Methods of Identifying Ions Flashcards

All 37 cards in this deck

What flame colour do lithium compounds give in a flame test?

Crimson.

What flame colour do sodium compounds give in a flame test?

Yellow.

What flame colour do potassium compounds give in a flame test?

Lilac.

What flame colour do calcium compounds give in a flame test?

Orange-red.

What flame colour do copper compounds give in a flame test?

Green.

True or false? If a sample contains a mixture of metal ions, a flame test always shows each colour clearly.

False. Colours can be mixed or masked, so some ions are not seen.

Which solution is added to a solution of a metal salt to identify the metal ion by the colour of a precipitate?

Sodium hydroxide solution.

What is seen when sodium hydroxide solution is added to a solution containing copper(II) ions?

A blue precipitate.

What is seen when sodium hydroxide solution is added to a solution containing iron(II) ions, and to one containing iron(III) ions?

Iron(II): green precipitate. Iron(III): brown precipitate.

Which three metal ions form white precipitates with sodium hydroxide solution?

Aluminium, calcium and magnesium ions.

What are the steps to show that a solution contains aluminium ions rather than calcium or magnesium ions?
e.g. testing aluminium chloride solution

1. Add a few drops of sodium hydroxide solution: a white precipitate forms.
2. Add excess sodium hydroxide solution.
3. The precipitate dissolves — aluminium ions are present.

What is the balanced symbol equation, with state symbols, for the precipitate formed when sodium hydroxide solution is added to a solution of iron(III) ions?

$Fe^{3+}(aq) + 3OH^-(aq) \rightarrow Fe(OH)_3(s)$

Which gas is produced when a carbonate reacts with a dilute acid?

Carbon dioxide.

What are the steps to test a substance for carbonate ions?
e.g. testing a white solid thought to be calcium carbonate

1. Add dilute acid — the solid fizzes.
2. Bubble the gas given off through limewater.
3. Limewater turns milky (cloudy) — carbonate ions are present.

True or false? A carbonate reacting with dilute acid gives off hydrogen gas.

False. It gives off carbon dioxide, which turns limewater milky.

What are the steps to test a solution for halide ions?
e.g. testing sodium chloride solution

1. Add dilute nitric acid.
2. Add silver nitrate solution.
3. A precipitate forms — white here, so chloride ions are present.

What colour precipitate does silver nitrate solution give with chloride ions?

White (silver chloride).

What colour precipitate does silver nitrate solution give with bromide ions?

Cream (silver bromide).

What colour precipitate does silver nitrate solution give with iodide ions?

Yellow (silver iodide).

True or false? Dilute hydrochloric acid should be used to acidify a solution before adding silver nitrate in the test for halide ions.

False. It contains chloride ions and would give a white precipitate; dilute nitric acid is used.

What are the steps to test a solution for sulfate ions?
e.g. testing potassium sulfate solution

1. Add dilute hydrochloric acid.
2. Add barium chloride solution.
3. A white precipitate forms — sulfate ions are present.

Which white precipitate forms in a positive test for sulfate ions?

Barium sulfate.

True or false? Dilute sulfuric acid can be used to acidify a solution before adding barium chloride in the test for sulfate ions.

False. Sulfuric acid contains sulfate ions; dilute hydrochloric acid is used.

What are the steps to carry out a flame test?
e.g. testing lithium chloride solution

1. Clean a nichrome wire loop in dilute hydrochloric acid and hold it in the flame until no colour shows.
2. Dip the clean loop into the sample.
3. Hold it in a blue Bunsen flame and observe the colour — crimson for lithium.

Why must the wire be cleaned between flame tests?

Ions left from the previous sample would give their own colour, mixing with or masking the new one.

What are the steps to identify both ions in an unknown single ionic compound?
e.g. an unknown white solid dissolved in water

1. Test for the metal ion: flame test and/or sodium hydroxide solution.
2. Test for the negative ion: dilute acid + limewater, acidified silver nitrate, or acidified barium chloride.
3. Combine the two results to name the compound, e.g. a lilac flame and a white precipitate with acidified barium chloride means potassium sulfate.

Why is dilute acid added to the solution before the silver nitrate or barium chloride in the halide and sulfate tests?

To remove carbonate ions, which would otherwise form a precipitate and give a false positive.

An unknown compound gives a yellow flame test and a white precipitate with dilute hydrochloric acid and barium chloride solution. Name the compound and give its formula.

Sodium sulfate, $Na_2SO_4$.

An unknown compound gives a lilac flame test and fizzes with dilute acid, giving a gas that turns limewater milky. Name the compound and give its formula.

Potassium carbonate, $K_2CO_3$.

State three advantages of instrumental methods compared with chemical tests.

They are more accurate, more sensitive and more rapid.

True or false? Chemical tests such as a flame test can measure the concentration of a metal ion in a solution.

False. Chemical tests only identify the ion; instrumental methods can measure its concentration.

True or false? Instrumental methods can detect and identify both elements and compounds, and work on very small samples.

True. They are sensitive, so only a tiny sample is needed.

Which instrumental method is used to analyse metal ions in solutions?

Flame emission spectroscopy.

What are the steps in flame emission spectroscopy?
e.g. analysing a solution containing lithium ions

1. Put the sample solution into a flame.
2. Pass the light given out through a spectroscope.
3. Analyse the line spectrum produced — its lines identify lithium and show its concentration.

What two pieces of information can a flame emission spectroscopy line spectrum give about a solution?

Which metal ions are present, and their concentrations.

What are the steps to identify the metal ions in a solution from its flame emission line spectrum?
e.g. an unknown solution compared with reference spectra for sodium and potassium

1. Compare the positions of the lines with the reference spectra.
2. Lines matching the sodium reference mean sodium ions are present, and so on for potassium.
3. The more intense the lines, the higher the concentration of that ion.

True or false? Flame emission spectroscopy can identify several different metal ions in the same solution.

True. Each metal gives its own set of lines, so colours are not masked as in a flame test.