A student is asked to plan an experiment to investigate how the surface area of calcium carbonate affects the rate of its reaction with dilute hydrochloric ...

This planning question is about how surface area of calcium carbonate affects its rate of reaction with hydrochloric acid, followed by the carbon dioxide given off: \( \text{CaCO}_3 + 2\text{HCl} \rightarrow \text{CaCl}_2 + \text{H}_2\text{O} + \text{CO}_2 \).

(a) The gas is carbon dioxide [1]; the test is to bubble it through limewater, which turns milky / cloudy [1].

(b) To collect and measure the gas, connect the reaction (conical) flask by a delivery tube to a gas syringe, or to an inverted measuring cylinder or burette filled with water over a trough [2] (one mark for a sound collection method, one for a way of measuring the volume).

(c) A detailed method scores up to six marks [6]: use the same mass of calcium carbonate in different forms, for example large chips, small chips and powder [1]; use the same volume of acid [1]; use the same concentration of acid [1]; add the acid, start the stopclock and connect the gas syringe at once [1]; record the volume of gas collected at fixed time intervals, or the time to collect a fixed volume [1]; keep the temperature constant and repeat each run, then average [1].

(d) Suitable headings, each with a unit where needed [3]:

(e) The independent variable is the surface area (form) of the calcium carbonate [1]; the dependent variable is the volume of gas collected in a fixed time, that is the rate [1].

(f) Display the results by plotting the volume of gas against time for each form on the same axes, or by drawing a bar chart of the rates, so the forms can be compared directly [1].

(g) Hazard: dilute hydrochloric acid is irritant or corrosive [1]; precaution: wear eye protection (goggles) [1].