Changes in States | Chemistry for Grade 11 (IGCSE) PDF Download

State Changes

Melting

• Melting refers to the process where a solid transforms into a liquid. 
• It requires heat energy, which is converted into kinetic energy, enabling the particles to move freely. 
• This transition occurs at a specific temperature called the melting point (m.p.).

Boiling

• Boiling is the transformation of a liquid into a gas. 
• Heat is essential for this change, leading to the formation of gas bubbles beneath the liquid surface, allowing liquid particles to escape. 
• Boiling transpires at a distinct temperature known as the boiling point (b.p.).

Freezing

• Freezing refers to the transformation of a liquid into a solid state.
• This process is the opposite of melting and happens at the same temperature as melting does. For instance, pure water freezes and melts at 0°C.
• It necessitates a considerable reduction in temperature (or loss of thermal energy) and takes place at a specific temperature.

Evaporation

• Evaporation is the change of a liquid into a gas and occurs at various temperatures.
• This phenomenon transpires exclusively at the surface of liquids, where high-energy particles can break free from the surface at low temperatures, typically below the liquid's boiling point.
• The rate of evaporation is influenced by the liquid's surface area and temperature, with larger surfaces and warmer temperatures leading to quicker evaporation.

Condensation

• Condensation is the process where a gas changes into a liquid upon cooling.
• This transition occurs over a range of temperatures.
• When a gas is cooled, its particles lose energy, causing them to group together and form a liquid.

Sublimation

• Sublimation is when a solid changes directly into a gas.
• This phenomenon is observed in specific solids like iodine or solid carbon dioxide.
• The reverse process, where a gas changes directly into a solid, is known as desublimation or deposition.

State Changes & Kinetic Theory

• When a substance is heated, its particles absorb thermal energy which is then converted into kinetic energy. This concept forms the basis of the kinetic theory of matter.
• Heating a solid leads to an increase in particle vibration. As the temperature rises, the particles vibrate more intensely, causing the solid to expand until its structure breaks and it melts.
• Upon further heating, a liquid substance expands further. Some particles at the surface gain enough energy to overcome intermolecular forces and evaporate.
• At the boiling point temperature, all particles acquire adequate energy to escape, resulting in the liquid boiling.
• State changes can be represented on a graph known as a heating curve.
• Cooling a gas has the opposite effect of heating and is depicted on a cooling curve.
• Cooling curves also show the impact of temperature changes on states of matter.
• The horizontal sections occur when there is a change of state but there is no change in temperature