Matter is anything that takes up space and has mass. It consists of tiny particles, such as ions, atoms, or molecules. Matter exists in various forms, called states of matter, depending on how these particles are arranged and move. ^[1-4]

The Four Main States of Matter

Most of the matter we see daily exists in one of four main states: solid, liquid, gas, or plasma. Scientists have also discovered a special state called the Bose-Einstein condensate, which forms only at extremely low temperatures. ^[1-9]

1. Solid

A solid has a fixed shape and volume. It keeps its shape unless something forces it to change, like cutting or breaking. The particles of a solid are very close because they attract each other strongly. These strong attractions keep the particles from moving freely, so they vibrate in place. This arrangement gives solids their shape, hardness, and strength and prevents them from flowing. That is why diamonds are hard and come in various shapes.

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

2. Liquid

A liquid has no fixed shape but has a fixed volume. It takes the shape of whatever container it is in. Its particles are close together but not tightly packed as in solids. Therefore, the particles can slide past one another, allowing 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

3. Gas

A gas has no fixed shape or volume. It can change its volume, and expand or compress easily to fit any container. Its particles are far apart and move rapidly in all directions. That explains why smells, like perfume or food cooking, can spread through the air.

Examples: Oxygen, carbon dioxide, nitrogen, and steam

4. Plasma

A plasma has no fixed shape or volume and consists of high-energy, ionized particles. The atoms in plasma have lost electrons through a process termed ionization. These particles move extremely fast and have the highest velocity among all the states. Plasma conducts electricity and responds to electric and magnetic fields. It is not as common on Earth as solids, liquids, and gases, but it is the most common state of matter in the universe.

Examples: Sun, stars, lightning, neon signs, plasma TVs, and some high-temperature flames

Change of State

Matter can change from one state to another, such as from a solid to a liquid or from a liquid to a gas. This process is known as a phase change or phase transition. ^[1-9]

Types of Phase Change with Examples

Here are some common phase changes with examples: ^[1-9]

Name of the Process	Initial State → Final State	Example
Melting	Solid → Liquid	Ice melts into water when left outside.
Freezing	Liquid → Solid	Water turns into ice in the freezer.
Vaporization	Liquid → Gas	Boiling: Water boils into steam in a kettle. Evaporation: Water evaporates from a puddle.
Condensation	Gas → Liquid	Water droplets form on a cold soda can.
Sublimation	Solid → Gas	Dry ice changes directly into carbon dioxide gas.
Deposition	Gas → Solid	Frost forms on windows during cold mornings.

Characteristics of Phase Change ^[1-9]

1. No Change in Chemical Identity: A phase change is a physical change, meaning the substance changes its form, but its chemical composition stays the same. For example, when water turns into steam, it is still H₂O.

2. Energy Is Involved: Phase changes happen when energy in the form of heat is added or removed.

Endothermic: Heat is added. For example, adding heat to ice changes it into water.

Exothermic: Heat is removed. For example, removing heat from water turns it into ice.

3. Temperature Remains Constant: During the phase change, the temperature of the substance does not change. The energy is used to break or form bonds between the particles, not to change the temperature.

4. Reversible Process: Phase changes can be reversed. The substance can return to its original state if the temperature or pressure changes. For example, water freezes into ice and melts back into water.

How Do Temperature and Pressure Affect Phase Change

Phase changes fundamentally depend on how tightly the particles are held together. Changes in temperature and pressure can affect how they behave and interact with each other. ^[1-9]

Effect of Temperature

Raising the temperature gives particles more energy. They move faster and can break free from each other.

• In solids, particles vibrate faster until they loosen and form into a liquid.
• In liquids, particles gain even more energy and escape into the air as a gas.

Lowering the temperature removes energy, causing particles to move more slowly.

• In gases, the particles slow down and move closer together to form a liquid.
• In liquids, further cooling causes them to become solids.

Effect of Pressure

Pressure affects how close particles are to each other.

Higher pressure pushes particles closer together. It can prevent a gas from forming easily and may raise the temperature at which the liquid boils (boiling point).

• Example: In a pressure cooker, water boils at a higher temperature because the steam is trapped and cannot escape easily.

Lower pressure allows particles to spread out more easily. It can make it easier for a liquid to turn into a gas.

• Example: On a mountaintop, water boils at a lower temperature because the air pressure is lower.

Understanding the basic states is just the beginning. A phase diagram clarifies when and why a substance changes from one state to another, especially in places with very high or low temperatures or pressures.