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Testing for Ions

Lerne mit deinen Freunden und bleibe auf dem richtigen Kurs mit deinen persönlichen Lernstatistiken

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The water that you drink, the pool that you swim and the food that you eat all have many different chemicals. However, there are some chemicals that we can ingest but this is to a certain point. When looking at swimming pools, for example, chlorine is added to kill any bacteria, but how can we ensure that it is in there? This is where testing for ions comes in, as we can take a sample of water and test for the chloride ions. We will be going through this test, as well as many others.

• We will first go over the test for metal ions; this will include the flame test and using sodium hydroxide solution.
• After, we will explore the test for carbonate ions.
• We will then go on to the test for sulphate ions.
• Then we will explore the test for halide ions.
• Finally, we will go through the test for ammonium ions.

Test for metal ions

Metal ions are cations. This means that they are positively charged and have lost electrons. To detect metal ions we can use two types of tests; using a flame and using sodium hydroxide. Metal ions are quite interesting because they are different bright colours, this makes it easier to distinguish between them and also makes it more fun to learn. So let us explore them both.

Flame test

The flame test is quite exciting and you might even carry it out in your classroom. During this test, each metal cation produces a different coloured.

Before we explore those different colours let us go through how we experiment:

1. First, get a nichrome wire loop and dip it into concentrated hydrochloric acid, then heat it. This is to clean the wire.
2. Then dip it into the acid again and then into the metal compound that is being tested.
3. Now hold the loop in the blue flame of a Bunsen burner.
4. You will see a specific colour, and you can compare this to when we explore the different colours. If a metal compound has a mixture of metals, some of the flame colours may be dominated by others.

Now to explore the different colours you will see and match them to the right metal ion. Lithium ions, Li+, produce a crimson flame colour, sodium ions (Na+) produce a yellow flame colour, potassium ions, K+, produce a lilac flame colour, calcium ions (Ca2+) produce an orange-red flame colour, and copper ions, Cu2+, produces a green flame colour. This is summarised in the table below.

 Metal Ion Flame Colour Lithium ions, Li+ Crimson Sodium ions, Na+ Yellow Potassium ions, K+ Lilac Calcium ions, Ca2+ Orange-red Copper, Cu2+ Green

Table 1: Different metal ions and the coloured flame they produce in the flame test.

Using sodium hydroxide

We can also use sodium hydroxide solution to detect some unknown compounds. But this has a smaller range and we do not have the same reaction for all the metal ions. The first group is aluminium ions Al3+, calcium ions Ca2+ and magnesium ions Mg2+. When sodium hydroxide is added to the compound with these ions, a white precipitate is formed.

If you carry on adding sodium hydroxide solution and the precipitate dissolves, this means the substance has aluminium ions. However, if the precipitate does not dissolve it can be calcium ions or magnesium ions, which can be tested further with the flame test that we explored above.

For the second group of metal ions, we can sodium hydroxide solution to detect produced coloured precipitates. If a blue precipitate appears, copper (II) ions are in the substance, if a green precipitate is produced, iron (II) ions are present and if a brown precipitate is formed, iron (III) is present. If you go on to study A-Level chemistry you will also cover this topic so it is important that you also understand it at the GCSE level.

Test for carbonate ions

When testing for carbonate ions, we have a two-stage process:

1. We first start with the solution we are testing, to which we add some dilute acid. This will produce carbon dioxide gas.
2. We can then test this gas by bubbling it through a test tube with lime water. If carbonates are present the limewater will turn from a clear solution to a milky cloudy white colour.

Looking at the diagram above you can see that the test tube with our sample has a bung and a tube that connects it to the tube with the limewater. This is to ensure none of the carbon dioxide escapes and can be used to test the presence of carbonate ions.

If you were exploring a compound that is not unknown such as magnesium carbonate MgCO3, when this reacts with the acid, magnesium chloride produces MgCl2, water and carbon dioxide.

$MgCO_3(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2O(l) + CO_2(g)$

This carbon dioxide can be bubbled into lime water and confirm that we have carbonate ions.

Test for halide ions

Our next test is to detect the different halide ions, these are chloride ions, Cl-; bromide ions, Br-; and iodide ions, I-.

For this reaction, we start by adding some dilute nitric acid to the substance we are testing. This removes any carbonate ions produce, which can affect our results.

We then add some silver nitrate and if a precipitate forms, this means halide ions are present.

This test also allows us to determine exactly which halide ion is present, so there is no need to carry out any additional tests. To distinguish between the different halides, we will observe the colour of the precipitate produced. If a yellow precipitate is produced, iodide ions are present, if a cream precipitate is produced, bromide ions are present, and if a white precipitate is produced, chloride ions are present. This is summarised in the tablet below.

 Halide Ion Precipitate colour Chloride ions Cl- White Bromide ions Br- Cream Iodide ions I- Yellow

Table 2: Different halide ions and the coloured precipitate they produce when reacting with nitric acid and silver nitrate.

