What If? Lab
What if oil and water mixed?
Shake oil and water together. Wait a bit. The oil floats back to the top! What if oil and water mixed? Let's find out with Olly.
Oil and water won't stay mixed because water molecules cling to each other much more strongly than they cling to oil. But helpers like egg yolk and soap can hold tiny oil drops in water. Imagine oil and water that mixed easily and follow what might change.
Oil and water are immiscible: weak attractions between polar water and nonpolar oil cannot overcome water's hydrogen bonding. Emulsifiers with water-loving and oil-loving ends stabilize droplets. This thought experiment makes them miscible and explores dressings, emulsions and soap.
The big question
Pour some oil into a glass of water. The oil floats on top.
Stir it hard. Soon the oil floats back up! What if oil and water mixed?
Shake a bottle of oil-and-vinegar salad dressing and it turns cloudy. Leave it on the table and, within minutes, the oil drifts back up and makes its own layer on top.
Oil and water just don't want to stay together. So what if they mixed as easily as water and juice? This lab is a thought experiment, and the clues take us into molecules, mayonnaise and the soap by your sink.
OpenStax Chemistry calls two liquids that do not mix to an appreciable extent immiscible; poured together, they form separate layers. Oil is one of many nonpolar liquids immiscible with water, alongside gasoline, benzene and some paints.
This thought experiment asks what would change if oil and water were miscible. It uses the question to explore polarity and intermolecular forces, how emulsions are made and stabilized, and how soap bridges the two kinds of matter.
Explore the idea
Make a prediction
You shake oil and water in a jar and wait. What happens?
You shake oil and water together in a jar, then wait a few minutes. What happens?
A shaken mixture of oil and water is left to stand. What happens?
- They separate into two layers
- They stay mixed forever
- They turn into soap
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Established fact
Oil and water don't mix, and oil floats.
Oil and water are immiscible: they form two layers with oil on top.
Weak polar-nonpolar attractions cannot overcome water's hydrogen bonding, so oil and water separate (OpenStax).
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Established fact
Egg yolk and soap help oil stay in water.
Emulsifiers such as egg yolk, milk's casein and soap hold tiny oil droplets in water.
Emulsifying agents inhibit droplet coalescence; amphiphilic soaps bridge oil and water (OpenStax).
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An idea scientists discuss
Oil and water mixing easily is pretend.
What a world where oil and water mixed would be like is an idea, worked out from how they behave today.
Consequences of miscible oil and water are reasoned, not observed.
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Established fact
Oil floats on water in its own layer.
Oil and water form two separate layers, with the less dense oil floating on top.
Water and oil are immiscible and form two layers with the less dense oil on top (OpenStax Chemistry 2e).
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An idea scientists discuss
Mixed oil and water would never make layers.
If oil and water mixed, they would make one liquid with no layers, like water and juice.
Miscible oil and water would form a single homogeneous phase; an inference from the premise.
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Established fact
Shaken oil drops soon join up again.
Shaking makes tiny oil droplets, but in many mixtures they soon join into big drops and separate.
In many emulsions the dispersed phase tends to coalesce, form large drops and separate (OpenStax).
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Established fact
Spilled oil floats on the sea.
Crude oil doesn't mix with water and is less dense, so spilled oil rises to the surface, where floating barriers and skimmer boats can collect it.
Crude oil is immiscible with and less dense than water, so spilled oil rises; booms and skimmers remove it from the surface (OpenStax).
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An idea scientists discuss
Mixed oil might be harder to clean up.
If oil and water mixed, spilled oil might spread through the water and be much harder to collect.
Dissolved oil would evade surface recovery methods; the environmental consequences are speculation.
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Established fact
Milk is tiny fat drops in water.
Milk is an emulsion of butterfat in water, with a protein called casein keeping the fat drops spread out.
Milk is an emulsion of butterfat in water with the protein casein as emulsifying agent (OpenStax).
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Established fact
Egg yolk helps mayonnaise stay mixed.
Mayonnaise is oil in vinegar, kept mixed by parts of the egg yolk.
