Key Terms and pH Scale

Chemical changes involve completely transforming substances into entirely new materials with different properties. Unlike physical changes (like melting ice), you can't easily reverse these transformations.

Understanding pH values is crucial for predicting how substances will react. The pH scale runs from 0 to 14, where numbers below 7 indicate acidic solutions (like lemon juice), 7 is neutral (pure water), and above 7 means alkaline or basic solutions (like soap).

The visual pH scale uses colours to make identification easier - reds and oranges show acidic substances, green represents neutral, and blues and purples indicate alkaline materials. This colour-coding system helps you quickly assess whether a substance will be corrosive, safe to handle, or useful for specific reactions.

Quick Tip: Remember that each pH unit represents a 10-fold change in acidity - so pH 2 is 10 times more acidic than pH 3!

Electrolysis and Energy Changes

Electrolysis is the process of using electricity to split ionic compounds - it's how we extract metals like aluminium from their ores. The setup involves electrodes (electrical conductors) placed in an electrolyte (molten or dissolved ionic compound).

At the cathode (negative electrode), positive ions gain electrons, whilst at the anode (positive electrode), negative ions lose electrons. For aluminium extraction, the ore is dissolved in cryolite to lower the melting point and save energy costs.

Energy changes during reactions follow the conservation of energy principle - energy is never created or destroyed, only transferred. Exothermic reactions release energy (making surroundings warmer), like combustion or hand warmers. Endothermic reactions absorb energy (making surroundings cooler), like thermal decomposition or sports injury packs.

Activation energy is the initial energy barrier that particles need to overcome before reacting. Breaking chemical bonds requires energy input, whilst forming new bonds releases energy - the balance determines whether the overall reaction is exothermic or endothermic.

Remember: In electrolysis, opposites attract - positive ions move to the negative cathode, whilst negative ions move to the positive anode!