4.2.1 Tests to identify gases

Hydrogen burns with a squeaky pop

Collect the gas in a test tube and hold a lit splint at the mouth of the tube.
Hydrogen gives a squeaky pop, which is the sound of hydrogen and oxygen from the air burning very rapidly.
1. The reaction forms water: $2H2(g)+O2(g)→2H2O(l)2\text{H}_2\text{(g)} + \text{O}_2\text{(g)} \rightarrow 2\text{H}_2\text{O(l)}$
Hydrogen is produced when a metal reacts with a dilute acid, or with water in the case of very reactive metals.
Hydrogen has a low density, so a tube of hydrogen is held with its mouth pointing down to stop the gas escaping.

Common Mistake

Do not describe the hydrogen test with a glowing splint, because a glowing splint is the test for oxygen.

Oxygen relights a glowing splint

Definition

Glowing splint

A wooden splint that has been lit and blown out, so that its tip glows without a flame.

Light a wooden splint, blow it out so that only the tip glows, then put it into the gas.
Oxygen relights the splint, because it lets the hot wood burn much faster than it does in air.
1. Air contains only about 21% oxygen, which is not enough to relight a glowing splint.
Oxygen can be made by adding manganese(IV) oxide, a catalyst, to hydrogen peroxide solution.
1. The hydrogen peroxide decomposes into water and oxygen: $2H2O2(aq)→2H2O(l)+O2(g)2\text{H}_2\text{O}_2\text{(aq)} \rightarrow 2\text{H}_2\text{O(l)} + \text{O}_2\text{(g)}$

Practical

Collecting a gas to test it

Aim: collect a sample of gas so that it can be tested.
Apparatus: a conical flask with a bung and delivery tube, a trough of water, test tubes with bungs, a test tube rack, the reactants that make the gas, wooden splints and a lighter.

Method

Fill a test tube with water and turn it upside down in the trough, keeping its mouth under the water.
Set up the reaction in the conical flask and fit the bung and delivery tube.
Hold the test tube over the end of the delivery tube.
Let the first few bubbles escape, because they are mostly air pushed out of the apparatus.
When the tube is full of gas, put a bung in it while its mouth is still under water.
Lift out the tube, remove the bung and test the gas straight away, before it escapes or mixes with air.

Accuracy and safety

Collect a gas that dissolves well in water, such as chlorine, in a dry tube instead, because it would dissolve in the water in the trough.
If the flask is heated, lift the delivery tube out of the water before you stop heating, so that water is not sucked back into the hot flask.
Test chlorine in a fume cupboard, because it is toxic.
Wear eye protection throughout.

Carbon dioxide turns limewater milky

Definition

Limewater

A colourless solution of calcium hydroxide in water, used to test for carbon dioxide.

Definition

Precipitate

An insoluble solid that forms when two solutions are mixed and react.

Bubble the gas through limewater using a delivery tube.
Carbon dioxide turns the limewater milky, or cloudy.
1. The cloudiness is a white precipitate of calcium carbonate: $CO2(g)+Ca(OH)2(aq)→CaCO3(s)+H2O(l)\text{CO}_2\text{(g)} + \text{Ca(OH)}_2\text{(aq)} \rightarrow \text{CaCO}_3\text{(s)} + \text{H}_2\text{O(l)}$
Carbon dioxide is released when a carbonate reacts with an acid and when fuels containing carbon burn.
A lit splint cannot confirm carbon dioxide, because other gases, such as nitrogen, also put out a flame.

Common Mistake

Do not describe the limewater result as fizzing or bubbling, because the observation that identifies carbon dioxide is the milky colour.

Chlorine bleaches damp litmus paper

Definition

Litmus paper

Paper containing an indicator that is red in acidic conditions and blue in alkaline conditions.

Hold a piece of damp blue litmus paper in the gas, using tongs.
Chlorine turns the paper red and then bleaches it white.
1. The paper must be damp, because chlorine dissolves in the water on the paper to form an acidic solution that bleaches.
Chlorine is a pale green gas with a sharp smell, and it is toxic, so the test is done in a fume cupboard.
Chlorine is produced at the positive electrode during the electrolysis of a solution containing chloride ions.

Common Mistake

Do not call the gas chloride, because chloride is the name of the ion and chlorine is the name of the element and the gas.

How a gas was made suggests its test

A metal reacting with an acid suggests hydrogen, so test the gas with a lit splint.
A carbonate reacting with an acid suggests carbon dioxide, so bubble the gas through limewater.
Hydrogen peroxide decomposing, or pondweed photosynthesising, suggests oxygen, so test the gas with a glowing splint.
The electrolysis of a chloride solution can give chlorine, so test the gas with damp litmus paper.
A negative result rules a gas out, so a gas that leaves limewater clear is not carbon dioxide.

