Qualitative Analysis: Tests for Ions Flashcards

All 35 cards in this deck

What flame colour does a lithium ion, $\mathrm{Li^+}$, give in a flame test?

Red.

What flame colour does a sodium ion, $\mathrm{Na^+}$, give in a flame test?

Yellow.

Which metal ion gives a lilac flame in a flame test?

The potassium ion, $\mathrm{K^+}$.

What flame colour does a calcium ion, $\mathrm{Ca^{2+}}$, give in a flame test?

Orange-red.

Which metal ion gives a blue-green flame in a flame test?

The copper ion, $\mathrm{Cu^{2+}}$.

What are the steps to carry out a flame test on a solid?
e.g. a solid containing lithium ions

1. Clean the flame test wire by dipping it in dilute hydrochloric acid.
2. Dip the wire into the solid.
3. Hold the wire in the hottest part of a blue (roaring) Bunsen flame, at the tip of the blue cone, and note the colour — red for lithium.

What do you see when sodium hydroxide solution is added to a solution containing copper ions, $\mathrm{Cu^{2+}}$?

A blue precipitate.

What do you see when sodium hydroxide solution is added to a solution containing iron(II) ions, $\mathrm{Fe^{2+}}$?

A green precipitate.
It slowly turns brown in air as $\mathrm{Fe^{2+}}$ is oxidised to $\mathrm{Fe^{3+}}$.

What do you see when sodium hydroxide solution is added to a solution containing iron(III) ions, $\mathrm{Fe^{3+}}$?

A brown precipitate.

What do you see when sodium hydroxide solution is added to a solution containing aluminium ions, $\mathrm{Al^{3+}}$?

A white precipitate that redissolves in excess sodium hydroxide solution to give a colourless solution.

What do you see when sodium hydroxide solution is added to a solution containing calcium ions, $\mathrm{Ca^{2+}}$?

A white precipitate, which does not redissolve in excess sodium hydroxide solution.

What are the steps to write the ionic equation for a metal hydroxide precipitating with hydroxide ions?
e.g. copper ions, $\mathrm{Cu^{2+}}$

1. Write the metal ion with its charge: $\mathrm{Cu^{2+}}$
2. Add enough $\mathrm{OH^-}$ ions to balance the charge: $\mathrm{Cu^{2+} + 2OH^-}$
3. The product is the hydroxide precipitate: $\mathrm{Cu^{2+} + 2OH^- \rightarrow Cu(OH)_2}$

What is the chemical test for ammonia gas, and the result?

Hold damp red litmus paper in the gas — it turns blue.

What are the steps to test a solid for ammonium ions, $\mathrm{NH_4^+}$?
e.g. testing solid ammonium chloride

1. Add sodium hydroxide solution to the solid.
2. Warm the mixture gently.
3. Hold damp red litmus paper at the mouth of the tube — it turns blue, showing ammonia, so $\mathrm{NH_4^+}$ is present.

True or false? Ammonium ions give a white precipitate when sodium hydroxide solution is added.

False.
No precipitate forms; ammonia gas is released on warming.

Why must the litmus paper be damp in the test for ammonia gas?

The gas must dissolve in the water to show its alkaline effect on the litmus.

Why is the mixture warmed when testing for ammonium ions with sodium hydroxide solution?

To release the ammonia gas so it can be detected.

What is the test for carbonate ions, $\mathrm{CO_3^{2-}}$, and the result?

Add dilute acid — it fizzes; the gas given off turns limewater milky (cloudy), showing carbon dioxide.

What is the test for sulfate ions, $\mathrm{SO_4^{2-}}$, and the result?

Add dilute hydrochloric acid, then barium chloride solution — a white precipitate forms.

What is the test for chloride ions, $\mathrm{Cl^-}$, and the result?

Add dilute nitric acid, then silver nitrate solution — a white precipitate forms.

What colour precipitate does a bromide ion, $\mathrm{Br^-}$, give with dilute nitric acid and silver nitrate solution?

Cream.

What colour precipitate does an iodide ion, $\mathrm{I^-}$, give with dilute nitric acid and silver nitrate solution?

Yellow.

True or false? Dilute hydrochloric acid should be added before silver nitrate solution when testing for chloride ions.

False.
Hydrochloric acid contains chloride ions, so dilute nitric acid must be used instead.

Why must the test for any ion be unique?

If more than one ion gave the same result, the test could not identify which ion is present.

True or false? A white precipitate with sodium hydroxide solution identifies the calcium ion.

False.
More than one cation gives a white precipitate (e.g. aluminium, calcium, magnesium, zinc, lead).

True or false? Sodium hydroxide solution can be used to test for anions.

False.
Sodium hydroxide solution only tests for cations.

Why is dilute acid added before barium chloride solution in the test for sulfate ions?

To remove carbonate ions, which would also give a white precipitate and a false positive.

What safety precaution is needed when using sodium hydroxide solution in ion tests, and why?

Wear eye protection — sodium hydroxide solution is corrosive/an irritant.

What are the steps to identify both ions in an unknown salt?
e.g. a solid gives an orange-red flame, and a white precipitate with dilute nitric acid and silver nitrate solution

1. Test for the cation (flame test or sodium hydroxide solution): orange-red flame means calcium ions.
2. Test for the anion: white precipitate with silver nitrate means chloride ions.
3. Name the salt: calcium chloride.

What three advantages do instrumental methods of analysis have over chemical tests?

They are more sensitive, more accurate and faster.

Which instrument is used to identify and measure metal ions in dilute solutions?

A flame photometer (flame emission spectroscopy).

How are metal ions in a sample identified from flame photometer data?

By comparing the sample's emissions with reference data for known metal ions.

How is a flame photometer calibration curve produced?

By measuring the readings of solutions of known concentration and plotting reading against concentration.

True or false? A flame test can show that both sodium and potassium ions are present in the same sample.

False.
The bright yellow sodium flame masks the lilac potassium flame; a flame photometer shows both sets of emissions.

True or false? The green precipitate formed when sodium hydroxide solution is added to iron(II) ions turns brown on standing in air.

True. The iron(II) hydroxide is oxidised by air to brown iron(III) hydroxide.