Extreme Drawing

Oil and water experiment for kids: why they don't mix

Kitchen science experiment

Shake it, stir it, watch it split again

Oil is lighter than water and doesn't mix with it, so it floats on top in a layer. Shake them together and they go cloudy, then separate again. Add egg yolk, mustard or soap and the tiny droplets stay mixed: an emulsion, like mayonnaise.

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  • Ages 4+
  • 20 minutes
  • With an adult
Painted illustration of the painter shaking a jar, beside a jar of golden oil floating on blue water, a jar of creamy mayonnaise, an egg yolk and mustard.

Quick facts

Age
4 to 10
Time
20 minutes, plus a day for the drying race
Mess
Medium: oily, so use a tray
Adult needed
Yes, for the egg and the clean-up
You need
Cooking oil, water, a jar with a lid, mustard or washing-up liquid (cheap)
On screen / for real
Both

In Extreme Drawing

Pick Cooking oil on the Kitchen tab: pour it or let it drip. Pour some water first (Water on the Bathroom tab) and put the oil on it: it floats as a golden, glossy film that spreads out further and thinner on the water than on the paper. Drip water onto oil lying on the paper and the water goes under it, while the oil comes up on top.

Then make it mix:

  • Vinaigrette. Pour a small puddle of water, drip vinegar into it (Science tab), pour a little oil on top and stir round with your finger (the hand). It goes cloudy, and within a minute or two the oil is a layer again.

  • Mayonnaise. Crack an egg (Kitchen), pour oil on the yolk and stir. Poured on, the oil just lies there; stirred in, it turns into one pale, creamy mass that holds. Mustard (Squeeze) or a drop of dish soap (Science) holds it too.

  • Grease stain. Leave oil on bare paper and it soaks in as a darker, see-through stain. Water and the sponge won't shift it; dish soap, water, a rub and the sponge will.

  • The drying race. Pour two puddles and put oil on one. The bare one dries up; the one under the oil stays wet.

Melted butter and candle wax float on water like oil, and set into a skin as they cool.

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Painted illustration of the painter sniffing a lemon behind a counter with ketchup, mustard, honey, an egg, oil, butter, flour, salt, sprinkles and a rolling pin.

Do it for real: the jar, the mayo and the paper

You need

  • A clear jar with a tight lid

  • Water, with a drop of food colouring to tint it

  • 3 to 4 tablespoons of cooking oil

  • 1 teaspoon of mustard, or a squirt of washing-up liquid

  • For mayonnaise: 1 egg yolk (pasteurised if anyone will taste it), a whisk and a bowl

  • A sheet of kitchen paper or a brown paper bag

  • Two saucers

Why you never put water on hot oil

  1. Layer it

    Fill the jar a third full with tinted water, then pour in the oil. Where does the oil go?

  2. Shake it hard

    Screw the lid on tight and shake for 20 seconds. It goes cloudy. Put it down and time how long it takes to split into two layers again.

  3. Add a helper

    Add the mustard or washing-up liquid and shake again. Does it stay mixed longer this time?

  4. Make mayonnaise (with an adult)

    Whisk the egg yolk with the mustard in a bowl. Add the oil a drop at a time, then in a thin stream, whisking all the time. It turns thick and pale.

  5. The grease spot test

    Put a drop of oil and a drop of water side by side on the kitchen paper. After 10 minutes, hold it up to a window: the water spot has dried, the oil spot is still see-through.

  6. The drying race

    Fill both saucers with the same amount of water. Float a few drops of oil on one. Leave them on a windowsill for a day or two. Which one dries first?

Painted illustration of the painter shaking a jar, beside a jar of golden oil floating on blue water, a jar of creamy mayonnaise, an egg yolk and mustard.

Why it works

For kids: water sticks to water much more strongly than it sticks to oil, so it squeezes the oil out, and the oil is lighter, so it floats up. Egg yolk, mustard and soap have one end that likes water and one end that likes oil. They wrap round tiny drops of oil and keep them apart, so the mix can't split again.

For grown-ups: water molecules are polar and hold each other by hydrogen bonds; oil is non-polar, so the two separate, and oil (about 0.92 g/cm³) floats on water (1.00 g/cm³). Shaking breaks the oil into droplets, a temporary emulsion, but the droplets soon run together again (coalescence). An emulsifier settles at the oil–water surface and stops that: lecithin in egg yolk, compounds in mustard seed, surfactants in soap. Mayonnaise is an oil-in-water emulsion that is about three-quarters oil. A grease stain looks see-through because the oil fills the air gaps between the paper fibres, so light passes through instead of being scattered. And an oil film slows evaporation because the water underneath can no longer reach the air.

In the game, each material has a weight for its size and a measure of how much it shuns water, and the emulsifiers have a strength. Nothing is scripted for mayonnaise: it comes out of those numbers.

Try next: predict, then test

Which helper holds best?

Shake three jars: one with mustard, one with washing-up liquid, one with nothing. Which stays cloudy longest?

Where does food colouring go?

Drip food colouring onto the oil layer. Does it stay in the oil or sink through to the water?

Which paper shows grease best?

Kitchen paper, printer paper or a brown paper bag: which one goes most see-through?

Warm or cold water?

Does the shaken jar separate faster with warm tap water or with cold water from the fridge?

For teachers

Oil and water fits KS2 Year 5 "properties and changes of materials" (mixtures, and which ones separate) and NGSS 2-PS1-1 (describing materials by their properties). The melting butter and wax that set into a skin on water also work for states of matter.

States of matter lesson

Questions about oil and water

Why don't oil and water mix?

Water molecules hold on to each other much more strongly than they hold on to oil, so they push the oil out. Shaking only breaks the oil into droplets for a minute or two.

Why does oil float on water?

Oil is less dense: a litre of cooking oil weighs about 920 grams and a litre of water about 1,000 grams, so the oil rises to the top.

What makes oil and water mix?

An emulsifier such as egg yolk, mustard or soap. It coats the tiny oil drops so they stay spread through the water instead of joining up again.

Is mayonnaise oil and water?

Yes. It is drops of oil held in the water from the yolk and the vinegar or lemon juice, kept apart by lecithin from the egg yolk.

Why does oil make paper see-through?

The oil fills the tiny air gaps between the paper fibres, so light goes straight through instead of bouncing around. Water does the same, but it dries; oil stays.

More with oil, soap and colour

Painted illustration of the painter lifting a sheet of paper with a red, blue and yellow marbled pattern off a tray of floating inks.

Oil marbling

Float inks on water, swirl them and lift the pattern off with paper.

Painted illustration of the painter touching a dish of milk with a soapy cotton bud while red, blue, yellow and green food colouring swirls outwards.

Magic milk

Soap breaks the surface of milk, and the colours rush away.

Pour, stir and watch it hold

Float the oil, beat it into a yolk and see a grease stain soak in, all without a single oily tea towel.

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