Building Your Own Solar Still
Sun-Powered Sea Water:
Where sunlight, evaporation, and condensation team up to make salt water drinkable.
Time: 25 minutes to set up
3–6 hours to observe
Complexity: Beginner
Setup: Bowl, cup, plastic wrap, sunlight
Overview
Imagine being stranded at sea with nothing to drink but salt water. The fix is genuine science you can recreate on your kitchen counter in half an hour. A solar still uses sunlight, a bowl, and plastic wrap to separate fresh water from salt water — the same idea behind emergency kits and large-scale desalination plants. You will build a working miniature still and collect a small cup of genuinely fresh water by day’s end.
A sealed bowl still, ready to turn sunlight into fresh water.
Solar Still Science Project Materials Required
| Large mixing bowl
wide and shallow |
Small cup or shot glass
lower than bowl’s rim |
| Salt water
2 tbsp salt per litre |
Plastic cling wrap
enough to cover the bowl |
| A small weight
a pebble or coin |
Rubber band or tape
to seal the edges |
| Sunny spot
windowsill or outdoors |
Food colouring (optional)
shows the separation |
Solar Still Science Project: Step-by-Step Instructions
| 1 | Mix your salt water: Stir two tablespoons of salt into a litre of water. Add food colouring for a visual proof later. |
| 2 | Pour into the bowl: Fill the bowl a third full with salt water, leaving room above the surface. |
| 3 | Place the collecting cup: Set the empty cup in the centre so its rim stays above the salt water level. |
| 4 | Seal it with plastic wrap: Stretch wrap tightly over the bowl and secure the edges, leaving no gaps. |
| 5 | Add the centre weight: Place a pebble on the wrap above the cup, pressing it into a shallow downward cone. |
| 6 | Set it in the sun: Move the setup to a sunny spot and leave it undisturbed for several hours. |
| 7 | Check and collect: Watch for droplets sliding into the cup, then remove the wrap and taste-test your fresh water. |
Four stages from bowl to bottled sunshine.
| Pro Tip
No droplets after an hour? Check the seal around the bowl’s edge — even a small gap lets humid air escape, which is the most common reason a still underperforms. |
The Science Behind the Solar Still Activity
A solar still works because salt does not evaporate along with water. Sunlight gives surface molecules enough energy to break free as invisible vapour, but salt ions are too heavy and strongly bonded to lift off, so they stay behind.
That vapour hits the plastic wrap, cooler than the water below, and loses energy — turning back into liquid droplets, the same condensation you see on a cold drink. Your angled weight guides each droplet to the lowest point and into the cup instead of back into the salty bowl.
Evaporation rises, condensation falls — gravity does the rest.
This same principle scales up to real desalination plants that use solar stills or evaporation ponds in sunny, dry regions. Your kitchen-counter version and an industrial plant differ mostly in size, not in science.
Expected Results
Within the first hour, expect a light mist on the underside of the wrap. By hour two or three in good sun, droplets slide toward the weight and fall into your cup. A well-sealed still left out for an afternoon typically collects a few tablespoons — enough to prove the principle, with clear water even though the bowl below is tinted.
Three ways to test and compare your still design.
Fun Variations to Try
- Dig a shallow pit outside and build a ground-level still.
- Compare a still in direct sun versus one left in shade.
- Try real seawater instead of homemade salt water.
- Test how bowl size and shape affects total water collected.
Conclusion
With just a bowl, plastic wrap, and a sunny afternoon, you turned undrinkable salt water into a small cup of fresh water — using the same evaporation-condensation cycle behind rain clouds, morning dew, and industrial desalination plants worldwide.
What Did You Learn?
Evaporation is selective. Sunlight lifts water into vapour while leaving dissolved salt behind.
Condensation needs a cooler surface. Vapour turns back to liquid when it touches something cooler, like the wrap.
Slope matters. An angled surface channels droplets toward your cup instead of back into the bowl.
Seals control efficiency. Gaps let humid air escape, which is why stills sometimes underperform.
Small models mirror big systems. The same principle powers real solar desalination plants in sunny regions.
Frequently Asked Questions
Is the water collected actually safe to drink?
It is genuinely desalinated, but home setups are not sterile, so treat it as a demonstration rather than a drinking source.
Why does the plastic wrap need to be tilted, not flat?
A flat sheet lets droplets fall straight back into the salty water. Tilting it toward the cup gives gravity a clear path to your collection cup instead.
Why doesn’t salt evaporate along with the water?
Salt ions are heavier and more strongly bonded than water molecules, so they need far more energy to become airborne.
Can a Solar Still Science Project work on a cloudy day?
Yes, but it will produce much less fresh water. A solar still works best in direct sunlight because higher temperatures increase evaporation. On cloudy days, the process is slower, so you may need to leave the setup out for longer.
How much fresh water can a Solar Still Science Project produce?
A small homemade solar still usually collects only a few tablespoons of fresh water after several hours of bright sunlight. The exact amount depends on the size of the bowl, the intensity of sunlight, and how well the setup is sealed.
Can I use seawater instead of homemade salt water?
Absolutely! A Solar Still Science Project works with real seawater just as well as homemade salt water. The evaporation process leaves the salt behind and collects fresh water in the cup.
Why is my Solar Still Science Project not collecting water?
The most common reasons are insufficient sunlight, a loose plastic wrap seal, or a flat plastic cover that doesn’t guide water droplets into the cup. Make sure the wrap is tightly sealed and slopes downward toward the center.
What scientific concepts does a Solar Still Science Project teach?
This STEM activity demonstrates several important science concepts, including evaporation, condensation, the water cycle, desalination, solar energy, heat transfer, and gravity. It also shows how scientists use natural processes to solve real-world water challenges.
