Chemical analysis
50 exam-style questions with answers for 4.8 Chemical analysis in AQA GCSE Chemistry (spec 8462). Practise with flashcards, a topic quiz, a random quiz across the course, 5 crosswords, or read every question and answer below. Not official AQA material.
All 50 questions and answers
Question 1 · Spec 4.8.1.1 · Purity
What is a pure substance in chemistry? [1 mark]
Answer:A single element or compound, not mixed with any other substance.Question 2 · Spec 4.8.1.1 · Purity
How can melting point be used to test purity? [2 marks]
Answer:Pure substances melt at a specific, sharp temperature (1); impure substances melt over a range of temperatures, usually lower (1).Question 3 · Spec 4.8.1.1 · Purity
Why is 'pure' orange juice not pure in chemical terms? [1 mark]
Answer:It is a mixture of many substances, such as water, sugars and acids.Question 4 · Spec 4.8.1.2 · Formulations
What is a formulation? [2 marks]
Answer:A mixture designed as a useful product (1); with each component in measured quantities for a particular purpose (1).Question 5 · Spec 4.8.1.2 · Formulations
Give three examples of formulations. [3 marks]
Answer:Any three from: fuels (1); cleaning agents (1); paints (1); medicines (1); alloys (1); fertilisers (1); foods (1).Question 6 · Spec 4.8.1.3 · Chromatography
Describe the two phases in chromatography. [2 marks]
Answer:The stationary phase is the paper (1); the mobile phase is the solvent (1).Question 7 · Spec 4.8.1.3 · Chromatography
Define the Rf value. [1 mark]
Answer:Rf = distance moved by the substance ÷ distance moved by the solvent.Question 8 · Spec 4.8.1.3 · Chromatography
A spot moves 3.6 cm while the solvent moves 9.0 cm. Calculate the Rf value. [2 marks]
Answer:3.6 ÷ 9.0 (1); = 0.4 (1).Question 9 · Spec 4.8.1.3 · Chromatography
How does chromatography show a substance is pure? [1 mark]
Answer:A pure compound gives a single spot in all solvents.Question 10 · Spec 4.8.1.3 · Required practical 6
Why is the start line drawn in pencil? [1 mark]
Answer:Pencil is insoluble, so it will not run up the paper with the solvent.Question 11 · Spec 4.8.1.3 · Required practical 6
Why must the start line be above the solvent level? [1 mark]
Answer:So the spots do not dissolve into the solvent in the beaker.Question 12 · Spec 4.8.1.3 · Required practical 6
Describe how to carry out paper chromatography. [3 marks]
Answer:Draw a pencil line near the bottom of the paper and place spots of the samples on it (1); stand the paper in a solvent below the line, with a lid (1); remove when the solvent nearly reaches the top, mark the solvent front and calculate Rf values (1).Question 13 · Spec 4.8.2.1 · Gas tests
Describe the test for hydrogen. [2 marks]
Answer:Hold a burning splint at the mouth of the test tube (1); hydrogen burns with a squeaky pop (1).Question 14 · Spec 4.8.2.2 · Gas tests
Describe the test for oxygen. [2 marks]
Answer:Put a glowing splint into the test tube (1); oxygen relights it (1).Question 15 · Spec 4.8.2.3 · Gas tests
Describe the test for carbon dioxide. [2 marks]
Answer:Bubble the gas through limewater (calcium hydroxide solution) (1); it turns milky (cloudy) (1).Question 16 · Spec 4.8.2.4 · Gas tests
Describe the test for chlorine. [2 marks]
Answer:Hold damp litmus paper in the gas (1); it is bleached white (1).Question 17 · Spec 4.8.3.1 · Flame tests
State the flame test colours for lithium, sodium and potassium. [3 marks]
Answer:Lithium: crimson (1); sodium: yellow (1); potassium: lilac (1).Question 18 · Spec 4.8.3.1 · Flame tests
State the flame test colours for calcium and copper. [2 marks]
Answer:Calcium: orange-red (1); copper: green (1).Question 19 · Spec 4.8.3.1 · Flame tests
Why are flame tests unreliable for mixtures? [1 mark]
Answer:Some colours can mask others.Question 20 · Spec 4.8.3.2 · Metal hydroxides
Describe the test for Cu²⁺ ions. [2 marks]
Answer:Add sodium hydroxide solution (1); a blue precipitate forms (1).Question 21 · Spec 4.8.3.2 · Metal hydroxides
What colour precipitates do Fe²⁺ and Fe³⁺ form with sodium hydroxide? [2 marks]
Answer:Fe²⁺: green (1); Fe³⁺: brown (1).Question 22 · Spec 4.8.3.2 · Metal hydroxides
How can you tell Al³⁺ from Ca²⁺ and Mg²⁺ using sodium hydroxide? [2 marks]
Answer:All form white precipitates (1); only the aluminium hydroxide precipitate dissolves in excess sodium hydroxide (1).Question 23 · Spec 4.8.3.3 · Carbonates
Describe the test for carbonate ions. [2 marks]
Answer:Add dilute acid (1); bubbles of carbon dioxide are produced, which turn limewater milky (1).Question 24 · Spec 4.8.3.4 · Halides
Describe the test for halide ions. [2 marks]
Answer:Add dilute nitric acid, then silver nitrate solution (1); chloride gives a white, bromide a cream and iodide a yellow precipitate (1).Question 25 · Spec 4.8.3.4 · Halides
