Core practical: measuring rates of reaction

calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide
1. Put a fixed mass of marble chips in a conical flask and add a fixed volume of acid.
2. Connect a gas syringe (or collect over water in an upturned measuring cylinder) and start the timer.
3. Record the volume of gas every 30 seconds.
4. Repeat with powder instead of chips (surface area), or with a different acid concentration.
Plot volume against time; a steeper curve means a faster rate.

sodium thiosulfate + hydrochloric acid → sodium chloride + sulfur dioxide + sulfur + water
Na₂S₂O₃ + 2HCl → 2NaCl + SO₂ + S + H₂O
1. Put a flask of sodium thiosulfate solution on a paper marked with a black cross.
2. Warm the solution to a set temperature, add the acid and start the timer.
3. Sulfur forms as a yellow precipitate, making the mixture cloudy. Stop the timer when the cross disappears from view.
4. Repeat at different temperatures.
rate ∝ 1 ÷ time. Work in a ventilated room: sulfur dioxide is toxic.

mean rate = amount of product ÷ time (for example cm³/s).
For the cross experiment, use 1 ÷ time as a measure of rate (unit s^-1).
For a curved graph (Higher), the rate at one moment is the gradient of a tangent at that point.