Chemical Tests (Edexcel IGCSE Science (Double Award): Chemistry): Flashcards

A flame test is a technique used to identify metal cations by the colour of the flame they produce when a sample is held in a Bunsen burner flame on a clean metal wire.

Li⁺ produces a red (crimson) flame in a flame test.

In a flame test, Na⁺ produces a yellow flame and K⁺ produces a lilac flame.

False.

Ca²⁺ produces an orange-red flame. It is Cu²⁺ that produces the blue-green flame in a flame test.

Before carrying out a flame test, the nichrome wire loop must be dipped in dilute acid and held in the flame until there is no colour change.

The wire must be cleaned to prevent contamination. Residual ions on the wire would mix with the sample, causing colours to overlap and making it impossible to identify the metal ion correctly.

True.

Na⁺ (sodium ion) produces a distinctive yellow flame, which is one of the easiest flame colours to recognise.

1. Li⁺: red

2. Na⁺: yellow

3. K⁺: lilac

4. Ca²⁺: orange-red

5. Cu²⁺: blue-green

Cu²⁺ produces a blue-green flame and Ca²⁺ produces an orange-red flame in a flame test.

The blue flame is used because it is hotter, ensuring the sample is heated strongly enough to excite the metal ions and produce a clear, visible flame colour.

A precipitate is an insoluble solid that forms when two solutions are mixed, such as when sodium hydroxide is added to a solution containing metal cations.

Cu²⁺ forms a light blue precipitate of copper(II) hydroxide when sodium hydroxide solution is added.

Fe²⁺ forms a pale green precipitate and Fe³⁺ forms an orange/brown precipitate when NaOH is added.

False.

Fe³⁺ produces an orange/brown precipitate with NaOH solution. It is Fe²⁺ that produces the pale green precipitate.

NaOH solution is added and the mixture is warmed. NH₄⁺ is identified by the release of ammonia gas, which turns damp red litmus paper blue.

To identify NH₄⁺, NaOH is added and the mixture is warmed. The gas released turns damp red litmus paper blue.

True.

Cu²⁺ reacts with OH^- ions from NaOH to form copper(II) hydroxide, which is an insoluble light blue precipitate.

1. Cu²⁺: light blue precipitate

2. Fe²⁺: pale green precipitate

3. Fe³⁺: orange/brown precipitate

Metal cations are identified by the colour of the precipitate formed when sodium hydroxide (NaOH) solution is added.

The metal ions react with hydroxide ions (OH^-) from NaOH to form a metal hydroxide. If the metal hydroxide is insoluble in water, it forms as a solid precipitate.

An anion is a negatively charged ion formed when a non-metal atom gains electrons, such as Cl^-, SO₄^2- or CO₃^2-.

The sample is acidified with dilute hydrochloric acid and a few drops of barium chloride solution are added. A positive result is a white precipitate of barium sulfate.

To test for carbonate ions, dilute acid is added and the gas produced is bubbled through limewater. A positive result is that it turns cloudy white.

True.

Silver nitrate solution, acidified with nitric acid, is added to the sample. A coloured precipitate forms if a halide ion is present: white for Cl^-, cream for Br^- and yellow for I^-.

1. Cl^-: white precipitate (silver chloride)

2. Br^-: cream precipitate (silver bromide)

3. I^-: yellow precipitate (silver iodide)

In the test for sulfate ions, hydrochloric acid is added first to remove any carbonate ions that could interfere with the results.

False.

Nitric acid must be used to acidify the sample before the halide ion test. Using hydrochloric acid would introduce Cl^- ions, which would interfere with the results.

The sample is acidified with dilute hydrochloric acid to remove any carbonate ions that may be present. Carbonates would also form a precipitate with barium chloride, giving a false positive result.

CO₃^2- ions react with dilute acid to produce carbon dioxide gas. This gas turns limewater cloudy white.

Barium chloride solution (acidified with dilute HCl) is used to test for SO₄^2- ions. A positive result is a white precipitate of barium sulfate, BaSO₄.

Anhydrous copper(II) sulfate is a white solid that turns blue in the presence of water, as it absorbs water to form hydrated copper(II) sulfate. It is used as a chemical test for water.

The chemical test for water is to add anhydrous copper(II) sulfate to the sample. A positive result is that the solid turns from white to blue, indicating water is present.

Anhydrous copper(II) sulfate is white in colour. In the presence of water it turns blue.

False.

Anhydrous copper(II) sulfate turns from white to blue in the presence of water. The white anhydrous form absorbs water to become the blue hydrated form.

The boiling point of the sample is measured. Pure water boils at exactly 100 °C. Impurities raise the boiling point above 100 °C, indicating the water is not pure.

Pure water has a boiling point of exactly 100 °C. Any impurities will raise the boiling point.

True.

Dissolving substances in water raises its boiling point above 100 °C. This is why the boiling point test can be used to check whether a water sample is pure.

Tasting or smelling unknown substances is hazardous in a laboratory. Other colourless liquids may also have no taste or smell, so these properties would not reliably identify water.