Identifying ions and gases, titrations — GCSE Chemistry practice questions

Question 1

A lab technician is making a wall chart of gas tests for a Year 10 class. She wants to check each row before printing it.

Which row of the chart is correct?

Accepted Answer

C: ammonia · damp red litmus paper · turns blue. Ammonia is an alkaline gas, so it turns damp red litmus paper blue. Oxygen relights a glowing splint, hydrogen burns with a squeaky pop with a lighted splint, and limewater is the test for carbon dioxide; chlorine bleaches damp litmus paper.

Question 2

Dried apricots are often kept bright orange with a sulfite preservative. A food inspector shakes some apricots with water and tests the solution for sulfite ions.

Which row shows the reagent she adds and the result if sulfite ions are present?

Accepted Answer

C: acidified aqueous potassium manganate(VII) · purple to colourless. Sulfite ions are tested with a small volume of acidified aqueous potassium manganate(VII), which turns from purple to colourless. Acidified barium nitrate is the test for sulfate, and limewater is used for carbon dioxide.

Question 3

Water from old iron pipes in a farmhouse is tested. A few drops of aqueous sodium hydroxide give a green precipitate, which turns brown near the surface on standing.

Which row identifies the ion in the water and explains why the precipitate turns brown?

Accepted Answer

A: Fe²⁺ · the iron ions are oxidised by oxygen in the air. A green precipitate with sodium hydroxide shows iron(II), Fe²⁺. At the surface, oxygen in the air oxidises iron(II) to iron(III), which forms the red-brown hydroxide, so the top turns brown.

	ion in the water	why the precipitate turns brown
A	$\ce{Fe^2+}$	the iron ions are oxidised by oxygen in the air
B	$\ce{Fe^2+}$	the iron ions are reduced by oxygen in the air
C	$\ce{Fe^3+}$	the iron ions are oxidised by oxygen in the air
D	$\ce{Fe^3+}$	the iron ions are reduced by oxygen in the air

	gas	test	result
A	hydrogen	glowing splint	relights
B	chlorine	bubble through limewater	limewater turns milky
C	ammonia	damp red litmus paper	turns blue
D	oxygen	lighted splint	squeaky pop

	reagent added	result
A	acidified aqueous barium nitrate	white precipitate
B	acidified aqueous potassium manganate(VII)	colourless to purple
C	acidified aqueous potassium manganate(VII)	purple to colourless
D	limewater	colourless to milky