Identifying the Products of Chemical Reactions (OCR GCSE Chemistry A (Gateway)): Flashcards

Sodium hydroxide solution (NaOH) is added. The colour of the precipitate formed identifies the metal cation present.

Adding NaOH to a solution containing Cu²⁺ ions produces a blue precipitate. Adding NaOH to a solution containing Fe³⁺ ions produces an orange-brown precipitate.

These distinctive colours help identify the metal cation present.

True.

Both Al(OH)₃ and Zn(OH)₂ re-dissolve in excess NaOH. This distinguishes them from calcium and magnesium hydroxides, which do not dissolve in excess.

Iron(II) ions produce a green precipitate of iron(II) hydroxide when sodium hydroxide is added.

Some precipitates dissolve in excess NaOH. Adding too much too quickly risks dissolving the precipitate before its colour can be observed, giving a false negative result.

True.

Both give white precipitates initially. Ca(OH)₂ and Mg(OH)₂ do not dissolve in excess NaOH, but this does not distinguish them from each other — a flame test is needed for that.

A solution produces a white precipitate with NaOH that does not dissolve in excess. The metal cation could be Ca²⁺, Mg²⁺, Al³⁺ or Zn²⁺.

To distinguish Al³⁺ and Zn²⁺ from Ca²⁺ and Mg²⁺, add excess NaOH.

Al(OH)₃ and Zn(OH)₂ re-dissolve in excess NaOH; Ca(OH)₂ and Mg(OH)₂ do not.

Add sodium hydroxide solution and warm gently. If ammonium ions are present, ammonia gas is produced, which turns damp red litmus paper blue.

The carbonate ion is CO₃^2-, a negatively charged ion that releases carbon dioxide gas when reacted with dilute acid.

Add dilute acid to the sample. A positive result is effervescence and the gas produced turns limewater milky, confirming that CO₂ has been released.

To test for sulfate ions, first acidify the sample with dilute hydrochloric acid, then add aqueous barium chloride solution. A white precipitate confirms the presence of sulfate ions.

The white precipitate is barium sulfate, BaSO₄.

True.

Nitric acid (HNO₃) is used because hydrochloric acid contains chloride ions, which would interfere with the test and produce a false white precipitate.

Chloride ions give a white precipitate, bromide ions give a cream precipitate and iodide ions give a yellow precipitate. All precipitates are silver halides (AgX).

The formula for the ion tested in the halide test is X^- (where X = Cl, Br or I). The reagent added after acidification is silver nitrate (AgNO₃) solution.

The silver ions react with the halide to form a coloured precipitate of AgX.

True.

HCl is used before the barium chloride test to remove carbonate ions that would also form a white precipitate and interfere with the result. Hydrochloric acid does not introduce ions that would produce a false positive for sulfate.

The acid removes any carbonate ions that may be present. Carbonates would also form a white precipitate with barium ions, giving a false positive result for sulfate.

A silver halide (AgX) is the insoluble precipitate formed when silver nitrate is added to a solution containing a halide ion. Its colour identifies the halide present.

Lithium (Li⁺) produces a red flame.

Match the metal ion to its flame colour:

• Sodium: yellow

• Potassium: lilac

• Copper: blue-green

True.

Cleaning the wire removes contaminating ions from previous tests. The wire is clean when it produces no colour change in the flame, ensuring only one ion is tested at a time.

A flame test is a technique used to identify metal ions by observing the colour they produce when heated in a roaring blue Bunsen flame.

The colours from different metal ions mix, making it impossible to identify either ion correctly. The test only works reliably when a single type of ion is present.

In a flame test, the sample is placed on a nichrome wire and held at the edge of the blue Bunsen flame. The wire must be unreactive so that it does not affect the flame colour.

False.

The bright yellow of Na⁺ can mask the lilac colour of K⁺ ions if both are present. A yellow result may conceal a second metal ion — further testing would be needed.

The roaring blue flame is hotter and provides enough energy to excite the metal ions. The blue flame is also not very bright, so it does not mask the colour produced by the metal ion.

A line emission spectrum is a pattern of bright coloured lines on a dark background produced when an element is heated. Each element has a unique pattern, like a fingerprint.

A flame photometer measures the wavelength and intensity of light emitted by metal ions in a flame. It can identify ions even when their colours are similar, and it can analyse multiple ions in the same sample.

In mass spectrometry, the peak with the highest mass-to-charge ratio is called the molecular ion peak and gives the relative molecular mass of the compound.

This peak is labelled M⁺ and is found furthest to the right on the spectrum.

True.

Instrumental methods offer greater accuracy, greater sensitivity (they can work with very small samples) and are faster than traditional chemical tests. They can also perform multiple simultaneous analyses.

Standard solutions of known concentration are analysed and their light intensity is plotted against concentration. An unknown sample's intensity is then measured and its concentration is read from the graph.

In mass spectrometry, as molecules pass through the instrument they can fragment into smaller parts. Each fragment produces a peak with a specific mass/charge (m/z) ratio.

The pattern of fragments can help identify the structure of a compound.

The emission spectrum of the unknown sample is compared to reference spectra for known elements. Matching the pattern of lines identifies which metal ions are present in the mixture.

True.

Mass spectrometry measures the abundance and mass of each isotope of an element. The relative atomic mass is calculated from these values.

Instrumental analysis offers greater accuracy, higher sensitivity (works with very small samples) and greater speed. It can also perform multiple simultaneous tests automatically.