Eggshells consist of calcium carbonate mixed with other substances that are insoluble.
Dilute hydrochloric acid reacts with calcium carbonate. The reaction is represented by the equation below.
$\text{CaCO}_3\text{(s)} + 2\text{HCl(aq)} \rightarrow \text{CaCl}_2\text{(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}$
The remaining substances in eggshells do not react with dilute hydrochloric acid.
A student determines the percentage by mass of calcium carbonate in an eggshell.
Fig. 1.1 illustrates the first three stages of the method used by the student.
step 1: Crush the eggshell into tiny pieces.
step 2: Measure the mass of the tiny pieces of eggshell.
step 3: Put the tiny pieces of eggshell into excess dilute hydrochloric acid and heat the mixture while stirring with a glass rod.
Fig. 1.1 labels: mortar, pestle, eggshell, small pieces of eggshell, balance, glass rod, conical flask, dilute hydrochloric acid.
(a(i))[1]
Describe what is seen when the calcium carbonate in the eggshell reacts with dilute hydrochloric acid.
(a(ii))[2]
Describe what the student should do to make sure the acid is in excess and all of the calcium carbonate has reacted.
(b(i))[2]
Draw a labelled diagram of the apparatus the student should use to filter the mixture.
(b(ii))[2]
Identify the two substances removed from the residue by washing.
(b(iii))[2]
Use the data in Table 1.1 to calculate the percentage by mass of calcium carbonate in the eggshell.
Worked solution & mark scheme
This 9-mark question has a full step-by-step worked solution and mark scheme. One marking point: “bubbling/fizzing/effervescence” …