Melting and freezing
Water can exist as a solid (ice), liquid (water) or gas (vapour or gas). Adding heat can cause ice (a solid) to melt to form water (a liquid). Removing heat causes water (a liquid) to freeze to form ice (a solid). When water changes to a solid or a gas, we say it changes to a different state of matter. Even though the water’s physical form changes, its molecules stay the same.
Water molecule
Water molecules contain two hydrogen atoms (pictured in green) bonded to one oxygen atom (blue). The large oxygen atom has a stronger attraction for electrons than the small hydrogen atoms. Because the oxygen atom has a stronger pull on the negative bonding electrons the oxygen atom has a slightly negative charge and hydrogen atom a positive charge. This unequal sharing of electrons is called a polar bond or dipole. This causes the hydrogen atoms in one water molecule to be attracted to the oxygen atom in another water molecule. This attraction is known as hydrogen bonding. Water can change from a liquid to a solid or a gas and back to a liquid, but its molecular structure always stays the same.
Water is a molecule
A molecule is a group of two or more atoms that bond or ‘stick’ together. Water is a molecule. It is made up of two hydrogen (H) atoms and one oxygen (O) atom that are chemically bonded together. The H and O are symbols for the atoms that make up water. This is why people often refer to water as H2O. Water can change from a liquid to a solid or a gas and back to a liquid, but its molecules always stay the same. A water molecule is always H2O whether it is liquid water, ice or water vapour.
Nature of science
Numerous everyday words, like energy, have a different meaning in science. We talk about running out of energy during a race or children having too much energy. Scientists define energy as the potential to do work – such as heating or cooling water to make it change state.
Changing states of matter and energy
Water, like all other types of matter, requires the addition or removal of energy to change states. A block of ice is solid water. When heat (a form of energy) is added, the ice melts into liquid water. It has reached its melting point – 0°C. Continue to apply heat, and the water will turn into water vapour, which is water in its gaseous state. The water has reached its boiling point – 100°C.
A snowflake
A snowflake is frozen water. When heat is added, the snowflake melts and changes state to become liquid water.
If heat is removed from water vapour, the gas cools down and it condenses back into liquid water. Continue to cool the water (by removing heat), and it becomes solid ice. This is its freezing point.
Water can change state over and over again
You may have heard that the water you drink today is the same water that dinosaurs drank thousands of years ago. Nature recycles water. It can be melted, frozen and evaporated again and again. There is no loss of water during or after it changes form. Frozen water (ice) takes up more space than liquid water because it is less dense, but when the ice melts, it is the same amount of water as before although some of it may be in the form of water vapour.
Related content
Changes of state go beyond melting and freezing. Ice can turn straight to a gas, and gas can change state to plasma or Bose-Einstein condensate. Read about the states of matter.
The article Solids, liquids and gases – uses water as the context to explain states of matter.
Activity ideas
Explore states of matter with these activities:
Water molecules in drama – students learn about the physical characteristics of water in its different states.
Looking at water – solid, liquid or gas – a hands-on experience of water in its different states.
Solid to liquid to gas – examines the role of heat energy as water changes states.
