Why Ice Floats and Iron Sinks (It's One Formula)

The Barbecue Experiment That Explains Buoyancy

I dropped a can of Coke and a can of Diet Coke into a cooler full of water at a barbecue. One floated. One sank. Same can size, same shape, same brand — different behavior.

Regular Coke has about 39 grams of sugar dissolved in it. Diet Coke replaces that with a fraction of a gram of aspartame. The cans weigh almost the same empty, but the sugar makes regular Coke denser than water. Diet Coke, without that sugar payload, is slightly less dense. Float vs. sink comes down to one division problem.

One Formula, One Division

A can of regular Coke: about 384 grams in a 355 mL can.

Water's density: 1.000 g/cm³. Coke is denser. It sinks.

Diet Coke: about 355 grams in the same 355 mL can.

Right at the boundary — and the air pocket at the top of the can tips it just under 1.0. It floats. Barely.

That's the entire density formula: mass divided by volume. The Greek letter $\rho$ (rho) is just a shorthand. Nothing scary about it.

So Why Does a Steel Ship Float?

Steel's density is about 7.8 g/cm³ — nearly eight times denser than water. A solid steel bolt sinks instantly. But a steel ship floats. Same material. What gives?

The ship isn't solid steel. It's a steel shell enclosing a massive volume of air. The average density of the entire ship (steel + air + cargo + everything inside) is what matters, not the density of the steel alone.

A cargo ship might weigh 50,000 tons but displace 60,000 tons of water. Its average density is less than water's. Archimedes figured this out around 250 BC — an object floats when it displaces a weight of fluid equal to its own weight. The ship's hull shape creates enough volume to keep the average density below 1.0 g/cm³.

Load too much cargo and the average density rises. The ship sits lower. Load too much more and it sinks. The Plimsoll line painted on every ship's hull marks exactly where that threshold is.

Why Ice Floats (And Why That's a Big Deal)

Water does something almost no other substance does: it gets less dense when it freezes. Liquid water at 4°C has a density of 1.000 g/cm³. Ice at 0°C: 0.917 g/cm³. That 8.3% difference is why ice floats.

If ice were denser than water — like most solids are denser than their liquid form — lakes would freeze from the bottom up. Fish would die. The insulating layer of surface ice that keeps aquatic ecosystems alive through winter wouldn't exist. Life on Earth as we know it depends on water being weird.

The molecular explanation: water molecules form a crystalline lattice when they freeze, with hydrogen bonds holding them in a structure that's actually more spread out than liquid water. More volume, same mass, lower density. Chemistry being poetic for once.

Density vs. Specific Gravity: What's the Difference?

Specific gravity is density without units — it's the ratio of a substance's density to water's density. Since water is 1.000 g/cm³, the numbers are the same in metric. Gold has a density of 19.3 g/cm³ and a specific gravity of 19.3.

In imperial units, density gets messy (pounds per cubic foot, slugs per cubic foot...). Specific gravity stays clean because it's a ratio. That's why engineers and brewers and gemologists prefer it — one number, no unit conversion headaches.

Homebrewers use specific gravity to track fermentation: wort starts around 1.050 (denser than water due to dissolved sugars) and drops to ~1.010 as yeast converts sugar to alcohol and CO₂. The density change tells you how much alcohol was produced.

Common Densities Worth Knowing

Everything above water in the table floats. Everything below sinks. One number, one comparison, one answer. The formula that governs geometric volume calculations feeds directly into density — you need accurate volume to get accurate density.

Frequently Asked Questions