Test for ammonium ions

The final test that we are going to explore is how to detect ammonium ions (NH4+). You can find ammonium ions in a variety of different substances such as ammonium solution or any ammonium salt like ammonium chloride.

We start the process by adding dilute sodium hydroxide to our substance, we then go on to gently heat this combination.

If ammonium ions are present, the ions will convert to ammonia gas. You can then test this using damp red litmus paper, where if there are ammonium ions, will turn blue.

Testing for Ions - Key takeaways

• To test for metal ions we can use the flame test and sodium hydroxide.
• Depending on the metal a different colour will appear, allowing us to identify the metal present.
• To test for carbonate ions we use dilute acid and bubble the gas produced into lime water, if it turns milky white, carbonate ions are present.
• To test for sulphate ions we use hydrochloric acid and barium chloride solution, if sulphate ions are produced you will see a white precipitate produced.
• To detect halide ions we used dilute nitric acid and silver nitrate, the result will tell us if there are halide ions present and which ones.
• To detect ammonium ions we use sodium hydroxide, if ammonium ions are present damp red litmus paper will turn blue.

We can use the flame test or sodium hydroxide solution

To detect chloride  ions we used dilute nitric acid and silver nitrate, if a white precipitate is used chloride ions are present.

We use barium chloride as when it reacts with sulphate, barium sulphate is produce, a white precipitate is produced allowing us to determine if sulphate ions are in a substance.

There are many ways to test for ions depending on which ion you are looking for. For metal ions you can use the flame test or sulphuric acid. To test for carbonate ions you use dilute acid and bubble the gas through lime water. To test for sulphate ions we use hydrochloric acid and barium chloride solution, if sulphate ions are produced you will see a white precipitate produced. To detect halide ions we used dilute nitric acid and silver nitrate, the result will tell us if there are halide ions present and which ones. To detect ammonium ions we use sodium hydroxide, if ammonium ions are present damp red litmus paper will turn blue.

To determine the difference between different ions

Testing for Ions Quiz - Teste dein Wissen

Question

What is the sodium hydroxide test?

An analytical technique.

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Question

What do you use the sodium hydroxide test to test for?

Positive cations

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Question

Which metal cations CAN'T be identified using the sodium hydroxide test?

Group 1 cations

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Question

Outline how you carry out the sodium hydroxide test to identify a metal cation in solution.

1. Pour approximately 10 cm3 of a metal cation solution into a test tube.
2. Add a few drops of sodium hydroxide solution to the test tube using a pipette.
3. Observe any changes, noting down the results.
4. Gradually add more sodium hydroxide until it is in excess, once again noting any observations.
5. Repeat the experiment with different metal cation solutions.

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Question

How does the sodium hydroxide test work?

In the sodium hydroxide test, hydroxide ions react with aqueous cations to form an insoluble ionic hydroxide salt, which precipitates out of solution. You can use the colour and solubility of the precipitate to identify the cation present.

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Question

In the sodium hydroxide test, calcium ions form a ____ precipitate.

White

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Question

In the sodium hydroxide test, magnesium ions form a ____ precipitate.

White

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Question

In the sodium hydroxide test, aluminium ions form a ____ precipitate.

White

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Question

In the sodium hydroxide test, copper(II) ions form a ____ precipitate.

Blue

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Question

In the sodium hydroxide test, iron(II) ions form a ____ precipitate.

'Dirty' green

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Question

In the sodium hydroxide test, iron(III) ions form a ____ precipitate.

Red-brown

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Question

1. Give three ions that form a white precipitate in the sodium hydroxide test.
2. Explain how you can distinguish between these ions.

1. Magnesium (Mg2+), calcium (Ca2+), aluminium (Al3+).
2. Add further sodium hydroxide until it is in excess. The aluminium hydroxide precipitate will dissolve, whilst the calcium and magnesium hydroxide precipitates are insoluble. You can then tell the calcium and magnesium ions apart by carrying out a flame test.

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Question

Give the chemical formula of magnesium hydroxide.

Mg(OH)2

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Question

What is the charge of an OH- ion?

-1

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Question

What are anions?

Negative ions

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Question

Which of the following are anions?

Group 1 metal ions

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Question

We test for anions in ____.

Giant covalent molecules

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Question

Give the method for testing an unknown solution for carbonate ions.

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Question

Give the method for testing an unknown solution for sulfate ions.

Add hydrochloric acid and barium chloride solution.

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Question

Give the method for testing an unknown solution for halide ions.

Add dilute nitric acid and silver nitrate solution.

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Question

You carry out a test for carbonate ions. How do you know if you have a positive result?

Carbon dioxide gas is given off. You can confirm that this gas is carbon dioxide by bubbling it through clear limewater - the limewater should turn cloudy.