Exam technique

When a question asks you to describe a gas test, give both what you do and what you see, such as putting a glowing splint into the gas and seeing it relight.
Name the observation precisely, because 'relights', 'squeaky pop', 'milky' and 'bleached' identify a gas, while 'goes out' or 'changes colour' do not.

Self review

How is hydrogen identified?
Why does a glowing splint relight in oxygen but not in air?
What forms when carbon dioxide reacts with limewater?
Why must the litmus paper be damp in the test for chlorine?
Which test would you use on the gas given off when marble chips react with hydrochloric acid?

4.2.2 Tests for aqueous ions and flame tests

Carrying out a flame test

Definition

Flame test

A test that identifies some metal ions from the colour they give to a blue Bunsen burner flame.

Clean a nichrome wire loop by dipping it in hydrochloric acid and holding it in a blue Bunsen flame until it gives no colour.
1. Cleaning removes traces of other ions, which would add their colours to the flame.
Dip the clean loop into the solid sample or a solution of it.
Hold the loop at the edge of a blue flame and note the colour produced.
1. A blue flame is used because the yellow safety flame would hide the colour of the ion.
Clean the wire again, or use a fresh loop, before testing the next sample.

Practical

Aim: identify the metal ion in a solid from the colour it gives a flame.
Apparatus: a Bunsen burner, a heatproof mat, a nichrome wire loop with a handle, dilute hydrochloric acid in a small beaker or watch glass, the solids to test, known samples of lithium, sodium, potassium, calcium and copper(II) compounds, and wooden splints.

Method

Light the Bunsen burner and open the air hole to give a roaring blue flame.
Dip the wire loop in dilute hydrochloric acid and hold it in the flame, repeating until the flame shows no colour.
Dip the clean loop in the acid again, then into a small amount of the solid so that some sticks to it.
Hold the loop at the edge of the flame and record the colour.
Clean the loop again, or use a new one, before each sample.
Compare each colour with the flames from the known samples.

Accuracy and safety

A clean loop and a blue flame stop other colours hiding the colour of the metal ion.
Testing known samples helps to tell apart similar colours, such as the reds from lithium and calcium ions.
If no nichrome wire is available, soak a wooden splint in water, dip it in the solid and hold it in the flame, taking it out before the wood catches fire.
Wear eye protection when using hydrochloric acid, and tie back long hair near the flame.

Five metal ions give characteristic flame colours

Definition

Cation

An ion with a positive charge, such as a metal ion.

Lithium ions, $Li+\text{Li}^{+}$ , give a red flame.
Sodium ions, $Na+\text{Na}^{+}$ , give a yellow-orange flame.
Potassium ions, $K+\text{K}^{+}$ , give a lilac flame.
Calcium ions, $Ca2+\text{Ca}^{2+}$ , give an orange-red flame.
Copper(II) ions, $Cu2+\text{Cu}^{2+}$ , give a green flame.
The flame test identifies only the metal ion, so a second test is needed to identify the negative ion in the compound.

Common Mistake

Do not rely on a flame test for a mixture, because the strong yellow-orange of sodium ions can hide the colours of other ions.
Lithium and calcium give similar red colours, so compare an unclear flame with flames from known samples.

Sodium hydroxide gives coloured metal hydroxide precipitates

Definition

Precipitate

An insoluble solid that forms when two solutions are mixed and react.

Add a few drops of sodium hydroxide solution to a solution of the sample.
The metal ions react with hydroxide ions to form an insoluble metal hydroxide, which appears as a precipitate.
Copper(II) ions give a blue precipitate of copper(II) hydroxide: $CuSO4(aq)+2NaOH(aq)→Cu(OH)2(s)+Na2SO4(aq)\text{CuSO}_4\text{(aq)} + 2\text{NaOH(aq)} \rightarrow \text{Cu(OH)}_2\text{(s)} + \text{Na}_2\text{SO}_4\text{(aq)}$
Iron(II) ions give a green precipitate of iron(II) hydroxide.
1. The green precipitate slowly turns orange-brown at its surface as it reacts with oxygen in the air.
Iron(III) ions give an orange-brown precipitate of iron(III) hydroxide.

Example

A pale green solution gives a green precipitate with sodium hydroxide solution, so it contains iron(II) ions.

Tests for carbonate, halide and sulfate ions

Definition

Anion

An ion with a negative charge, such as a halide, sulfate or carbonate ion.

Definition

Halide ion

The negative ion formed when a halogen atom gains one electron, such as chloride, bromide or iodide.

