Density Column
Summary
Six household liquids that don’t mix will stack into neat layers, densest at the bottom, and small objects hover at whichever layer matches their own density. You’ll be able to explain what density is, why polar and nonpolar liquids refuse to mix, and how an object’s density sets where it floats.
History
Density — mass per unit volume — is one of the most fundamental physical properties of matter. Archimedes famously described buoyancy in the 3rd century BCE: an object immersed in fluid experiences an upward force equal to the weight of fluid it displaces. If the object is denser than the fluid, it sinks; less dense, it floats.
Sailors and winemakers in the ancient world used floating instruments (hydrometers) to measure fluid density long before the physics was formally understood. The density column makes this invisible property immediately visible by revealing that ordinary household liquids exist at a wide range of densities — some nearly twice as dense as water — and that they separate cleanly because they do not dissolve in each other.
Hazards & preparation
PPE: none needed — everything here is a household liquid.
- Rubbing alcohol is flammable — keep it away from flames.
- It’s messy, not dangerous: work over a wipeable surface, and don’t eat the objects after they’ve been in soap and alcohol.
Disposal: pour it down the drain with plenty of warm water (the soap helps carry the oil and syrup away). See the Safety page.
Materials
- Honey — 50 mL
- Corn syrup (light or dark) — 50 mL
- Dish soap — 50 mL
- Water (with food coloring) — 50 mL
- Vegetable oil — 50 mL
- Rubbing alcohol (70% isopropanol, tinted with food coloring if desired) — 50 mL
- Tall glass, graduated cylinder, or clear vase — at least 400 mL capacity
- Pouring spoon or pipette
- Small objects to float: grape, cherry tomato, small plastic bead, cork, raisin, rubber eraser piece
Procedure
- Pour honey into the bottom of the container — slowly, to avoid splashing.
- Tilt the container and pour corn syrup slowly down the inside wall.
- Add dish soap the same way.
- Gently spoon colored water onto the surface so it doesn’t disturb the soap layer.
- Slowly pour vegetable oil down the side — wait ~30 seconds between layers.
- Finally pour the tinted alcohol very slowly over the back of a spoon held just above the surface.
- Wait 2–3 minutes for the layers to settle and sharpen.
- Gently drop in the small objects one at a time and see where each comes to rest.
What you should see
Poured carefully, the liquids stack into six distinct bands with crisp boundaries, honey at the bottom and alcohol on top. Dropped objects don’t all sink or all float — each stops at the layer that matches its density, hovering at an interface as if suspended.
| Symptom | Likely cause | Fix |
|---|---|---|
| Layers merge / turbulent | Poured too fast | Pour slowly down the wall or over a spoon; wait between layers |
| Two layers won’t separate | They’re both water-based (e.g. syrup + water) | Give them time, or accept they may slowly blend — see Question 2 |
| Object sinks to the bottom | Denser than every layer | Try a lighter object, or add a denser bottom layer |
The Science
Density (ρ = m/V, in g/mL or kg/m³) determines how liquids stack. These liquids are immiscible — they do not dissolve in each other — so instead of mixing they form distinct horizontal layers, with the densest at the bottom.
Approximate densities:
| Liquid | Density (g/mL) |
|---|---|
| Honey | 1.36–1.45 |
| Corn syrup | 1.33–1.38 |
| Dish soap | 1.03–1.06 |
| Water | 1.00 |
| Vegetable oil | 0.91–0.93 |
| Rubbing alcohol (70%) | 0.87–0.89 |
Objects settle at the interface whose density matches their own. A grape (~1.10 g/mL) sinks below water but floats on dish soap. A cork (~0.12 g/mL) sits on top of the alcohol.
Immiscibility arises from polarity: water and alcohols are polar; oils are nonpolar. The saying “like dissolves like” applies to miscibility as well — polar liquids mix with each other, and so do nonpolar ones, but the two groups resist mixing. Honey and corn syrup are aqueous (water-based) and polar, so in theory they could mix with water given enough time and agitation — the column is a kinetic stability, not an absolute thermodynamic one.
Questions to Explore
Why don’t polar liquids mix with oil? Water and oil refuse to mix. What does “polar” mean at the molecular level, and why does polarity decide whether two liquids are miscible?
Hint / answer
A polar molecule has a lopsided charge — a slightly positive and slightly negative end — so polar molecules cling to each other. Oil molecules are charge-balanced (nonpolar) and can’t join that network, so water squeezes them out: “like dissolves like.”
How long will the column last? The layers aren’t at true equilibrium. Which pairs are most likely to eventually merge, and which are permanently separated?
Hint / answer
The water-based, polar layers (water, syrup, honey, soap) could slowly blend into one another because they’re mutually soluble — they stay separate only by density and lack of stirring. The oil layer, being nonpolar, is permanently separated from all of them.
What determines where an object floats? A grape floats at the soap–water interface. If you cut it in half, would each half float at the same level, higher, or lower?
Hint / answer
The same level. Floating depends on density (mass per volume), not total size — cutting the grape halves both its mass and its volume, so its density is unchanged and each piece settles at the same interface.
Why does honey have such a high density? Honey is ~80% dissolved sugar. What does dissolving that much sugar do to the density?
Hint / answer
Sugar molecules tuck into the gaps between water molecules, packing a lot of extra mass into nearly the same volume. More mass in the same space means higher density — which is why concentrated sugar syrups sink below plain water.
Could you separate mixed liquids? If someone stirred water and alcohol together, the column can’t un-mix them. What technique would, and what property does it exploit?
Hint / answer
Distillation — the two have different boiling points, so gently heating the mixture boils off the alcohol first and you can collect it separately. It exploits a difference in volatility rather than density.
Going further
- Predict, then test. Estimate the density of each floating object and predict its resting layer before you drop it in.
- Add a layer. Try saturated salt water (denser than plain water) or glycerin to squeeze in an extra band.
- Another invisible property made visible: distinguish a true solution from a colloid with the Tyndall Effect.