Why is nitric acid added before silver nitrate? [1 mark]
Answer:To remove carbonate ions, which would also form a precipitate.Question 26 · Spec 4.8.3.5 · Sulfates
Describe the test for sulfate ions. [2 marks]
Answer:Add dilute hydrochloric acid, then barium chloride solution (1); a white precipitate forms (1).Question 27 · Spec 4.8.3.6 · Instrumental methods
State three advantages of instrumental methods of analysis. [3 marks]
Answer:Accurate (1); sensitive (1); rapid (1).Question 28 · Spec 4.8.3.7 · Flame emission spectroscopy
Describe how flame emission spectroscopy works. [2 marks]
Answer:The sample is put into a flame and the light given out is passed through a spectroscope (1); the line spectrum is analysed to identify the metal ions and their concentrations (1).Question 29 · Spec 4.8.3.7 · Flame emission spectroscopy
Why is flame emission spectroscopy better than a flame test for mixtures? [1 mark]
Answer:It can identify several metal ions at once, because each gives its own set of lines.Question 30 · Spec 4.8.3.2 · Required practical 7
Describe how to identify an unknown salt as iron(III) sulfate. [2 marks]
Answer:Add sodium hydroxide: a brown precipitate shows Fe³⁺ (1); add hydrochloric acid then barium chloride: a white precipitate shows sulfate (1).Question 31 · Spec 4.8.3.2 · Required practical 7
Why should the flame test loop be cleaned between tests? [1 mark]
Answer:To remove traces of other metals that would change the flame colour.Question 32 · Spec 4.8.1.3 · Chromatography
Why do different substances travel different distances in chromatography? [2 marks]
Answer:They have different solubilities in the solvent (1); and different attractions to the paper (1).Question 33 · Spec 4.8.1.3 · Chromatography
Why is a lid placed on the beaker in chromatography? [1 mark]
Answer:To stop the solvent evaporating.Question 34 · Spec 4.8.1.3 · Chromatography
How can you identify a substance from a chromatogram? [1 mark]
Answer:Compare its Rf value with that of known substances in the same solvent.Question 35 · Spec 4.8.2.3 · Gas tests
Why does limewater turn milky with carbon dioxide? [1 mark]
Answer:An insoluble precipitate of calcium carbonate forms.Question 36 · Spec 4.8.3.1 · Flame tests
Describe how to carry out a flame test. [2 marks]
Answer:Dip a clean nichrome wire loop in the sample (1); hold it in a blue Bunsen flame and observe the colour (1).Question 37 · Spec 4.8.3.4 · Halides
What colour precipitate does sodium bromide give with silver nitrate? [1 mark]
Answer:Cream.Question 38 · Spec 4.8.3.2 · Metal hydroxides
Write the ionic equation for copper ions reacting with hydroxide ions. [1 mark]
Answer:Cu²⁺ + 2OH⁻ → Cu(OH)₂.Question 39 · Spec 4.8.3.6 · Instrumental methods
Why are instrumental methods useful in medicine and forensics? [1 mark]
Answer:They can detect very small amounts of substances quickly.Question 40 · Spec 4.8.3.5 · Sulfates
Why is hydrochloric acid added before barium chloride? [1 mark]
Answer:To remove carbonate ions, which would give a false positive.Question 41 · Spec 4.8.1.1 · Purity
How does the boiling point of an impure liquid compare with the pure liquid? [1 mark]
Answer:It is usually higher and boils over a range of temperatures.Question 42 · Spec 4.8.1.2 · Formulations
Why are formulations made with exact quantities? [1 mark]
Answer:So the product has the properties needed and works reliably every time.Question 43 · Spec 4.8.2.4 · Gas tests
Why should the chlorine test be done in a fume cupboard? [1 mark]
Answer:Chlorine is toxic.Question 44 · Spec 4.8.3.2 · Metal hydroxides
What colour is magnesium hydroxide precipitate? [1 mark]
Answer:White.Question 45 · Spec 4.8.3.7 · Flame emission spectroscopy
How does flame emission spectroscopy measure concentration? [1 mark]
Answer:The intensity of the spectral lines is compared with those from known concentrations.Question 46 · Spec 4.8.1.3 · Chromatography
Why can Rf values never be greater than 1? [1 mark]
Answer:A substance cannot move further than the solvent.Question 47 · Spec 4.8.3.3 · Carbonates
Write the word equation for calcium carbonate reacting with hydrochloric acid. [1 mark]
Answer:Calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide.Question 48 · Spec 4.8.2.1 · Gas tests
Which gas is produced when magnesium reacts with hydrochloric acid, and how is it tested? [2 marks]
Answer:Hydrogen (1); a lighted splint gives a squeaky pop (1).Question 49 · Spec 4.8.3.4 · Halides
What colour precipitate does sodium iodide give with silver nitrate? [1 mark]
Answer:Yellow.Question 50 · Spec 4.8.3.4 · Halides
How could you tell sodium chloride from sodium iodide? [2 marks]
Answer:Add dilute nitric acid and silver nitrate solution (1); sodium chloride gives a white precipitate and sodium iodide a yellow precipitate (1).