What if metal melted in your hand?

Spoons, keys and bikes are made of metal. They stay hard even on hot days.

But one metal melts in a warm hand! What if all metal did?

Metal seems about as solid as anything can be. Spoons, keys, bike frames and bridges keep their shape on the hottest summer day. Yet there's a silvery metal that turns into a liquid from the warmth of a hand.

So what if metal melted in your hand? This lab is a thought experiment. It isn't something to try: scientists handle unusual metals with special care. But we can follow the clues about why different metals melt at such different temperatures.

Chemist John Emsley, writing for the Royal Society of Chemistry, notes that a piece of gallium metal will melt in your hand; its melting point, 29.7646 °C, is one of the standard reference points on the International Temperature Scale.

This thought experiment asks what would change if everyday metals behaved like gallium. It uses the question to compare melting points across the metals, to explore gallium's unusual properties, and to look at how people have long used molten metal to shape objects.

Each metal melts at its own temperature. Iron needs a super hot furnace.

Gallium melts when it's only a little warm. Mercury is already runny!

A melting point is the temperature at which a solid turns into a liquid. For metals, these are wildly different. Mercury is already liquid at room temperature and melts at about −39 °C. Tin melts at about 232 °C, aluminum at about 660 °C and iron at 1,538 °C. Tungsten holds out until 3,414 °C.

Gallium is the surprise. It melts at about 29.8 °C. The average human body temperature is about 37 °C, so gallium melts in a person's hand.

Royal Society of Chemistry data give melting points of −38.829 °C for mercury, 29.7646 °C for gallium, 231.928 °C for tin, 660.323 °C for aluminum, 1,064.18 °C for gold, 1,084.62 °C for copper, 1,538 °C for iron and 3,414 °C for tungsten.

Gallium's melting point lies below the generally accepted average human body temperature of 37 °C (98.6 °F), with normal values ranging from about 36.1 to 37.2 °C (MedlinePlus). Gallium is solid at 20 °C but liquefies just above room temperature.

Pretend all metal melts on a warm day. Spoons droop in hot soup!

Bikes, cars and bridges couldn't be made of metal. That would be a very different world.

If all metals melted at about 30 °C, a spoon might melt in hot soup, and a car parked in the sun might sag. Bridges, rails and buildings couldn't rely on steel, and wires might drip on a warm day.

Some things might get easier, though: shaping metal would need no furnace at all. These are ideas, worked out from how metals behave today. In the real world, gallium is the rare exception, and it is used where its odd properties help.

Structural metals are useful partly because their melting points sit far above ambient temperatures: iron at 1,538 °C, aluminum at 660 °C. Metals melting near 30 °C could not bear loads on warm days, so construction, transport and electrical wiring would need other materials.

Casting would become trivial, but solid metal objects would be fleeting in warm climates. These consequences are reasoned speculation; real metals with low melting points, such as gallium and mercury, are exceptions used for specific jobs.

Gallium is a soft, silvery metal. You could cut it with a knife.

It gets bigger when it freezes, just like water does!

Gallium is a soft, silvery-white metal, soft enough to cut with a knife. It does something most metals don't: it gets bigger when it freezes, just like water does when it turns to ice. That means solid gallium floats on liquid gallium.

It can also stay liquid below its melting point, down to as low as 0 °C, if nothing triggers it to freeze. And once it is liquid, it stays liquid over a range of about 2,000 °C, the longest useful liquid range of any element.

Gallium is silvery white, soft enough to cut with a knife, expands on solidifying and supercools readily, remaining liquid as low as 0 °C (Britannica). Solid gallium is less dense (5.9 g/cm³) than the liquid (6.1 g/cm³), so it floats on its melt.

It remains liquid over about 2,000 °C with very low vapor pressure up to about 1,500 °C, the longest useful liquid range of any element, and the liquid wets glass. Mendeleyev had predicted its properties as "eka-aluminum" before Paul-Émile Lecoq de Boisbaudran discovered it in 1875.

People melt metal in very hot furnaces. Then they pour it into molds.

Long ago, iron was poured into molds to make pots and pans!

Because most metals melt only at very high temperatures, people use furnaces to melt them and pour the liquid metal into molds. At the Saugus Iron Works in Massachusetts, now a national historic site, molten iron was poured into molds of clay and sand to make cooking pots, kettles and skillets.

Low melting points are useful too. Tin melts at about 232 °C, which is why it's used in soft solder for joining metals. And most window glass is made flat by floating it on melted tin.

At Saugus Iron Works, now a U.S. National Park Service site, gray iron was poured into dried molds of a clay-sand mix called loam to make cast-iron pots, kettles and skillets, and mottled iron was cast in sand as bars called pig iron. Cast iron, 3 to 5 percent carbon, is still used for pipes, valves and pumps.

Gallium's own uses depend on its compounds: gallium arsenide in red LEDs and the solar panels of the Mars Exploration Rovers, and gallium nitride in blue and green LEDs, which made white LED lighting possible (Royal Society of Chemistry).