Required Practical: Identifying Ions Flashcards

All 6 cards in this deck

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.

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, $K_2CO_3$.