AQA GCSE FOUNDATION Chemistry: Changes of State

AQA GCSE Foundation Chemistry: Changes of State

This tutorial will explore the changes of state and how they relate to energy changes.

What are changes of state?

Changes of state refer to the physical transformation of matter from one form to another, such as solid, liquid, or gas. These changes are caused by changes in temperature and pressure. Here are the common changes of state:

• Melting: Solid to Liquid (e.g., ice melting into water)
• Freezing: Liquid to Solid (e.g., water freezing into ice)
• Boiling/Evaporation: Liquid to Gas (e.g., water boiling into steam)
• Condensation: Gas to Liquid (e.g., steam condensing into water droplets)
• Sublimation: Solid to Gas (e.g., dry ice sublimating into carbon dioxide gas)
• Deposition: Gas to Solid (e.g., frost forming on a cold surface)

Energy changes during changes of state

Melting and Boiling:

• Both processes require energy input.
• This energy is used to overcome the forces holding the particles together in their original state.
• For example, when ice melts, the energy input breaks the hydrogen bonds between water molecules, allowing them to move more freely as a liquid.

Freezing and Condensation:

• Both processes release energy.
• This energy is released as the particles slow down and become more tightly packed.
• For example, when water freezes, the energy is released as the water molecules form hydrogen bonds, becoming a more ordered structure in the solid state.

Sublimation and Deposition:

• These processes also involve energy changes but are less common than melting, freezing, boiling, and condensation.

Summary

Key points to remember:

• Changes of state are physical changes, meaning the chemical composition of the substance remains the same.
• Energy is absorbed during endothermic changes, while energy is released during exothermic changes.
• The temperature of a substance remains constant during a change of state, even though energy is being added or removed.

Examples:

• Melting: Imagine you put an ice cube on a warm surface. The ice cube absorbs energy from the surface, causing the ice molecules to vibrate more, eventually breaking the hydrogen bonds and turning into liquid water.
• Boiling: When you heat water on a stove, the water molecules absorb energy and move faster. Eventually, they have enough energy to overcome the forces holding them together as a liquid, and they escape into the air as steam.
• Freezing: If you put a glass of water in the freezer, the water molecules lose energy and slow down. This causes them to form hydrogen bonds, becoming more tightly packed and turning into ice.

Additional notes:

• The temperature at which a substance melts or boils is its melting point or boiling point, respectively.
• The amount of energy required to melt or boil a substance is called its enthalpy of fusion or enthalpy of vaporization, respectively.

Further exploration:

• Research the different types of intermolecular forces and how they affect the changes of state.
• Explore the concept of vapor pressure and how it relates to boiling point.
• Investigate how pressure affects the changes of state, such as the effect of pressure on the boiling point of water.