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Question

You carry out a test for sulfate ions. How do you know if you have a positive result?

A white precipitate forms.

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Question

You carry out a test for halide ions. How do you know if you have a positive result?

A precipitate forms. Chloride ions give a white precipitate, bromide ions give a cream precipitate, and iodide ions give a yellow precipitate.

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Question

When testing for many anions, you look for precipitates. Which positive anion tests produce a white precipitate?

Test for chloride ions.

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Question

True or false? The test for halide ions uses barium chloride solution.

False. The test for halide ions uses dilute nitric acid and silver nitrate solution. The test for sulfates, on the other hand, uses barium chloride solution (alongside hydrochloric acid).

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Question

Why do we add hydrochloric acid before barium chloride solution when testing for sulfates?

The hydrochloric acid removes any aqueous carbonate ions. These also react with barium chloride solution to produce a white precipitate, and so would give a false positive result.

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Question

True or false? You can use hydrochloric acid instead of dilute nitric acid when testing for halides.

False. Hydrochloric acid contains chloride ions, which would react in the test and produce a false positive result.

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Question

True or false? The test for halides is a precipitation reaction.

True

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Question

Give two advantages of tests for anions.

For example:

1. They are cheap.
2. They are simple to carry out.
3. They use readily accessible reagents.

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Question

Give two disadvantages of tests for anions.

For example:

1. They are less sensitive and accurate than other analytical techniques.
2. They can't be used for trace amounts.
3. They can produce ambiguous results.
4. They are qualitative, not quantitative.

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Question

What are metal ions?

Metal ions are positively charged particles formed when a metal atom loses one or more electrons.

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Question

Metals form ____ ions.

Positive

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Question

Metals form _____.

Cations

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Question

Which of the following are tests for metal ions?

Flame tests

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Question

Outline a method for identifying an unknown metal solution using a flame test.

1. Dip a wire loop into concentrated hydrochloric acid, then hold it in a blue Bunsen burner flame.
2. Dip the loop in the concentrated acid a second time before dipping it into the unknown metal ion solution.
3. Hold the loop in the Bunsen burner flame once again and use the colour of the flame to identify the metal.

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Question

True or false? ALL metals burn with a coloured flame.

False

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Question

True or false? ALL metals produce a precipitate in the sodium hydroxide test.

False

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Question

Give the colours of the flames associated with the following metal ions:

1. Li+
2. Na+
3. K+
4. Ca2+
5. Cu2+

1. Crimson
2. Yellow
3. Lilac
4. Orange-red
5. Green

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Question

Outline a method for testing an unknown metal ion solution using the sodium hydroxide test.

1. Pour approximately 10 cm3 of a metal cation solution into a test tube.
2. Add a few drops of sodium hydroxide solution to the test tube using a pipette, observing any changes.
3. Add more sodium hydroxide until it is in excess, again noting any observations.
4. Use your observations to identify the unknown metal ion.

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Question

Describe the changes you'd expect to see when you add sodium hydroxide to solutions containing the following metal ions:

1. Ca2+
2. Mg2+
3. Al3+
4. Fe2+
5. Fe3+
6. Cu2+

1. White insoluble precipitate.
2. White insoluble precipitate.
3. White precipitate, soluble in excess sodium hydroxide.
4. Green insoluble precipitate that slowly turns brown.
5. Red-brown insoluble precipitate.
6. Blue insoluble precipitate

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Question

Why do you dip the wire loop in hydrochloric acid before each flame test?

To clean the loop and remove any impurities.

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Question

Why do you use a platinum or nichrome wire loop in flame tests?

Platinum and nichrome are both stable under high temperatures and don't affect the colour of the flame.

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Question

True or false? All metals give unique, distinctive results in the sodium hydroxide test.

False

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Question

Ca2+ and Mg2+ ions both form an insoluble white precipitate in the sodium hydroxide test. How can you tell these two metals apart?

Carry out further analytical tests, such as flame tests. Ca2+ ions produce an orange-red flame, whilst Mg2+ ions don't affect the colour of the flame.

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Question

Flame tests and the sodium hydroxide test are _____.

Qualitative

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Question

Give two advantages of chemical tests for metal ions.

E.g:

• They are cheap to run.
• They are simple to carry out.
• They don't require specialist equipment.

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Question

Give two disadvantages of chemical tests for metal ions.

E.g:

• They are not as sensitive, accurate, or fast as instrumental analytical methods.
• They are qualitative, not quantitive.
• They aren't useful for mixtures.
• They give ambiguous results and can't identify all metal ions.

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Question

In a test for carbonate ions, when the gas is bubbled through lime water, what colour does the lime water turn?

Milky cloud white

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Question

In a halide ion test, if a yellow precipitate is produced, what ion is present?

Iodide ions

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Question

In a halide ion test, if a white precipitate is produced, what ion is present?

Chloride ions

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