Paper 2 Experimental Techniques MCQ Practice

C

Distillation is used to separate a solvent (water) from a solution (seawater). The seawater is heated, the water evaporates and is then condensed and collected. Filtration cannot separate dissolved salts from water. Evaporation would leave the salt behind but the water would be lost as steam.

C

The squeaky pop test is the test for hydrogen gas. A burning splint ignites the hydrogen, which combines explosively with oxygen in air: 2H₂ + O₂ → 2H₂O. Oxygen relights a glowing splint. Carbon dioxide turns limewater milky. Chlorine bleaches damp litmus paper.

B

Rf = distance moved by the substance (spot) / distance moved by the solvent front. The Rf value is always between 0 and 1 (or exactly 1 if the substance moves with the solvent front). Each substance has a characteristic Rf value under specific conditions.

C

Fractional distillation is used to separate miscible liquids with different boiling points (such as ethanol and water). The mixture is heated and the liquid with the lower boiling point evaporates first and is collected. The fractionating column provides multiple condensation-evaporation cycles for better separation.

D

Sodium compounds produce a yellow/orange flame. The flame test colours to know are: lithium = red, sodium = yellow/orange, potassium = lilac/pale purple, calcium = orange-red, copper = blue-green. A nichrome wire dipped in the sample is held in a Bunsen flame.

A

To test for chloride ions, add dilute nitric acid (to remove any interfering carbonate or hydroxide ions) followed by silver nitrate solution. A white precipitate of silver chloride forms: Ag⁺ + Cl⁻ → AgCl. Bromide gives a cream precipitate and iodide gives a yellow precipitate.

C

To obtain crystals, the solution is heated gently to evaporate some water (until the crystallisation point, tested by dipping a glass rod and seeing if crystals form on it). The solution is then allowed to cool slowly so crystals form. The crystals are filtered off and dried between filter papers. Heating until dry gives powdery residue, not crystals.

D

Carbon dioxide turns limewater (calcium hydroxide solution) milky. The reaction is: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O. The white precipitate of calcium carbonate makes the limewater appear milky/cloudy. If excess CO₂ is passed through, the precipitate dissolves and the solution turns clear again.

D

A pipette is used to measure a fixed, accurate volume of solution (commonly 25.0 cm³) for transfer into a conical flask during a titration. A burette is used to add the other solution gradually. A measuring cylinder is less precise and not suitable for titrations. A beaker has no precise volume markings.

D

Ammonia (NH₃) is the only common gas that is alkaline. It turns damp red litmus paper blue. Ammonia is produced when ammonium compounds are heated with an alkali (e.g. NaOH). Hydrogen chloride turns damp blue litmus red. Carbon dioxide has no effect on litmus. Chlorine bleaches damp litmus paper.