Solutions are a core unit of middle school chemistry, and the concepts — saturated, unsaturated, solubility — blur together fast when students only memorize them. This experiment needs nothing but table salt, sugar and water to make every one of those ideas visible, and at the end it even turns the dissolved salt back into tiny square crystals.

There are no hazardous reagents; the only step that needs care is the final evaporation, which uses a flame or other heat source.

Safety first (read this before you start)

  • Evaporation crystallization involves heating and must be done or supervised by an adult throughout. Use a low flame or an electric hot plate, wear heat-resistant gloves, and never keep heating a container after it has boiled dry.
  • Never move a glass container straight from the heat into cold water — the thermal shock can shatter it. Set it on a heatproof mat and let it cool naturally.
  • No evaporating dish at home? A small stainless-steel spoon or a heat-resistant ceramic saucer works; do not heat an ordinary drinking glass directly.
  • The salt water in this experiment is harmless, but do not taste any liquid after the experiment, and don't serve food in containers used for experiments.
  • Wash your hands and all equipment when finished.

Materials and equipment

  • A small bowl of table salt (sodium chloride) and a small bowl of white sugar (for comparison);
  • 3 clear glasses and room-temperature water;
  • A measuring spoon or small tablespoon (use the same one every time so every scoop is equal), a glass rod or chopstick;
  • A cup of hot water (about 60–70 °C, prepared by an adult);
  • For evaporation: a small ceramic saucer or stainless-steel spoon, a heat source, a heatproof mat, heat-resistant gloves;
  • Pen and paper for records; a magnifying glass is optional (for looking at crystal shapes).

Step 1: Find the moment of saturation

  1. Pour 50 mL of room-temperature water into a glass.
  2. Add one level spoonful of salt and stir thoroughly with the glass rod until no solid grains are visible.
  3. Record "spoonful 1: fully dissolved." Add spoonful 2, stir until nothing more changes, and record the result.
  4. Keep going, one spoonful at a time. Stir each spoonful until it truly stops dissolving before adding the next, or you will misjudge the endpoint.
  5. Stop when students notice that no matter how long they stir, a layer of undissolved salt stays on the bottom. That glass now holds a saturated solution of salt at this temperature — and every glass before it was an unsaturated solution.
  6. Write down the number of spoonfuls added. That is a hands-on measurement of roughly how much salt 50 mL of water can dissolve at this temperature.
The most common mistake: adding the next spoonful before stirring long enough — sediment has been sitting on the bottom for a while, but everyone thinks the solution isn't saturated yet. The one and only criterion for saturation is "at this temperature, further stirring dissolves no more." It has nothing to do with how long you stirred or how patient you were.

Step 2: Two comparison experiments

Comparison 1 · Change the solute (salt vs. sugar)

Fill a second glass with the same 50 mL of room-temperature water, switch to white sugar, and again add spoonful by spoonful until saturated. Students will find that far more sugar goes into the water than salt did. Lesson: in the same amount of water, the maximum amount that dissolves differs enormously from substance to substance.

Comparison 2 · Change the temperature (room temperature vs. hot)

Take the saturated salt water with leftover salt on the bottom and warm it in a hot-water bath, or stir in hot water (adult's job), and watch whether the salt at the bottom shrinks. Then do the same with the sugar glass. You will see:

  • Sugar: once warmed, much more dissolves — the leftover sugar on the bottom disappears quickly;
  • Salt: warming lets a little more dissolve, but the change is nowhere near as dramatic as sugar's.

Step 3: Evaporate — and get the salt back

  1. Take some of the clear saturated salt water (leave the sediment behind) and pour a thin layer into the small saucer.
  2. Have an adult heat it slowly over a low flame — or simply leave it on a sunny, well-ventilated windowsill for a few days (safer, and recommended).
  3. As the water evaporates and the level drops, white solid begins to appear around the edges.
  4. Stop heating just before the water is completely gone and let it cool and dry on its own.
  5. Examine the crystals with the magnifying glass: salt crystals are neat little cubes.

What you should observe

  • The first few spoonfuls of salt vanish completely when stirred (unsaturated solution);
  • After a certain spoonful, solid remains on the bottom no matter what (saturated solution);
  • At the same temperature and water volume, sugar dissolves in far greater amounts than salt;
  • Raising the temperature greatly increases how much sugar dissolves, but only slightly increases the salt;
  • After evaporation, white cube-shaped salt crystals appear.

The science

When salt dissolves, water molecules surround the sodium chloride particles, pull them apart, and spread them evenly through the water, forming a solution. But water's capacity for particles is not unlimited: when the rates of dissolving and of crystallizing back out become equal, the solution is saturated — add more and it simply won't go in.

The textbook pins this down with solubility: the mass of a solid that dissolves in 100 g of solvent to reach saturation at a given temperature. That is why a solubility figure must always state two conditions — which temperature and which solvent — or the number is meaningless.

Why do sugar and salt differ so much? Because solubility is a property of the substance itself. At 20 °C, sodium chloride's solubility is about 36 g, while sucrose's is about 204 g — nearly six times greater. That is the gap you saw between the two glasses.

And why does warming affect them so differently? Most solids dissolve more as temperature rises, but by wildly different amounts. Sucrose and potassium nitrate respond strongly to temperature; sodium chloride barely moves (about 36 g at 20 °C, still only about 39 g at 100 °C).

This difference decides, in industry, which purification method is used:

  • Substances like table salt, whose solubility changes little with temperature, are purified by evaporation crystallization (drive the water off) — exactly what solar salt fields do;
  • Substances like potassium nitrate, whose solubility changes a lot with temperature, are purified by cooling crystallization (cool the hot saturated solution and let the crystals fall out on their own).

As for why the crystals come out as neat cubes: in solid sodium chloride the particles are packed in a strict, repeating pattern, and that microscopic order shows up macroscopically as a regular geometric shape. You can see this in the molecular simulation in our states-of-matter article — the orderly arrangement of particles in a solid is exactly where a crystal's regular shape comes from.

Notes for teachers and parents

  • Time and grouping: two class periods (the crystals need to sit); pairs.
  • Make clear before hands-on work: an adult handles the hot water.

The step most often skipped. Salt and sugar must be run side by side — that six-fold gap only shows up in the comparison. Let students design the stirring and heating comparisons themselves; it is an excellent workout in controlling variables.

How to know they really got it. Students meet the bar when they can explain that "saturated" only means something at a stated temperature, and why salt is purified by evaporation rather than cooling crystallization.

Take it further

  • Grow a big crystal: make a hot saturated solution of sugar or salt, hang a cotton thread from a chopstick into it, and leave it undisturbed for several days — visible crystals grow along the thread. The slower the cooling and the stiller the glass, the larger and more regular the crystals.
  • Three ways to dissolve faster: stir, warm it up, or grind the grains finer. Try each one; students will discover these change only how fast something dissolves, never how much can dissolve. This distinction is one of the most-tested points in the unit.
  • Purify crude salt: deliberately mix a little fine sand into some salt, then dissolve, filter (the sand stays behind), and finally evaporate to crystals — a complete walk-through of the textbook's crude-salt purification procedure.

When students can explain that saturation always assumes a fixed temperature, why solubility must state its temperature, and why salt is recovered by evaporation instead of cooling, the hardest ideas in this unit have all clicked into place.