Everything in the universe is made of tiny particles, which include atoms, molecules, and ions. Depending on the arrangement and movement of the particles within a substance, we classify them as one of three states of matter. These are solid, liquid, and gas.
Substances can have wildly different properties based on the state they are in, despite the actual particles themselves remaining unchanged. When you think of water, most people would think of the water that you drink from a cup or the water in the sea. But water can also exist as ice (a solid), and steam (a gas).
Why is it that one substance can take so many forms? To answer this question, we need to take a look at the concepts of matter and how particles behave differently in each state.
Matter is a term used to describe any substance that has mass and takes up space, i.e. has volume. In a practical sense, anything made of atoms is matter. In fact, virtually everything around you, from the air you breathe to the food you eat, can be considered matter. Following this definition, it is important to note that massless particles like photons are not matter, and neither are energy phenomena like sound or light.
Now that you have a grasp of what ‘matter’ is, we can discuss the three states of matter and how they differ. The states will be discussed in terms of molecular structure and intermolecular forces.
Solid matter contains tightly packed molecules, often in a regular assortment. These molecules vibrate about their fixed positions, but generally do not move from their place. This is due to very strong intermolecular forces of attraction, which also cause the molecules to be very close together.
As a result, solids often maintain a fixed shape and volume. They are also incompressible due to the high density, i.e. the compactness of the molecules. As you would expect, solids do not flow easily either, as the rigid arrangement prevents the molecules from sliding past one another.
Liquids contain molecules that are arranged randomly and irregularly. Like solids, the molecules in liquids are packed rather close together, but not as compactly. The intermolecular forces of attraction are also strong, but less so than in solids. Therefore, the molecules are able to move about freely and can slide over one another.
Due to its molecules’ freedom to move, liquids do not have a fixed shape. They can take the shape of the container that they are kept in, as water does with your cup. However, the molecules are still compact, meaning that liquids are incompressible and have a fixed volume.
Gases contain widely spaced molecules which are arranged irregularly and randomly. These molecules can vibrate and move around freely at high speeds, in random directions. The forces of attraction between the molecules in a gas are very weak.
Gases have no fixed shape or volume. Also, because of how far apart the molecules are, gases can be compressed. This is a property unique to gases, which the other two states of matter do not share.
Understanding the properties of the three states of matter and how to compare them is integral to the understanding of classical physics. Once you have grasped these fundamental concepts, you can then move on to more complex topics like the kinetic model of matter.
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