Experiment 5: The Density Column

Pour honey, soap, water, oil and alcohol into one tall glass and — instead of mixing into a mess — they stack into five neat stripes, and a grape dropped in sinks through some layers and stops dead at a boundary. The property running the show is density: mass per unit volume. This experiment teaches you to predict, for any liquid or object, exactly where it will come to rest.

The chemistry behind it

Density is defined as:

density = mass / volume        (units: g/mL or g/cm3)

Liquids sort themselves because a denser liquid sinks through a lighter one until buoyancy balances weight — the stack rebuilds itself in density order, densest at the bottom, even if you pour them in the "wrong" order (stirring only delays the sorting; most of these liquids also resist mixing, which keeps the layers tidy). An object dropped in sinks through every liquid less dense than itself and floats on the first liquid denser than itself — so its resting place is a density measurement you can see:

Variables. In the column itself the "experiment" is a prediction test: the independent variable is the object dropped in; the dependent variable is the layer where it rests. Controlled variables: same liquids, same layer thicknesses, objects lowered gently (not thrown) so momentum does not punch them through a boundary.

Safety first

• Rubbing alcohol is flammable — keep the column well away from stoves, candles and any flame.
• Pour slowly and wipe spills at once: oil and honey make floors and benches slippery.
• Wash hands after handling the liquids, and never taste anything from a lab container.

Doing the investigation

Hands-on. Pour about 3 cm of honey into a tall clear glass and let it settle. Add each liquid *very slowly* down the inside wall or over the back of a spoon — dish soap, tinted water, vegetable oil, tinted rubbing alcohol — waiting between layers. Photograph the stack, then lower in each test object gently and record where it rests.

In the virtual lab below (The Density Column) tick the liquids to build the column, pick an object from the list — grape (1.07 g/mL), cherry tomato (1.03), ice cube (0.92), cork (0.24), penny (8.90), marble (2.50) — and read off the object density and where it rests. Follow its guided procedure: find the two layers that bracket the grape, then untick honey and corn syrup and predict where the cherry tomato settles *before* you look. The bar chart marks your object as a dashed line against every liquid's density. Log the final layer order in your observation log.

Worked investigation: predicting the grape

Before dropping anything, measure the grape properly:

Mass of grape (kitchen scale)      = 8.6 g
Volume (displacement in a cylinder) = 8.0 mL
Density = 8.6 / 8.0 = 1.075 -> about 1.08 g/mL

Prediction: the grape (1.08) sinks through alcohol (0.79), oil (0.91), water (1.00) and dish soap (1.06), but floats on corn syrup (1.33) — it should rest at the boundary between the dish soap and the corn syrup. Observed resting places for the full set:

Conclusion. Every object stopped exactly where its density said it should: an object comes to rest *between* the densest liquid it can sink through and the lightest liquid that can hold it up. The column is a working density meter — an unknown object's resting boundary brackets its density between two known values.

Uncertainty. The displacement volume is the weak measurement: 8.0 ± 0.5 mL gives the grape's density as 1.08 ± about 0.07 g/mL (±6%). That is still plenty to place it between soap and corn syrup — but it would *not* reliably separate the grape (1.07) from the tomato (1.03): honest error bars stop you over-claiming.

Evaluation: how good is this evidence?

• Layers contaminate each other. Each pour drags a little of the previous liquid along the wall, so boundary liquids slowly change density. Improvement: pour over a spoon back, wait for full settling, and treat the column as fresh each session.
• Objects carry liquid with them. A grape lowered through alcohol and oil arrives coated, adding drag and trapped bubbles (bubbles lower the *effective* density). Improvement: lower objects on a thread or spoon, slowly, and knock off bubbles.
• Kitchen scale precision. ±0.1 g on an 8.6 g grape is fine (±1%), but a cork's tiny mass (about 1 g) carries ±10%. Improvement: weigh 10 corks and divide.
• The ice cube is a moving target. It melts into the water layer, diluting it, and its own density is fixed but its shape changes. Improvement: observe ice last and quickly, or use a sealed ice substitute of known density.

