The answer lies in a type of material known as a soft solid, which can behave either like a solid or like a liquid, depending upon the stress it is subjected to. Cake batter, molten chocolate, Marmite®, custard and the foamed concrete used in oil wells are all examples of these ‘dual personality’ materials.

Examples of Solid to Liquid (Melting) Ice to water – Ice melts back into water when it is left out at temperatures above the freezing point of 32 degrees. Rocks to lava – Rocks in volcanoes can be heated until they are molten lava. Metal to molten liquid – Metals such as steel and bronze can be molten down.

The batter is a liquid mixture of flour, sugar, water and other ingredients. When you heat it in the oven, the batter turns into a yummy solid. You cannot change it back into flour, sugar, water and the other ingredients. The batter has made a chemical change that is irreversible and it has become something new.

Molecular solids are characterized by relatively strong intramolecular bonds between the atoms that form the molecules and much weaker intermolecular bonds between these molecules. Because the intermolecular bonds are relatively weak, molecular solids are often soft substances with low melting points.

Most covalent network solids are very hard, as exemplified by diamond, which is the hardest known substance.

The molecular solid tends to have a lower melting points. Considering its properties, the molecules of this type of solid are held together by van der Waals forces rather than by ionic or covalent bonds.

Molecular solids are made of covalent molecules, and the solids are held together by Van der Waals forces between the molecules. These molecular solids usually have low melting points and are easy to break, because the forces between molecules are weak.

Molecular solids

Zirconium dioxide (ZrO2) is known to exist as a white crystalline solid under standard conditions. The most naturally occurring form of this compound is the mineral baddeleyite, which is known to have a monoclinic crystal structure.

Indium arsenide, InAs, or indium monoarsenide, is a semiconductor composed of indium and arsenic. It has the appearance of grey cubic crystals with a melting point of 942 °C. Indium arsenide is used for construction of infrared detectors, for the wavelength range of 1–3.8 µm.

An example of an ionic solid is table salt, NaCl. Molecular solids—Made up of atoms or molecules held together by London dispersion forces, dipole-dipole forces, or hydrogen bonds. Examples of this type of solid are diamond and graphite, and the fullerenes.