Carbonate ions, $CO32−\text{CO}_3^{2-}$ $CO_{3}^{2 -}$ : add dilute acid and bubble any gas given off through limewater.
1. Fizzing, with the limewater turning milky, shows that carbon dioxide formed, so carbonate ions are present.
Halide ions: add dilute nitric acid, then silver nitrate solution.
1. Chloride ions, $Cl−\text{Cl}^{-}$ , give a white precipitate of silver chloride.
2. Bromide ions, $Br−\text{Br}^{-}$ , give a cream precipitate of silver bromide.
3. Iodide ions, $I−\text{I}^{-}$ , give a yellow precipitate of silver iodide.
Sulfate ions, $SO42−\text{SO}_4^{2-}$ $SO_{4}^{2 -}$ : add dilute hydrochloric acid, then barium chloride solution.
1. A white precipitate of barium sulfate shows that sulfate ions are present: $Na2SO4(aq)+BaCl2(aq)→BaSO4(s)+2NaCl(aq)\text{Na}_2\text{SO}_4\text{(aq)} + \text{BaCl}_2\text{(aq)} \rightarrow \text{BaSO}_4\text{(s)} + 2\text{NaCl(aq)}$

Common Mistake

Do not acidify with hydrochloric acid before adding silver nitrate, because its chloride ions would give a white precipitate whatever the sample contains.
The acid goes in first to remove carbonate ions, which would otherwise form a precipitate of their own and give a false result.

Combining the results for both ions identifies a compound

Test separate portions of the sample for each ion, so that one reagent does not interfere with another test.
Use a flame test or sodium hydroxide solution to identify the positive ion.
Use the carbonate, halide and sulfate tests to identify the negative ion.
Put the two ions together, using their charges to name the compound and write its formula.
Give the ion names, such as bromide rather than bromine, because the sample contains ions and not the element.

Example

Identifying a compound from two ion tests

A white solid gives a red flame, and a solution of it gives a cream precipitate with dilute nitric acid and silver nitrate solution; identify the solid.
The red flame shows lithium ions, $Li+\text{Li}^{+}$ .
The cream precipitate with acidified silver nitrate shows bromide ions, $Br−\text{Br}^{-}$ .
The charges cancel in a $1 : 1$ ratio, so the solid is lithium bromide, $LiBr\text{LiBr}$ .

Self review

How is a nichrome wire cleaned before a flame test?
What flame colour do potassium ions give?
What colour is the precipitate that iron(III) ions give with sodium hydroxide solution?
Why is dilute nitric acid, rather than hydrochloric acid, added before silver nitrate?
A solution gives a blue precipitate with sodium hydroxide and a white precipitate with acidified barium chloride; what is the compound?

4.2.3 Instrumental methods of analysis

Instrumental methods use machines to detect and measure substances

Definition

Instrumental method

A method of analysis that uses a machine to detect and measure a property of a substance.

A chemical test, such as a flame test, depends on a person seeing and judging a colour, a precipitate or a sound.
An instrumental method uses a machine to measure a property of the sample, and a computer usually records the result.
Flame emission spectroscopy heats a sample in a flame and measures the light given out, which identifies the metal ions present.
Gas chromatography separates the substances in a mixture and records the time each one takes to pass through the instrument.
The result is a chart or table of measurements, which is then compared with results for known substances.

Example

Hospital laboratories analyse blood samples with instruments, and water companies use them to check drinking water for traces of metal ions.

Instrumental methods are sensitive, accurate and rapid

Definition

Sensitivity

How small an amount of a substance a method of analysis can detect.

They are sensitive, so they detect substances present in very small amounts, even in a tiny sample.
1. A flame test needs enough of the metal ion to colour the flame, whereas flame emission spectroscopy detects traces far too small to see.
They are accurate, giving measurements close to the true value, so they identify substances reliably and can show how much is present.
1. The result does not depend on a person's judgement of a colour, so a yellow-orange flame cannot be mistaken for an orange-red one.
They are rapid, so results come quickly and a machine can analyse many samples one after another, day and night.
The main drawback is that the instruments are expensive to buy and need trained people to run them.

Common Mistake

Do not give 'easier' or 'safer' as advantages of instrumental methods, because the accepted advantages are sensitivity, accuracy and speed.

Identifying substances by matching a reference chart

Definition

Flame emission spectroscopy

An instrumental method that heats a sample in a flame and splits the light given out into a line spectrum, which identifies the metal ions present.

Definition

Reference data

Results recorded for known substances, under the same conditions as an unknown sample, which are compared with the unknown to identify it.

