What Is a Phase Change? — Definition & Overview

What Are Phases?

Phases or states of matter are the different ways that substances can exist. Everything is made of matter which is defined by taking up space and having mass. Matter can exist in four different states, each one defined by its particle arrangement, movement, and shape fluidity:

• Solids are made of closely packed particles. So, solids have a defined shape.

• Liquids are made of particles that can move past each other. Liquids take on the shape of their container. But, they retain their same volume.

• In gases, the particles have a lot of kinetic energy and move quickly. A gas fills the volume of any container.

• Plasmas differ from gases because the molecules are ionized. So, there are cations and free electrons. The positive and negative charges cause electric and magnetic fields. So, they have unique properties. The particles move around freely and fill the space of their container, like a gas.

What are Phase Changes?

Phase changes occur when a substance moves from one phase/state to another. These changes are typically created by adding or removing energy. Remember, changing the energy at constant pressure is enthalpy. Changing the energy at constant volume is internal energy. These transitions are physical changes. So they alter the physical properties of a substance without changing its chemical bonding.

Changes between phases occur at melting points, freezing points, and boiling points. What's happening at these points? The amount of energy in the substance changes so that the particles take on new movements. We can visualize this with a phase change diagram.

A common phase change is vaporization or boiling. We convert a liquid to a gas. Often we think of boiling as taking a liquid and getting it "hot enough." For example, boiling water on a stove. However, boiling has nothing to do with our idea of hot and cold.

Let's take liquid nitrogen as an example. Liquid nitrogen is -321 degrees F. It is so cold it freezes things instantly! However, if you look at liquid nitrogen in a container at room temperature, you'll see bubbles moving through it. Those bubbles are because the liquid nitrogen is boiling! It is quickly turning from a liquid to a gas.

Example

Let's take water as an example! Water out of the faucet is in liquid form. If we decrease the enthalpy of the liquid water, we can create ice or solid water. If we add energy and increase enthalpy to the ice, we go back to liquid water. If we add enough energy, we create steam or the gas state of water. Add even more energy and ionized water or plasma gets created.

Most often phase changes occur from one state to the next state closest in energy: $$Solid\rightarrow\ Liquid\rightarrow\ Gas\rightarrow\ Plasma\\ or\\ Plasma\rightarrow\ Gas\rightarrow\ Liquid\rightarrow\ Solid$$ Sometimes, substances can "skip" over the next phase closest in energy. A common example is dry ice or solid carbon dioxide ($CO_2$). Dry ice is often used to keep things cold or frozen, often items are shipped with dry ice if they need to be kept cold. Dry ice undergoes sublimation which means it moves directly from a solid to a gas, it skips the liquid state.

Matter is generally categorized as either a solid, liquid, or a gas. At each phase, a substance will look and behave differently due to changes in intermolecular forces. A phase change is the term used to describe any time a sample changes from one form of matter to another.

Changes of States of Matter

By: Teacher's Pet

When a sample's molecules are at their slowest, it is said to be in its solid state. As these molecules are heated, they become excited and begin to move more rapidly.

If the sample continues to be heated, the molecules will begin to move so rapidly that the sample will change from a solid into a liquid state, through a process known as fusion. The temperature at which this occurs is unique to each compound and is known as the sample's melting/freezing point.

If these molecules continue to be heated even further, to the sample's boiling point, the liquid sample will undergo evaporation and eventually become a gas.

Cooling a sample can bring about a phase change as well.

When a gas is cooled to a temperature below its boiling point, it returns to its liquid state in a process called condensation. Cooling a liquid sample back down below its melting point causes it to return to its solid state in a process known as crystallization.

Under certain unique conditions, some compounds can bypass the liquid state and jump directly from solid to gas, or gas to solid.

For example, once compounds like CO$_{2}$ (dry ice) that have extremely low melting/freezing points are heated, they undergo sublimation and turn directly into gas from their solid state.

Another example is the process through which snow is formed in clouds. In sub-freezing air temperatures, the gaseous water vapor that makes up clouds undergoes deposition and transforms from gas into ice crystals within the cloud. Once these crystals grow to be to heavy, they fall down in the form of a snowflake.

(Video) Change of State | Matter | Physics | FuseSchool

by FuseSchool - Global Education

In this video, you will learn about the three states of matter commonly found on Earth. You will also see real-life examples of phase changes or phase transitions and how bonds change in response to heat.