Secondary 4 Combined Science Chemistry Practice Paper 3

Answer Key - Combined Science Chemistry Secondary 4 Practice Paper (Version 3)

Section A

Question 1 (a) pH 11 to 14 (Accept any value in this range). [1] (b) An acid that completely ionizes/dissociates in aqueous solution to produce a high concentration of hydrogen ions ($\text{H}^+$). [2] (c) Ethanoic acid is a weak acid; it only partially ionizes in water. [1] Therefore, it has a lower concentration of $\text{H}^+$ ions compared to $\text{HCl}$ (a strong acid), resulting in a higher pH. [1]

Question 2 (a) Copper(II) oxide and dilute sulfuric acid. [1] (b)

• Add excess copper(II) oxide to warm sulfuric acid until no more dissolves. [1]
• Filter the mixture to remove unreacted copper(II) oxide. [1]
• Heat the filtrate in an evaporating dish to the point of crystallization. [1]
• Allow the saturated solution to cool slowly to form crystals, then filter and dry them. [1] (c) To ensure the solution is saturated so that crystals form upon cooling. [1]

Question 3 (a) (i) $\text{Mg(s)} + 2\text{HCl(aq)} \rightarrow \text{MgCl}_2\text{(aq)} + \text{H}_2\text{(g)}$ [1] (ii) $\text{CaCO}_3\text{(s)} + 2\text{HNO}_3\text{(aq)} \rightarrow \text{Ca(NO}_3)_2\text{(aq)} + \text{CO}_2\text{(g)} + \text{H}_2\text{O(l)}$ [1] (iii) $2\text{NaOH(aq)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{Na}_2\text{SO}_4\text{(aq)} + 2\text{H}_2\text{O(l)}$ [1] (b) Reaction (iii). [1] It involves the reaction between an alkali (base) and an acid to produce a salt and water. [1]

Question 4 (a) Sulfur dioxide ($\text{SO}_2$). [1] (b) Bubble the gas through acidified potassium manganate(VII) solution. [1] The purple solution will be decolorized. [1] (c) $\text{H}^+\text{(aq)} + \text{OH}^-\text{(aq)} \rightarrow \text{H}_2\text{O(l)}$ [2]

Question 5 (a) Soluble salts can dissolve in water to form aqueous solutions. [1] Insoluble salts do not dissolve in water and form a precipitate. [1] (b) Mix two aqueous solutions containing barium ions (e.g., $\text{BaCl}_2$) and sulfate ions (e.g., $\text{Na}_2\text{SO}_4$). [1] A white precipitate of barium sulfate forms. [1] Filter the precipitate, wash with distilled water, and dry. [1]

Section B

Question 6 (a) $2\text{NaOH(aq)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{Na}_2\text{SO}_4\text{(aq)} + 2\text{H}_2\text{O(l)}$ [2] (b) $\text{Moles} = \text{Concentration} \times \text{Volume} = 0.10 \times (20/1000) = 0.002\text{ mol}$. [2] (c)

• Mole ratio $\text{H}_2\text{SO}_4 : \text{NaOH} = 1 : 2$. [1]
• $\text{Moles of NaOH} = 0.002 \times 2 = 0.004\text{ mol}$. [1]
• $\text{Concentration} = 0.004 / (25/1000) = 0.16\text{ mol/dm}^3$. [1]

Question 7 (a) A: Red, B: Green, C: Purple/Violet. [3] (b) pH increases (moves toward 7). [1] The indicator color changes from red to green. [1]

Question 8 (a) A base is any substance that neutralizes an acid. [1] An alkali is a base that is soluble in water. [1] (b) (i) Effervescence/bubbles of gas observed; white solid disappears. [1] (ii) The pH increases. [1] The basic magnesium oxide reacts with and neutralizes the $\text{H}^+$ ions in the acid. [1]

Question 9 (a) Acidic. [1] The salt is formed from a strong acid and a weak base; the conjugate base of the weak base undergoes hydrolysis. [2] (Accept: "The strong acid dominates the pH of the resulting salt solution"). (b) Add dilute hydrochloric acid (or nitric acid) to the solution. [1] Then add barium chloride solution. [1] A white precipitate forms. [1]

Question 10 (a)

• $\text{Moles of HCl} = 0.5 \times (100/1000) = 0.05\text{ mol}$. [1]
• Ratio $\text{CaCO}_3 : \text{HCl} = 1 : 2 \rightarrow \text{Moles of } \text{CaCO}_3 = 0.025\text{ mol}$. [1]
• $\text{Molar mass of } \text{CaCO}_3 = 40 + 12 + (16 \times 3) = 100\text{ g/mol}$. [1]
• $\text{Mass} = 0.025 \times 100 = 2.5\text{ g}$. [1] (b) Pressure increases. [1] Carbon dioxide gas is produced, increasing the number of gas particles colliding with the flask walls. [1]