Mayonnaise is an emulsion of oil in vinegar with egg yolk components as emulsifying agents (OpenStax).
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An idea scientists discuss
Mayonnaise might be made without egg.
If oil and water mixed, sauces like mayonnaise might not need egg yolk to hold together, though they might taste and feel different.
Emulsifiers would lose their function; the texture of foods built on emulsions would change unpredictably. Speculation.
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Established fact
Soap has an oil-loving end and a water-loving end.
Soap molecules have a tail that is attracted to oil and grease and a head that is attracted to water.
Soap and detergent molecules have a nonpolar hydrocarbon end attracted to oil and an ionic end attracted to water; they are amphiphilic (OpenStax).
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Established fact
Soap helps rinse oily dirt away.
Soap molecules line up between grease and water like a bridge, so oily dirt can be washed away.
Oriented at the interface, soap molecules bridge nonpolar and polar matter, so dirt particles are suspended as colloids and readily washed away (OpenStax).
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An idea scientists discuss
Maybe we'd need less soap.
If oil and water mixed, we might need less soap to wash greasy hands and dishes.
Detergency would matter less for oily soils, though not for all dirt; this is reasoning, not data.
Why oil floats on water
Oil is lighter than water for its size. So oil floats on top.
The oil and water stay in two layers, like a sandwich.
When oil and water sit in a jar, they form two separate layers, and the oil is always on top. That's because oil is less dense than water: a cupful of oil weighs less than a cupful of water.
Some liquids, like water and the antifreeze used in cars, mix in any amount. Others, like oil and water, separate. Scientists call liquids that mix miscible and liquids that separate immiscible.
Miscible liquids mix in any proportion: ethanol, sulfuric acid and ethylene glycol are completely miscible with water. Immiscible liquids form separate layers; water and oil separate with the less dense oil floating on top.
The distinction is one of extent. Miscible liquids have infinite mutual solubility, while immiscible ones have very low, though not zero, mutual solubility. Partially miscible pairs, such as bromine and water, also form two layers, each saturated with a little of the other (OpenStax).
Let them mix
Pretend oil and water mix like juice and water. Salad dressing never separates!
But what would happen to greasy pans? Or to oil spilled in the sea?
If oil and water mixed, salad dressing would never need shaking, and greasy pans might rinse clean with plain water. But spilled oil might spread through rivers and seas instead of floating on top, where it can be scooped up.
Cooks might need new tricks too, because a lot of recipes, from mayonnaise to milk-based sauces, depend on oil and water staying separate until something holds them together. These are ideas, worked out from how oil and water behave today.
Miscible oil and water would make dressings homogeneous and grease removal trivial, and would remove the need for emulsifiers. But floating, recoverable oil on water would vanish: OpenStax notes that spilled crude oil rises to the surface because it is immiscible and less dense, which is what booms and skimmers exploit.
These consequences are reasoned speculation. In reality, the immiscibility follows from molecular polarity, so changing it would mean changing water or oil themselves.
Why they won't mix
Water bits love to hold on to each other. They hold on tight!
Oil bits can't break in. So the oil gets pushed out.
Water molecules are polar, and they cling tightly to one another. Oil molecules are nonpolar.
When oil and water meet, the oil molecules don't attract water strongly enough to push between water molecules that are clinging to each other. So the water stays together and the oil is squeezed out into its own layer. Chemists sum it up as "like dissolves like": polar mixes with polar, and nonpolar with nonpolar.
Liquids with similar polarities tend to be miscible, because solute-solvent attractions are at least as strong as those within each pure liquid. This is the axiom "like dissolves like."
For nonpolar liquids in water, the relatively weak attractions between polar water molecules and nonpolar molecules are not adequate to overcome the much stronger hydrogen bonding among water molecules. Mixing would disrupt that network without compensating attractions, so the liquids separate (OpenStax Chemistry 2e).
Helpers that hold them together
Shaking breaks oil into tiny drops. Egg yolk helps keep them mixed in mayonnaise.
