Matter is anything that takes up space and has mass. It exists in different forms called states of matter, which depend on how the tiny particles inside matter are arranged and how they move. ^[1-4]

The four main states of matter are solid, liquid, gas, and plasma. Most of the matter we see every day exists in these four states. However, scientists have also discovered other special states, like Bose-Einstein condensates, which form at extremely low temperatures.

Solid

A solid is a state of matter that has a fixed shape and volume. This means solids do not change their shape unless something forces them to do so, like breaking or cutting. For example, a wooden table keeps the same shape, whether it is in a classroom or a park. ^[1-9]

Solids keep their shape because their particles are packed tightly together. These tiny particles (atoms or molecules) are arranged in an orderly way and do not move freely. Instead, they vibrate in place. This strong arrangement gives solids their hardness and strength.

Examples: Ice, rocks, metal, wood, and plastic

Liquid

A liquid is a state of matter that has a fixed volume but no fixed shape. This means a liquid takes the shape of the container it is in. For example, if you pour water into a glass, it fills the shape of the glass. If you pour the same water into a bowl, it spreads out to match the bowl’s shape. However, the amount (volume) of the liquid stays the same no matter where you put it. ^[1-9]

The particles in a liquid are close together, like in a solid, but they are not tightly packed. Instead, they can slide past each other, which allows the liquid to flow. This is why water, juice, and milk can be poured from one place to another.

Examples: Water, oil, juice, and honey

Gas

A gas is a state of matter that has no fixed shape and no fixed volume. Gases can expand or compress easily. For example, when you blow up a balloon, the air inside expands to fill the shape of the balloon. If you pop the balloon, the air quickly spreads out into the room. Unlike solids and liquids, gases do not stay in one place or keep a constant shape. ^[1-9]

The particles in a gas are far apart and move very fast in all directions. Because of this, gases can expand to fill a large space or be compressed into a smaller one. It also explains why smells – like perfume or food cooking – can spread through the air.

Examples: Oxygen, carbon dioxide, nitrogen, and steam

Plasma

Plasma is a special state of matter that is similar to a gas but has much more energy. In plasma, the particles move very fast and have so much energy that some atoms lose electrons and become charged particles. This process is called ionization. Because of this, plasma can glow and conduct electricity. The charged particles also react to electric and magnetic fields. Scientists use plasma in fusion energy research, which could one day provide unlimited clean energy. ^[1-9]

Plasma is not as common on Earth as solids, liquids, and gases, but it is the most common state of matter in the universe. The sun, stars, and lightning are made of plasma. The bright glow of neon signs and plasma TVs also comes from plasma. Even fire contains small amounts of plasma. Plasma is also used in medical treatments, electronics, and space technology.

The opposite of ionization is called recombination.

Examples: The Sun, stars, lightning, neon signs, plasma TVs, and fire

Changes in State

Matter can change from one state to another. This process is called a phase transition. Here are some common examples: ^[1-9]

• Melting (Solid → Liquid): When a solid is heated, it melts into a liquid. 

Example: Ice melts into water when left in the sun.

• Freezing (Liquid → Solid): When a liquid is cooled, it freezes into a solid. 

Example: Water turns into ice in the freezer.

• Evaporation & Boiling (Liquid → Gas): When a liquid gains heat, it changes into a gas. 

Example: Water evaporates from a puddle on a hot day.

• Condensation (Gas → Liquid): When a gas cools down, it turns into a liquid. 

Example: Water droplets form on a cold glass of juice.

• Sublimation (Solid → Gas): Some solids turn directly into gas without becoming liquid. Example: Dry ice (solid carbon dioxide) turns into gas.

• Deposition (Gas → Solid): Some gases turn directly into a solid without becoming liquid. 

Example: Frost forms on windows in winter.