Besides their chemical make-up (what kind of atoms and molecules they are made of), solid materials have different qualities or properties. These include, for example, whether they are hard or soft, heavy or light, or shiny or dull. Some materials are stronger than others. Some are denser (have more mass per cubic metre) than others. Some are more elastic (stretchy) than others. Some are more malleable (can be bent or moulded without breaking) than others. Other properties are: how well the material conducts heat and electricity, whether it dissolves in liquid or another liquid or not, and what its boiling and melting points are. 

Crystals

Many solid materials are not irregular lumps, but have fixed shapes. Their atoms or molecules are arranged in regular patterns, known as crystals. These have sharp edges and flat sides. Some solids, for example glass, have no regular pattern in their molecules.

Some solids can also be broken up by stress, while others will stretch. Using a force or adding a load to something leads to stress. In a metal, stress causes the layers of atoms to shift out of position, sliding across one another. This is what makes pure metals malleable (bendable). In a pure metal, the atoms can shift quickly—or slowly over a long period of time.

Elasticity

Most solids, even metals, are elastic. They can be stretched longer, then go back to their original length. Some materials, for example, rubber, and some synthetic (man-made) materials, such as nylon, are highly elastic.

Plasticity

There is a limit to how elastic a material can be. If the material is stretched beyond that limit, the material will not return to its original shape. This property of changing shape permanently is called plasticity. (Note: this is not the same as being made of plastic.) Materials such as dough (a mixture of flour and water for making bread) and clay are highly plastic. Other plastic materials, such as most metals, need a great deal of force to change their shapes, unless they are heated.

Brittleness

Some materials, such as glass, break almost immediately when they are struck. They have no elasticity. This property is called brittleness. Other materials become brittle after they have been heated. Clay, for example, becomes brittle after being baked in a kiln.

Conductivity

Material made of crystals, such as metals, are good conductors of heat and electricity. The electrons in their atoms can move easily from atom to atom. Non-crystal materials, like plastics, glass and wood, are poor conductors. They are, however, good insulators—materials that prevent the flow of electricity or heat. This is why plastic, for example, is used to cover wires in an electric cable or in handles for saucepans in the kitchen.

Melting

Solids made of crystals can be broken up by heating. This makes the atoms vibrate (shake) more until they fall apart. The solid either then burns or melts, becoming a liquid.