Soap grabs oil with one end and water with the other!
Shake oil and water hard and the oil breaks into tiny droplets spread through the water. That's an emulsion. On its own, the droplets soon join up and separate. But an emulsifier keeps them apart. In mayonnaise, parts of egg yolk do the job; in milk, it's a protein called casein holding butterfat in water.
Soap is an emulsifier too. Each soap molecule has one end that sticks to oil and another that sticks to water, so it can carry grease away in the rinse water.
Shaking or blending immiscible liquids makes an emulsion of colloid-sized droplets, but the dispersed phase tends to coalesce and separate unless an emulsifying agent inhibits coalescence. Milk is butterfat in water stabilized by casein; mayonnaise is oil in vinegar stabilized by egg yolk components; a little soap stabilizes kerosene in water.
Soaps and detergents are amphiphilic: a nonpolar hydrocarbon tail attracted to oil and grease and an ionic head attracted to water. They orient at the interface, bridging polar and nonpolar matter, so oily dirt is suspended and washed away.
Amazing facts
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Milk is tiny drops of fat in water.
Milk is an emulsion: tiny drops of butterfat spread through water, kept apart by a protein called casein.
Milk is an emulsion of butterfat in water stabilized by casein (OpenStax).
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Waves can mix spilled oil with sea water.
Ocean waves can whip spilled oil and seawater into an emulsion, which is one reason oil spills are hard to clean up.
Wave action can emulsify spilled oil, which partly explains why spills are difficult to clean up (OpenStax).
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Some liquids mix with water perfectly.
Car antifreeze and water mix in any amount, unlike oil and water.
Ethanol, sulfuric acid and ethylene glycol are completely miscible with water (OpenStax).
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Soap works less well in hard water.
In hard water, soap forms solid bits that don't dissolve, but detergents don't have that problem.
Soaps form insoluble calcium and magnesium compounds in hard water, while detergents form water-soluble products (OpenStax).
Words to know
- immiscible
- describing liquids that won't mix and form separate layers, like oil and water
- miscible
- describing liquids that mix together in any amount
- polar
- having a slightly positive end and a slightly negative end, like a water molecule
- emulsion
- a mixture with tiny droplets of one liquid spread through another
- emulsifier
- a substance, like egg yolk or soap, that keeps the droplets in an emulsion from joining up
- density
- how much mass is packed into a certain amount of space
Discovery challenge
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Which one floats on top?
When oil and water separate, which one ends up on top?
In an oil-water mixture at rest, which phase lies on top?
- Oil
- Water
- They take turns
Show the answer
Oil
Oil floats on top.
Oil, because it's less dense than water.
Oil, the less dense phase.
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What helps mayonnaise stay mixed?
What keeps the oil drops in mayonnaise from separating?
What emulsifies mayonnaise?
- Parts of egg yolk
- Salt
- Ice
Show the answer
Parts of egg yolk
Egg yolk.
Parts of the egg yolk act as emulsifiers.
Egg yolk components acting as emulsifying agents.
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Why does soap help wash away grease?
Why does soap help water wash away grease?
Why are soaps effective against oily dirt?
- One end grabs oil and the other grabs water
- It makes the water hot
- It makes grease heavier
Show the answer
One end grabs oil and the other grabs water
It holds on to oil and water at once.
Its tail sticks to grease and its head sticks to water, making a bridge.
They are amphiphilic and orient at the oil-water interface.
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Milk is tiny drops of fat in water. What is that called?
Milk is tiny drops of butterfat spread through water. What is this kind of mixture called?
What type of colloid is milk?
- An emulsion
- A crystal
- A gas
Show the answer
An emulsion
An emulsion.
An emulsion, held together by the protein casein.
An emulsion of butterfat in water stabilized by casein.
Experiments to try
- Watch What If? Lab What if gold were as common as sand? Gold makes up only a few parts in every billion of Earth's crust, while silicon, the main element in sand, makes up more than a quarter. Gold never rusts, so tiny amounts of it help our phones work. Imagine gold as common as sand and follow what might change.
