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Hoar frost forms when water vapour changes directly to a solid. Image by jamsthebest from pixabay

In the last lesson, you learned that there are five different states of matter — solids, liquids, gases, plasma, and Bose-Einstein condensates. In this lesson, you will learn that matter may change its physical state due to changes in kinetic energy and/or molecular forces of attraction. This change does not alter its atomic or molecular properties.

Can matter change its physical state?

Change in the physical state of matter has to do with the properties of the molecules and atoms that it is made of. Molecules are simply combinations of atoms that bond together to make various elements. The motion or energy of these molecules and the attraction between them create various states of matter, each with its own particular physical characteristics and behaviours.

Molecules are a collection of atoms. They have unique physical and chemical properties.

Theories about Molecules

The Kinetic Molecular Theory

All matter is made of atoms or molecules.
The particles are always in motion. The higher the temperature of the matter, the more it is in motion. Temperature is a measure of the motion of this matter.
Kinetic energy is the energy of motion. Temperature is a measure of the amount of kinetic energy

Another theory about how molecules move is that similar molecules and atoms are attracted to each other; they want to be close together. This process is called molecular attraction.

You can see that these two theories have the opposite effect on molecules: One pushes them away from each other and the other one draws them closer to each other.

Depending on which force is stronger, the physical state of matter may change from one form to another.

If a material has low kinetic energy, it is the molecular attraction that keeps the matter in a solid state.
If a material has high kinetic energy, this energy can overwhelm the molecular attraction so it is only able to keep the material together in a liquid state.
When the kinetic energy of molecules is so powerful that it is much greater than the attractive force, the material will take the form of a gas.
In the Bose-Einstein condensate, the molecules are at such an extremely low temperature that they barely move and so are able to bond very closely together.

What happens to molecular structure when a material changes its form? Although matter can change its physical state, its molecular or atomic properties will remain the same, no matter what form it is in.

Water is an excellent way to illustrate this:

Solid: ice
Liquid: water
Gas: steam

Water is a compound made up of two hydrogen atoms and one oxygen atom, and that structure remains the same although its physical state may change.

Matter can take different physical forms without changing its chemical structure. This means the structure of its molecules remains the same. In other words, it may look different on the outside, but it is the same on the inside.

It is useful to know some of the physical characteristics of matter. For example, if you are building a house, you need to think about whether the materials you will use suit the purpose. If the house is to be built in a hot climate, you will use materials that help keep the house cool. For a house in Alberta, you will use materials that keep the house warm in the winter.

Scientists look for various physical characteristics when they describe matter. Different substances have various properties that can help us to identify them or tell them apart from other substances.

Scientists use many physical characteristics or properties to describe matter as listed below.

Physical state: Is the substance a solid, liquid, or gas?
Melting point: At what temperature does it change from solid to liquid?
Boiling point: At what point does it change from liquid to gas?
Hardness: How easily can it be scratched?
Malleability: How easily can it be shaped (bent, folded, pounded, etc.)?
Ductility: Can it be stretched?
Solubility: Can it be dissolved in another substance?
Density: How much mass will it have in a given volume?
Conductivity: Can it conduct electricity or heat?
Colour: What colour is it?
Odour: Does it have a particular smell?
Clarity: Is it clear or cloudy?
Lustre: Is it shiny or dull?