Why does an egg float in salt water?

Part 1 of 4 · The kitchen test

Salt, one spoon at a time

A glass with 250 mL of water and one egg. The arrows show two pushes: the egg's weight pulling down, and the water pushing up.

Try: add salt a teaspoon at a time until the egg lifts off the bottom.

Part 2 of 4 · Where the push comes from

Shove some liquid aside

Push a wooden block down into a full tank. The liquid it shoves aside spills into a cup on a scale. Watch two numbers: the spilled liquid's weight, and how hard the liquid pushes back up.

Try: drag the block down, then let go. Then switch liquids.

Part 3 of 4 · Build an instrument

A straw that reads density

If a floater sits lower in a less dense liquid and higher in a denser one, then how deep it sinks measures the liquid. That's a hydrometer: a straw with a lump of modelling clay at the bottom.

1. Pick how much clay. It must float upright in every liquid.
2. Calibrate: dip it in two liquids you know and mark the lines.
3. Read three mystery liquids.
4. Use it for real: finish a batch of maple syrup.

Part 4 of 4 · Ship it

Load a laker without running aground

A steel freighter weighing 9,000 tonnes empty takes on iron ore at Sept-Îles, Quebec, on the salty Gulf of St. Lawrence. It's bound for the steel mills in Hamilton, up the St. Lawrence Seaway, where the river and lakes are fresh water. The Seaway's channels and locks let a ship sit at most 8.08 m deep.

Try: load as much ore as you safely can, then sail.

Putting it back together

Floating is a comparison

• The push upA liquid pushes up on anything in it with the weight of the liquid it shoves aside. That push is buoyancy.
• Denser liquid, bigger pushThe same egg shoves aside the same 55 cm³, but 55 cm³ of salt water weighs more than 55 cm³ of fresh water.
• Float or sinkLess dense than the liquid: it floats, and sinks only until the push matches its weight. Denser than the liquid: it sinks. Weight alone doesn't decide; a 30,000-tonne steel ship floats.
• Not depthA bathtub or a lake, the block floats exactly as deep. Only the liquid's density matters.

Real ships carry these ideas painted on their sides: the load-line marks, with a fresh-water line above the salt-water one. Next: why honey pours so much slower than water, and why "thick" is not the same as "dense".