Summary
Changes to materials involve chemical reactions that create new substances and are often irreversible. These reactions can be identified by indicators such as color change, effervescence, precipitation, temperature change, smell, and sound.
- Chemical Reaction — a process where substances change to form new substances.
Example: Combustion of wood. - Exothermic Reaction — a reaction that releases energy to the surroundings, increasing the temperature.
Example: Combustion of fuels. - Endothermic Reaction — a reaction that absorbs energy from the surroundings, decreasing the temperature.
Example: Photosynthesis. - Oxidation — a reaction where oxygen is added to a substance.
Example: Rusting of iron. - Reduction — a reaction where oxygen is removed from a substance.
Example: Extraction of metals from ores. - Solubility — a measure of how much of a substance can dissolve in a solvent at a given temperature.
Example: Sugar dissolving in water.
Exam Tips
Key Definitions to Remember
- Chemical Reaction: A process where reactants form products.
- Exothermic Reaction: Releases heat to surroundings.
- Endothermic Reaction: Absorbs heat from surroundings.
- Oxidation: Addition of oxygen to a substance.
- Reduction: Removal of oxygen from a substance.
- Solubility: Amount of substance that can dissolve in a solvent.
Common Confusions
- Confusing exothermic and endothermic reactions.
- Misunderstanding the concept of solubility and saturation.
Typical Exam Questions
- What are two types of reactions that are usually exothermic? Combustion and neutralisation.
- Give an example of a change which is endothermic. Photosynthesis.
- Explain why the temperature increases during an exothermic reaction. Energy is released to the surroundings, increasing temperature.
What Examiners Usually Test
- Understanding of energy changes in reactions.
- Ability to identify signs of chemical reactions.
- Knowledge of oxidation and reduction processes.