Why solids soften
If you had lived in a country village over a hundred years ago the noisiest place would probably have been the blacksmith's workshop. Inside the workshop was a forge. This was a deep tray of hot coals similar to a barbecue of charcoal today. The blacksmith's job was to make horseshoes and repair farm implements such as ploughs. The metal he used was steel.

Steel is not a pure metal. It is a mixture of iron and carbon. Solid mixtures differ from pure mixtures when they get hot. They soften first before they melt. Water is a pure substance. It does not soften before it melts. You never find a soft ice cube. When an ice cube starts to melt, you see a layer of liquid water on its surface. Underneath the water the ice is still hard.

A mixture softens when it gets hot owing to the substances it contains. The substances do not all have the same melting point. The melting points of some substances in the mixture are lower than the melting points of other substances. When a solid mixture is heated, the substances with the lower melting points melt, while the substances with the higher melting points remain as solids. These solids and liquids in the mixture make the mixture become softer. As the temperature of the mixture rises, more of the substances melt and the mixture becomes very soft. Eventually, if the temperature is raised high enough, all the substances melt and the mixture becomes a liquid.

Steel is a very hard material and it has a very high melting point of 1,536°C. Because it is a mixture it does not turn to liquid on its surface while the inside remains solid: the whole metal gradually softens as it gets hotter.

When a blacksmith wanted to make a horseshoe he took a bar of metal and placed it in some tongs with long handles. He held the metal bar in the tongs and put it in the forge. The heat from the forge passed into the metal. The blacksmith heated the metal until it was red-hot. When it was so hot it had also softened. The blacksmith then took the red-hot bar out of the forge and held it on his anvil. He took a heavy hammer and knocked the metal bar into a horseshoe shape on his anvil. If the metal cooled fast it became difficult to shape so the blacksmith returned it to the forge to soften it up again.

When the blacksmith had finished shaping the horseshoe he let it cool down. As the temperature of the metal fell the metal became hard again. Although we no longer use horses as our main means of transport the heating, softening and shaping of metal continues. These processes are used to make sheets of metal and bend them into shapes for car bodies.

Are car bodies made by using a hammer?
No. The large steel bars are heated until they soften then they are pushed between rollers. The rollers squeeze the soft metal and make it form a broad flat sheet. This is then bent into the shape needed for part of a car body.

How are metal pipes made?
They are made by putting a sheet through several sets of rollers. As the soft metal sheet passes through the rollers its sides are turned upwards and inwards until the edges meet and a pipe forms.

How is metal wire made?
A rod of metal is heated until it is soft then its end is pushed through a small hole. A machine then pulls on the end of the rod and the rest of the soft metal passes through the hole to make a thin wire.

What happens to the particles in a mixture when it gets warm?
The particles of the different substances are found in groups throughout the mixture. When the mixture gets warm, the particles of the substance with the lowest melting point use the energy in the heat to break their links. This allows the particles to slide over each other and the substance becomes a liquid. At this time, the particles of the substances with higher melting points use the energy to vibrate. These substances remain as solids. At this temperature the mixture has a small amount of liquid flowing round a large number of pieces of solid. The action of the liquid makes the mixture softer.

What happens to a mixture if it gets hot?
Substances with higher melting points turn to liquids. There are more particles sliding over each other than are held firmly together and the mixture become very soft. If the temperature of the mixture rises high enough all the substances may melt. When this happens, the mixture is made entirely of particles sliding round each other and has become a liquid.

On very hot days in summer the surface of a road can become sticky. Why is this?
A road surface is made from a thick tar-like substance called bitumen. It is mixed with sand and small stones to make a hardwearing surface. Bitumen is a mixture of substances and in very hot weather some of them turn into liquids. This makes the bitumen soften and become sticky.

If you take butter and margarine out of the fridge you find that margarine is easier to spread than butter. Why?
Both are made from mixtures. Some of the substances in margarine have lower melting points than those in butter. In the cool conditions of a fridge the temperature is lower than the melting points of most of the substances in the butter so the butter is almost completely solid. The melting points of some of the substances in the margarine are actually lower than the temperature in the fridge so they exist as liquids. The mixture of liquids and solids in the margarine make it softer than the butter.