Required practicals review

The practicals at a glance

• Making a soluble salt, titration, electrolysis, temperature change, rates (two ways), chromatography, identifying ions, water purification.
• For each one, know: what you change, what you measure, what you keep the same, the main source of error, and one hazard with its control.

Improving an experiment usually means removing a source of error you can name, not "doing it more carefully".

Examples: replace judging a disappearing cross by eye with a light sensor and data logger; insulate the cup and use a lid when energy is lost to the room; put a reactant in a small tube inside the sealed flask and tip it over, so no gas escapes before the bung goes in.

A risk assessment is written before a practical, so the hazards are known and controlled before anyone starts. Hazard symbols tell you what kind of hazard a chemical is:

• Flammable (a flame): catches fire easily, such as ethanol. Keep away from flames.
• Oxidising (a flame over a circle): makes other things burn more fiercely, such as concentrated hydrogen peroxide. Keep away from flammable materials.
• Toxic (a skull): can kill in small amounts, such as chlorine. Use a fume cupboard.
• Corrosive (liquid eating a hand and a surface): destroys skin and eyes, such as concentrated acids. Wear gloves and eye protection.
• Irritant or harmful (an exclamation mark): makes skin or eyes sore, such as dilute sodium hydroxide. Wear eye protection.