Common mistakes

• Pouring fast "to save time" — the layers churn, mix at the boundaries and may take an hour to re-sort, if they ever look clean again.
• Dropping objects from a height: momentum carries a borderline object through the layer where it should have stopped, faking a wrong density.
• Assuming the order you *poured* is the order of the layers — the liquids sort by density, not by your sequence.
• Confusing mass with density: a huge cork still floats, a tiny penny still sinks. Size is irrelevant; mass *per unit volume* is everything.

Exam Experience

Question 1 — calculation [4 marks]. A small plastic figure has a mass of 5.35 g and displaces 5.0 mL of water. (a) Calculate its density. (b) In the full column (honey 1.42, corn syrup 1.33, soap 1.06, water 1.00, oil 0.91, alcohol 0.79 g/mL), state where it comes to rest and justify your answer.

Scoring guide:

• Density = 5.35 ÷ 5.0 = 1.07 g/mL [1]
• It sinks through alcohol, oil, water and dish soap, all less dense than 1.07 [1]
• It floats on corn syrup (1.33 > 1.07) [1]
• So it rests at the soap–corn syrup boundary [1]

Question 2 — design [4 marks]. Design an investigation to measure the density of an unknown clear liquid as precisely as a school lab allows, so it can be placed correctly in the column.

Scoring guide:

• Measures the mass of a known volume using a balance and a measuring cylinder (or volumetric flask) [1]
• Uses a large volume (e.g. 50–100 mL) to shrink the percentage error in both readings [1]
• Repeats at least three times and averages; checks the cylinder is read at eye level (meniscus) [1]
• Calculates density = mass ÷ volume with units, and quotes an uncertainty from the instrument precisions [1]

Multiple choice 1. The layers in the column are ordered by:

• A. The order they were poured
• B. Colour, lightest at the top
• C. Density — the densest liquid sinks to the bottom
• D. How well they mix with water

Answer: C. Given time to settle, liquids always sort by density; pouring order only affects how clean the boundaries look.

Multiple choice 2. An ice cube (0.92 g/mL) is lowered into a glass containing only vegetable oil (0.91 g/mL) floating on water (1.00 g/mL). The cube will:

• A. Float on top of the oil
• B. Sink through the oil and rest at the oil–water boundary
• C. Sink to the bottom of the glass
• D. Hover in the middle of the oil

Answer: B. 0.92 > 0.91, so it sinks (just barely) through the oil, but 0.92 < 1.00, so the water holds it up — it rests exactly on the boundary.

Going deeper: density at work

The column on your bench is a toy version of the hydrometer, a sealed float that sinks deeper in less-dense liquids and rides higher in dense ones. Mechanics float one in battery acid to judge a car battery's charge; brewers float one in wort to track fermentation (as sugar turns to alcohol, the density falls — the column's logic, read off a scale). Both are your grape's trick reversed: instead of an object finding its layer, the float finds its depth.

Two consequences of the numbers are worth testing in the virtual lab. First, ice in alcohol: ice (0.92 g/mL) floats on water (1.00) but the alcohol in the column is only 0.79 g/mL — so in pure rubbing alcohol an ice cube *sinks*. "Ice floats" is not a law; "things float on liquids denser than themselves" is. Second, icebergs: seawater is about 1.025 g/mL, so the submerged fraction of an iceberg is:

Fraction under water = 0.917 / 1.025 = 0.895 -> about 90%

Nine-tenths of the berg hides below the surface — the origin of the phrase "tip of the iceberg", and a number you can now derive rather than quote. Finally, revisit the melted ice cube in your hands-on column: the meltwater joins the water layer, slightly diluting its neighbours at the boundary — a reminder that in a real column the "known" densities themselves drift as the experiment runs, which is why the evaluation section insists on fresh columns and quick observations for anything that melts, evaporates or mixes.