Changes of State

A change of state is when matter changes its physical form, such as when a solid melts to form a liquid, or when a liquid is heated to form a gas. Changes of state are physical, not chemical, changes. The matter itself does not change its chemical composition.

The physical state of matter is related to the thermal energy of the particles and the amount of movement of the particles. Matter that is solid has the least amount of thermal energy and the particles have very little movement. Liquids have more thermal energy and the particles exhibit more movement. Gases contain the most thermal energy and the particles are in constant motion.

Changes of state occur when matter gains or loses thermal energy. For example, a liquid can gain thermal energy when it is heated. The particles move faster, and the liquid matter changes to a gas. If a liquid is cooled, it loses energy and the particles slow down. The liquid then changes to a solid. Changes that occur when energy is added to matter are called endothermic, and changes that occur when energy is removed or lost are called exothermic.

We are familiar with the changing physical forms of water in our everyday life. This diagram shows the different physical forms of water and the processes that occur when water changes states. Melting and evaporation occur when heat or thermal energy is added. Freezing and condensation occur when heat or thermal energy is removed.

Melting is the process of changing from a solid to a liquid state. When thermal energy is added to the solid, the temperature rises and the particles vibrate faster and faster. The particles reach a point when they move out of their fixed positions and the matter becomes a liquid. The temperature at which the solid becomes a liquid is called the melting point. Different materials have different melting points.

Freezing is the process of changing from a liquid to a solid state. It is the opposite process of melting. When thermal energy is removed or lost from a liquid, the temperature becomes cooler and the particles move more slowly. When the particles reach a point when they are no longer moving and are in fixed positions, the matter becomes a solid. The temperature at which the liquid becomes a solid is called the freezing point, which is actually the same temperature as the melting point.

Vaporization is the process of changing from a liquid to a gas. When thermal energy is added to a liquid, the particles move faster and faster, until they form a gas. There are two types of vaporization, evaporation and boiling. Evaporation occurs when particles on the surface of a liquid are moving fast enough to escape into the air as gas. Boiling occurs when the particles throughout a liquid are moving fast enough to become gas. Water molecules that are heated will vaporize and form bubbles that rise to the surface. The boiling point is the temperature at which a liquid begins to boil.

The boiling point of liquids is different for different materials. The molecular structure of a substance determines how much energy is needed for it to change from a liquid to a gas. Air pressure also affects the boiling point. When the air pressure surrounding the liquid is higher, more energy is needed for the particles to break free and change to gas. If the air pressure is lower, then less energy is required, and the liquid will boil at a lower temperature.

Condensation is the process of changing from a gas to a liquid. It is the opposite process of vaporization. An example of condensation is when we see water droplets form on the surface of a cold glass. When the air around the glass is cooled, the particles of water vapor in the air move more slowly. The particles begin clumping due to molecular attraction, and the gas changes to liquid water droplets. The temperature at which a gas becomes a liquid is called its condensation point, and it is the same temperature as its boiling point.

Sublimation is a process that occurs when a material changes from a solid directly into a gas. An example is dry ice, which is extremely cold and is composed of carbon dioxide. When dry ice is removed from a freezer, it acquires thermal energy from the warm air surrounding it, and the solid changes directly to a gas. The cold gas causes water vapor in the air to condense and form fog.