Classroom activity Grade 4 Science
Building a Solar Still for Tulmere Point's Salty Water
A grade 4 activity in which groups build two solar stills that differ only in the stone on the cover, compare what each collects, and test both waters for salt.
Objectives
- Students will build two solar stills that differ in only one feature.
- Students will compare how much water each still collects and name the design that worked better.
- Students will use a dried drop on dark paper as evidence that the salt stayed behind.
Vocabulary
Key words: evaporate, condense, solar still, fair test, variable
Teacher overview
Tulmere Point is a fictional village on a bay where the wells have turned salty and the only water close at hand cannot be drunk. In this two-session activity, groups of three build two solar stills that are identical except for one small stone resting on the plastic cover, then compare what each still collects and check both waters for salt. The mechanism is plain enough to state in order: the sun turns liquid water into vapor, the vapor condenses on the cool plastic, and the salt, which does not evaporate, stays behind in the bowl. The misconception to plan for is the belief that the salt travels with the water or that the plastic wrap strains it out. The dried-drop test on dark paper settles that question with evidence rather than with a teacher's assurance, so it is worth asking for predictions before step nine.
Activity Directions
Set up (5 min). Put students in groups of three and give each group two bowls, two small cups, plastic wrap, two rubber bands, one stone, and a marker. Groups label one bowl A and the other bowl B.
Mix the salty water (5 min). Each group stirs 4 tablespoons of table salt into 2 cups of warm tap water until the salt disappears. Fill the water from the warm tap yourself if your tap runs hot, since the water only needs to be warm to the touch.
Pour and place (5 min). Groups pour half the salty water into bowl A and half into bowl B, then stand one empty small cup upright in the center of each bowl. No salty water may get into the small cups, and any group that splashes should start that bowl over. If the cup is light, set a clean pebble inside it so it does not float.
Cover both stills (5 min). Stretch plastic wrap tightly over each bowl and hold it with a rubber band. The wrap must not touch the small cup.
Change one thing only (3 min). Place the stone on the wrap of bowl A, directly above the small cup, so the wrap dips into a low point. Leave the wrap on bowl B flat. Say plainly that the stone is the only difference between the two stills, which is what makes this a fair test.
Put both stills in the sun (2 min). Set both bowls side by side in the same sunny window, the same distance from the glass. Groups write down the time. Leave the stills until the next session, at least three hours later, and longer if the day is cloudy.
Look before you open (5 min, session two). Groups describe what they see on the underside of the wrap and inside each small cup without touching anything. Record these observations on the handout first.
Measure what was collected (10 min). Groups remove the rubber bands, lift each small cup out carefully so no bowl water spills into it, and measure the collected water with a spoon or a small measuring cup.
Test for salt (8 min). Each group puts one drop of bowl water on dark paper and one drop of cup water beside it, labels both, and sets the paper aside to dry. No one drinks or tastes any water in this activity, because classroom bowls and cups are not clean enough for drinking water.
Read the dried spots (5 min). After the drops dry, groups compare the two spots. The bowl water leaves a white salt ring, and the cup water leaves little or nothing.
Debrief (5 min). Ask two questions of the whole class: which still collected more water, and where the salt went. Then ask what a village would need in order to get enough drinking water this way, which points to how slow the process is.
Clean up (2 min). Pour the salty water down the sink, wash the bowls, and have every student wash their hands. The activity ends once both dried drops have been compared and the handout is complete.
Student Handout
Suppose your group works for the village of Tulmere Point, where the well water has turned salty. Answer these questions about your two stills.
How long did your stills sit in the sunny window?
Describe what you saw on the underside of the plastic wrap before you opened the stills.
How much water did you collect in the cup in Still A?
How much water did you collect in the cup in Still B?
Which still collected more water?
Describe how the two dried drops looked on the dark paper.
Where did the salt go? Use what you saw on the paper as your reason.
The stone was the only difference between Still A and Still B. Why did your group keep everything else the same?
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Answer Key
Answer: Accept the group's own elapsed time.
Check that the recorded time matches the class start time and the time of the second session. Most classes report three hours to one full school day.Answer: Small drops of water clinging to the underside of the wrap.
Accept any description of beads, drops, or fog on the plastic. Groups often notice that the drops in Still A slide down toward the low point under the stone.Answer: Accept the group's own measurement for Still A.
Yields are small, often a spoonful or two after several hours. A group with an empty cup should check whether the wrap sagged onto the cup or the seal came loose.Answer: Accept the group's own measurement for Still B.
Still B usually collects less than Still A, and sometimes nothing, because drops on the flat wrap run to the edges instead of dripping into the cup.Answer: Still A, the one with the stone, in most trials.
Accept whichever still the group actually measured as higher, as long as the answer matches their own numbers. A tie is a real result and should be reported as one.Answer: The bowl drop left a white ring, and the cup drop left little or nothing.
Accept any description naming a white, crusty, or cloudy mark for the bowl water and a clean or nearly clean spot for the cup water.Answer: The salt stayed behind in the bowl.
Accept any answer stating that only the water evaporated and the salt could not, supported by the white ring left by the bowl water and the clear spot left by the collected water.Answer: So the stone was the only thing that could explain the difference.
Accept any answer describing a fair test: same bowls, same amount of salty water, same window, same amount of time, so one change is the only possible cause of a different result.
Differentiation
Support
Assign these students the pouring and stone-placing jobs, which are the steps that make the single variable visible, and give them a picture-sequence card showing the six setup steps in order. Provide a recording sheet with the first two questions already framed as circle-the-answer items, and measure the collected water alongside them.
Extension
Have students design a third still that changes one different feature, such as a black paper liner under the bowl or a taller cup, and predict which change will beat the stone. They should state the failure point they are trying to fix and explain how they will keep the test fair.
ELL supports
Pre-teach evaporate, condense, and collect with a demonstration of a cold glass fogging on a warm day, and post the three words with arrows on a labeled diagram of the still. Provide the handout with a small drawing beside each question. Give sentence frames for questions 5 through 8: "Still ___ collected more because ___." and "The salt stayed in the ___ because ___."
Extensions
Set one still outdoors overnight and one in the classroom window, then compare the two the next morning and discuss what the temperature difference changed. Students can also work out how many days a still their size would take to fill one drinking glass, which explains why real seaside towns build reverse osmosis plants instead of covering a field with plastic sheets.