cup. The cup must sit firmly and be shorter than the bowl's sides.
Loose wrap. Vapour escapes from gaps.
Changing several things between tests.
Expecting a lot of water. A few teaspoons in an afternoon is normal.

Safety: the Sun is the only heat source. Remind your child never to look at the Sun, to use sunscreen and a hat, and not to drink the collected water.