Density experiments for kids work well because density is invisible until you set something in a liquid and it either sinks or floats — and that binary outcome is easy to observe and genuinely surprising. Density is mass per unit volume: how much stuff is packed into a given space.
The Core Idea
An object floats in a liquid if it is less dense than that liquid. It sinks if it is denser. This is why:
- A heavy log floats while a small pebble sinks — wood is less dense than water, stone is denser
- Ice floats in water — unusual, and most liquids do the opposite when frozen
- A steel ship floats — it is hollow, so its average density including the air inside is less than water
That ship point is the one that fixes the most confusion. It is not about weight, it is about weight relative to the space taken up.
1. The Density Tower
Pour these slowly, one down the side of a tall glass, in this order:
| Layer (bottom to top) | Density (g/mL) |
|---|---|
| Honey | ~1.42 |
| Corn syrup or dish soap | ~1.33 / ~1.06 |
| Water (coloured) | 1.00 |
| Vegetable oil | ~0.92 |
| Rubbing alcohol (coloured) | ~0.79 |
Pour slowly down the side of the glass or they mix. Then drop in small objects — a grape, a cork, a plastic bead, a paperclip — and predict where each will settle. Objects stop at the layer matching their own density.
2. Sink or Float Predictions
Gather a dozen objects. Before each test, write down the prediction. Then test.
The good trick items: an unopened can of regular soda versus diet soda. Regular sinks, diet floats. Why: the sugar in regular soda adds enough mass to push density above water, while the sweetener in diet soda is used in such tiny quantity that it does not. Kids love this one because the two cans look identical.
3. The Egg Test
Put a raw egg in plain water — it sinks. Add salt, a tablespoon at a time, stirring to dissolve. At around six to eight tablespoons the egg rises and floats.
Why: dissolving salt increases the water’s density without changing its volume much. Once the salt water is denser than the egg, the egg floats. This is exactly why floating in the Dead Sea is effortless.
4. Lava Lamp Without Electricity
Fill a bottle with oil, top up with water, add food colouring, then drop in part of an Alka-Seltzer tablet.
Why: oil is less dense than water and does not mix with it, so it sits on top. The tablet makes CO2 bubbles that lift coloured water through the oil; when the bubbles pop at the surface, the denser water sinks again.
5. Ice in Water vs Ice in Alcohol
Drop an ice cube in water — it floats. Drop one in rubbing alcohol — it sinks.
Why: ice is about 8% less dense than water, so it floats. But it is denser than alcohol. Same object, different liquid, opposite result. This is one of the cleanest demonstrations that floating is about comparison, not about the object alone.
6. Making a Hydrometer
Put a straw in a lump of clay so it stands upright in water. Mark the waterline. Then float it in salt water and mark again — it sits higher.
Why: denser liquid supports more of the straw’s weight above the surface. This is literally how brewers and winemakers measure sugar content.
7. Hot vs Cold Water
Fill one jar with hot water (red food colouring) and one with cold (blue). Put the cold one on top, mouth to mouth with a card between, and pull the card out. They stay separate — cold on the bottom.
Flip the arrangement: hot on top, cold on the bottom. They mix immediately.
Why: cold water is denser than hot water, so cold sinks. When dense liquid is already at the bottom, nothing moves; when it is on top, it falls through and convection mixes everything. This is the same mechanism driving ocean currents.
Turning It Into a Real Experiment
The best density experiments for kids end with a number, not just an observation.
- Measure, do not just observe. Use a kitchen scale and a measuring jug; density = mass ÷ volume.
- Vary one thing. How much salt before the egg floats? Repeat three times and average.
- Predict first, write it down. Wrong predictions are the interesting ones.
- Explain the exception. Ice floating is genuinely unusual — most substances get denser when they freeze. Being able to say why scores highly.
Related Reading
- Why does ice float? — the famous exception
- Easy chemistry experiments at home — more to try
- How do ocean currents work? — density at planetary scale
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