- Watch What If? Lab What if rubber were not stretchy? Rubber stretches because its molecules are long, tangled chains. Pull, and they straighten out. Let go, and they curl back into a tangle. Imagine rubber that would not stretch and follow what might change for tires, balloons and rubber bands.
- Watch What If? Lab What if soap did not exist? Water on its own slides right off greasy hands. Soap is different: each soap molecule has one end that holds on to grease and another end that holds on to water, so the grease, dirt and germs rinse away. Imagine a world without soap and follow what might change.
Explore next
- Watch What If? Lab What if gold were as common as sand? Gold makes up only a few parts in every billion of Earth's crust, while silicon, the main element in sand, makes up more than a quarter. Gold never rusts, so tiny amounts of it help our phones work. Imagine gold as common as sand and follow what might change.
- Watch What If? Lab What if rubber were not stretchy? Rubber stretches because its molecules are long, tangled chains. Pull, and they straighten out. Let go, and they curl back into a tangle. Imagine rubber that would not stretch and follow what might change for tires, balloons and rubber bands.
- Watch What If? Lab What if soap did not exist? Water on its own slides right off greasy hands. Soap is different: each soap molecule has one end that holds on to grease and another end that holds on to water, so the grease, dirt and germs rinse away. Imagine a world without soap and follow what might change.
- Watch Try it What If? Lab What if a balloon floated very high? As a balloon floats higher, the air around it gets thinner and pushes on it less, so the gas inside spreads out and the balloon swells. In our model, a perfectly stretchy balloon launched at sea level is about 14 times its starting size by the time it reaches 20 kilometers.
- Watch What If? Lab What if apples never turned brown? When an apple is cut, a natural chemical in it mixes with oxygen from the air, and the fruit turns brown. Lemon juice and the fridge slow it down, and some apples are now grown to brown less. Imagine apples that never browned and follow what might change.
- Watch What If? Lab What if bubbles could last forever? A soap bubble's skin is a thin sandwich of soap and water, and it pops when the water drains or dries away. Scientists in France made a bubble that lasted 465 days. Imagine bubbles that lasted forever and follow the clues: thin films, drying out and frozen bubbles.
- Watch What If? Lab What if cakes had no baking powder? Look closely at a slice of cake and you'll see it is full of tiny holes. Baking powder makes bubbles of a gas called carbon dioxide, and those bubbles puff up the batter as it bakes. Imagine baking with no baking powder and follow what might happen.
- Watch What If? Lab What if colors never faded? Leave a bright poster in a sunny window and, over weeks, its colors grow pale. Light slowly changes the colored molecules in dyes and inks. Imagine a world where colors never faded, and follow the clues: sunlight, paints and blocking ultraviolet.
For parents & teachers
What children will learn
- Oil and water separate into layers, with the less dense oil on top.
- Water molecules attract each other strongly, so nonpolar oil is pushed out.
- Emulsifiers such as egg yolk and casein hold oil droplets in water.
- Soap molecules have an oil-loving end and a water-loving end, so they help wash grease away.
Recommended ages: 7 to 17 · About 15 minutes
Talk about it
- Which foods in your kitchen are mixtures of oil and water?
- Why do you think you shake salad dressing before pouring it?
- Would a world where oil and water mixed be better or worse? Why?
Try at home or in class
- Pour water into a clear jar with a tight lid, add a few spoonfuls of cooking oil and close it firmly. Shake hard, then time how long it takes for the oil to make a layer again. Does shaking longer make a difference?
- Rub a few drops of cooking oil on your hands and try rinsing with plain water. Then wash with soap as you normally would. What changed? This shows soap bridging oil and water.
Key vocabulary: immiscible, miscible, polar, emulsion, emulsifier, density
Where these facts come from
- Chemistry 2e, 11.3 Solubility , OpenStax
- Chemistry 2e, 11.5 Colloids , OpenStax
Checked, not yet human-reviewed. Last updated October 26, 2026. Spotted a mistake? Tell us.