What if ice were heavier than water?

Drop an ice cube in water. It floats! Big icebergs float in the sea too.

But most solid things sink in their own liquid. What if ice sank too?

Drop an ice cube into a glass of water and it bobs at the top. Icebergs float in the sea the same way. That seems normal, but it is actually unusual. For most substances, the solid is denser than the liquid, so it sinks.

Water is different: when it freezes, it becomes less dense. So what if ice behaved like most solids and was heavier than water? In this lab you can change the density of a block of ice and watch what happens.

Whether an object floats depends on its average density compared with the liquid's. For most substances, the solid phase is denser than the liquid, so solids sink in their own melt. Water is an exception: the U.S. Geological Survey notes that on freezing its density drops by about 9 percent.

This lab asks what would change if ice followed the usual rule. It starts with a block of ice at 917 kg/m³ in fresh water (1,000 kg/m³), then raises the ice's density to an imaginary 1,100 kg/m³.

In Olly's lab, a big block of ice goes in the water.

It floats! But most of it is under the water. Only a little bit pokes out.

The model's ice block is 1 meter long, half a meter wide and 40 centimeters tall. In fresh water it floats, but low: about 92 percent of it is under the water, and only 3 centimeters stick out above.

That matches real ice. The U.S. Geological Survey says ice is about 90 percent as dense as water, so about 10 percent of an ice cube or an iceberg sits above the waterline. In salty sea water, which is denser, the model's block floats a little higher, with about 89 percent under.

The block (1 × 0.5 × 0.4 m) of ice at 917 kg/m³ has a mass of 183.4 kg. Archimedes' principle says it floats when it can displace its own mass of water; it settles at a draft d = m ÷ (ρ_water·L·W) ≈ 0.37 m. The model reports 91.7% submerged and a freeboard of 0.03 m.

The submerged fraction equals the density ratio, 917 ÷ 1,000. In sea water (1,025 kg/m³) the ratio falls and the model shows about 89% under. USGS adds that real ice varies, because it can contain air bubbles.

Now Olly makes pretend ice that is heavier than water.

Plop! It sinks right to the bottom.

Now change the ice's density to 1,100, making our pretend ice 10 percent denser than fresh water. Let it go and the block sinks straight to the bottom of the tank.

The water still pushes up on it, but not hard enough. In the model the block's weight is about 2,158 newtons, while the most the water can push up is about 1,962 newtons. With no way to push aside its own weight of water, it sinks.

At 1,100 kg/m³ the block's mass is 220 kg and its weight 2,158 N. Fully submerged, it displaces 0.2 m³ of water, giving a buoyant force of only 1,962 N. The net downward force of about 196 N sends it to the floor.

In the model, ice sinks in fresh water as soon as its density passes 1,000 kg/m³; at exactly 1,000 it hangs fully submerged. The pretend ice would still float in a much denser liquid: in the model's Dead Sea setting (1,240 kg/m³) the 1,100 block floats about 89% under.

Water pushes up on things in it. Light things float. Heavy things sink.

When water freezes, its tiny parts spread out a little. That makes ice lighter for its size.

Anything in water pushes some water out of the way, and the water pushes back up. If the push can match the object's weight, it floats. That depends on density: how heavy something is for its size.

Ice is less dense than water because of how it is built. The U.S. Geological Survey explains that in ice the water molecules line up in a regular pattern, which holds them farther apart than in liquid water. Our model is simple: one block that stays level, never melts and is the same all the way through.

Archimedes' principle gives the buoyant force as ρ_liquid·g·V_displaced; an object floats if that can equal its weight before it is fully submerged, which happens when its average density is below the liquid's. USGS attributes ice's low density to its structure: molecules lock into a regular lattice that spaces them farther apart than in the liquid.

Model limits: a rectangular block that stays upright and level, a single density for ice (real ice can hold air), no melting, and no temperature at all. It cannot show lakes, layers or freezing, so anything about lakes is a sourced fact or an idea, not a model result.

Lakes freeze on top first. The ice makes a lid.

Fish and tiny living things stay in the water below. Life goes on all winter!

There really is ice that sinks. "Heavy ice" is made from heavy water, a special kind of water, and the U.S. Geological Survey says it is 10.6 percent denser than normal water, so it sinks in a glass.

In a lake, ordinary floating ice matters a lot. Water is densest at about 4 °C, so in winter the coldest water stays on top and freezes there. Under the ice, scientists at the University of Wisconsin say fish slow down but keep moving, and a big study found algae and tiny animals still plentiful. If ice sank, USGS says, lakes could freeze from the bottom up.

USGS notes that water at about 4 °C is denser than water at 0 °C, so the densest water sinks and the coldest stays at the surface to freeze. If water were densest at its freezing point, the cold surface water would sink and a lake could freeze from the bottom up; because water is such a good insulator, some frozen lakes might not fully thaw in summer.

University of Wisconsin limnologists describe ice-covered lakes as roughly 3 to 4 °C throughout, with fish slowing down but still moving to breathe. A study of more than 100 lakes by 62 scientists found life under ice vibrant, complex and surprisingly active.