Liquid Ice Explained: The Physics of Pressure Melting and Cutting Through Ice
libre
Liquid ice is water kept in a liquid state below its normal freezing point by pressure rather than heat, and the simplest way to demonstrate it is an experiment that looks like a magic trick: cutting through a piece of ice without splitting it into two parts.
What is liquid ice
Liquid ice is the thin film of liquid water that forms when solid ice is squeezed hard enough to melt at a temperature below 0 °C. Under ordinary conditions ice melts only when it is warmed, but when strong pressure is applied, the same ice can turn to water while still cold; the moment the pressure is removed, that water freezes solid again. This behaviour is the key to a demonstration in which an icicle is "cut" yet stays whole.
The icicle experiment: how to cut ice without splitting it
The icicle experiment shows liquid ice in action by passing a loaded wire straight through a piece of ice while leaving it intact. The setup is quick and needs only a stand, a slender wire, a weight, and an ice icicle.
Preparing for the experiment
Preparation for this experiment is short. Fix an ice icicle roughly 2.5 centimetres in diameter to a stand. Near the point where the icicle is supported, loop a thin wire over it. To the hanging end of the loop attach a 5-kilogram weight, leaving a gap of about 5 centimetres between the weight and the table.
Calculating the load on the icicle
The pressure on the icicle is what makes the trick work, so it helps to work out the numbers. The area of the icicle covered by the wire is about 1/10 of a square centimetre. That means the weight presses on each square centimetre with a force of roughly 50 kilograms. The load on the cross-section of the ice is therefore considerable, and it is concentrated along the thin line of the wire.
The course of the experiment and the result
The result is that the wire sinks all the way through while the icicle stays in one piece. As the load takes effect the weight slowly descends, the wire cuts deeper and deeper into the icicle, and water glistens along the cut. You expect the severed piece to drop away — but it does not. When the weight and wire finally clatter onto the table, the icicle remains whole. The ice melts under the pressure exerted by the load, and the instant that pressure is lifted, it freezes together again above the wire.
Why ice melts under pressure
Ice melts under pressure because squeezing it lowers the temperature at which it can stay solid, so cold ice turns briefly to water and then re-freezes when the load is gone. Until this was understood, ice was thought to melt only when heated.
Faraday's discovery
The English natural scientist Faraday established more than a hundred years ago that ice melts under pressure as well as under heat. By keeping the water below its freezing point he prevented it from expanding and so obtained liquid ice; as soon as the pressure ceased, the water turned back into ice. His work explained why the icicle experiment behaves as it does rather than being mere play.
Melting under pressure (regelation)
Regelation is the name for this two-step process in which ice melts under pressure and then re-freezes once the pressure is released. In the icicle experiment the wire melts a groove beneath itself, slips down through the water, and the water above it instantly re-freezes into solid ice, sealing the cut so the icicle never actually falls apart.
The physics: pressure and freezing point
Water is unusual because it expands as it freezes, and pressure works against that expansion, which is why pressure lowers its freezing point. When a heavy load is squeezed onto a tiny area of ice, the ice is pushed toward the liquid state even though its temperature has not risen. Remove the load and expansion is again allowed, so the water freezes once more. This is the direct cause of the liquid film that forms beneath the wire.
Practical uses of the liquid-ice principle
The liquid-ice principle explains several everyday phenomena, from skating to the slow crawl of glaciers, because in each case pressure produces a thin, temporary layer of water.
Why skates glide over ice
Skates glide because the narrow blade concentrates the skater's weight into a pressure of several hundred kilograms per square millimetre, and that pressure melts a very thin layer of water beneath the blade. This film acts like the lubricant in a machine. As soon as the pressure passes, the water freezes again and the blade slides on. The same reasoning explains why you cannot skate across a mirror-smooth parquet floor or a floor of stone or glass — there is no ice to melt into a lubricating film.
The movement of glaciers
A glacier slides down into the valley as though on a gigantic skate, driven by the same liquid-ice principle. The enormous weight of the ice mass generates pressure at its base, a thin layer of meltwater forms, and the glacier creeps forward on it.
How snowballs form
Snowballs hold together because the pressure of your hands melts the snow slightly, and the meltwater re-freezes to bind the grains. A snowball is easiest to make when the air temperature is no lower than zero. When the snow is very cold, it is hard to pack, because the pressure of your hands alone is not enough to press it firmly together.
How air temperature affects the pressure needed
The colder the ice or snow, the greater the pressure required to melt it, and this is why snow behaves so differently on mild days versus deep-freeze days. Near 0 °C only light pressure is needed to form the liquid film, so snowballs pack easily and skate blades glide smoothly. At much lower temperatures the freezing point is harder to reach by squeezing, so more force is needed and the effect grows weaker.
Other examples of regelation in nature and technology
Regelation appears wherever heavy or sharp loads meet cold ice, not only in the icicle demonstration. Everyday and natural cases include:
- The base of a moving glacier, where pressure melts a lubricating film and the ice flows downhill.
- The blade of an ice skate or the runner of a sled, gliding on a momentary layer of meltwater.
- Snow being compacted into snowballs or hard-packed trails under foot and hand pressure.
- Two ice cubes pressed together, which fuse into one block as the crushed surfaces melt and re-freeze.
- A weighted wire drawn through a block of ice — the classic laboratory version of the icicle test.
How to repeat the experiment at home
You can reproduce the liquid-ice experiment at home with a few common items and a little patience. A practical procedure is:
- Freeze a solid piece of ice — an icicle about 2.5 cm across, or a firm ice block — and clamp or rest it securely so both ends are supported.
- Loop a length of thin, strong wire (steel or copper) over the ice.
- Hang a heavy weight of a few kilograms from the wire's free end, leaving space below for it to descend.
- Watch the wire slowly sink through the ice as the load melts a groove beneath it.
- Observe that the water re-freezes above the wire, so the ice remains a single piece even after the wire passes right through.
The demonstration works best when the ice is only slightly below 0 °C, because warmer ice needs less pressure to melt along the wire.
Safety precautions during the experiment
Safety matters because the experiment combines a suspended heavy weight with slippery ice. Keep your hands and feet clear of the space beneath the hanging weight so it cannot land on them if the wire slips. Secure the stand so it cannot topple, wear gloves when handling very cold ice to avoid frost burns, and wipe up any meltwater to prevent slips. If children take part, an adult should manage the weight and the wire.
Common questions about liquid ice
Liquid ice differs from ordinary melting in one crucial way: it is caused by pressure, not warmth, and it reverses the instant the pressure is removed. The icicle stays whole because the water produced under the wire re-freezes above it, so no piece is ever truly separated. The colder the ice, the more pressure is needed to achieve the effect, which is why the same trick is harder to perform with deeply frozen ice than with ice near its melting point. A related read: Dry ice.