In flame emission spectroscopy, the light from the flame is split into a line spectrum, a set of bright lines at particular wavelengths.
Each metal ion gives its own pattern of lines, like a fingerprint.
To identify an unknown sample, compare its lines with reference spectra for known metal ions, drawn on the same scale.
A metal ion is present only if every line in its reference spectrum appears in the spectrum of the sample.
1. Lines left over after one match belong to another ion, so one spectrum can identify each ion in a mixture.

Example

In a simplified set of reference spectra, calcium ions give a line at $nm423\,\text{nm}$ and sodium ions a line at $nm589\,\text{nm}$ , so a sample whose spectrum shows both lines contains calcium ions and sodium ions.

Identifying substances by matching a reference table

Definition

Retention time

The time a substance takes to pass through a gas chromatography instrument, from injection to detection.

In gas chromatography, each substance leaves the instrument after its own retention time, measured under fixed conditions.
The instrument records a peak for each substance, so the number of peaks shows how many substances the sample contains.
The retention times from the sample are compared with a reference table for known substances, measured under the same conditions.
A substance is identified when its retention time matches a reference value.
A value that matches no reference belongs to a substance that cannot be identified from the data given.

Example

Identifying alcohols in a sample from retention times

Under the same conditions, ethanol has a retention time of $2.1$ minutes, propan-1-ol $3.4$ minutes and butan-1-ol $5.0$ minutes; a sample gives peaks at $2.1$ and $5.0$ minutes, so identify the substances in it.
Count the peaks: two peaks show two substances.
The peak at $2.1$ minutes matches ethanol, and the peak at $5.0$ minutes matches butan-1-ol.
No peak appears at $3.4$ minutes, so the sample contains ethanol and butan-1-ol but no propan-1-ol.

Self review

Give three advantages of instrumental methods over chemical tests.
What does it mean for an instrumental method to be sensitive?
How is a metal ion identified from a line spectrum?
Why must reference data be recorded under the same conditions as the sample?
A gas chromatogram shows three peaks; what does this tell you about the sample?

4.2.1 Tests to identify gases

Hydrogen burns with a squeaky pop

Collect the gas in a test tube and hold a lit splint at the mouth of the tube.
Hydrogen gives a squeaky pop, which is the sound of hydrogen and oxygen from the air burning very rapidly.
1. The reaction forms water: $2H2(g)+O2(g)→2H2O(l)2\text{H}_2\text{(g)} + \text{O}_2\text{(g)} \rightarrow 2\text{H}_2\text{O(l)}$
Hydrogen is produced when a metal reacts with a dilute acid, or with water in the case of very reactive metals.
Hydrogen has a low density, so a tube of hydrogen is held with its mouth pointing down to stop the gas escaping.

Common Mistake

Do not describe the hydrogen test with a glowing splint, because a glowing splint is the test for oxygen.

Oxygen relights a glowing splint

Definition

Glowing splint

A wooden splint that has been lit and blown out, so that its tip glows without a flame.

Light a wooden splint, blow it out so that only the tip glows, then put it into the gas.
Oxygen relights the splint, because it lets the hot wood burn much faster than it does in air.
1. Air contains only about 21% oxygen, which is not enough to relight a glowing splint.
Oxygen can be made by adding manganese(IV) oxide, a catalyst, to hydrogen peroxide solution.
1. The hydrogen peroxide decomposes into water and oxygen: $2H2O2(aq)→2H2O(l)+O2(g)2\text{H}_2\text{O}_2\text{(aq)} \rightarrow 2\text{H}_2\text{O(l)} + \text{O}_2\text{(g)}$

Practical

Collecting a gas to test it

Aim: collect a sample of gas so that it can be tested.
Apparatus: a conical flask with a bung and delivery tube, a trough of water, test tubes with bungs, a test tube rack, the reactants that make the gas, wooden splints and a lighter.

Method

Fill a test tube with water and turn it upside down in the trough, keeping its mouth under the water.
Set up the reaction in the conical flask and fit the bung and delivery tube.
Hold the test tube over the end of the delivery tube.
Let the first few bubbles escape, because they are mostly air pushed out of the apparatus.
When the tube is full of gas, put a bung in it while its mouth is still under water.
Lift out the tube, remove the bung and test the gas straight away, before it escapes or mixes with air.

Accuracy and safety

Collect a gas that dissolves well in water, such as chlorine, in a dry tube instead, because it would dissolve in the water in the trough.
If the flask is heated, lift the delivery tube out of the water before you stop heating, so that water is not sucked back into the hot flask.
Test chlorine in a fume cupboard, because it is toxic.
Wear eye protection throughout.

Carbon dioxide turns limewater milky

Definition

Limewater

A colourless solution of calcium hydroxide in water, used to test for carbon dioxide.

Definition

Precipitate

An insoluble solid that forms when two solutions are mixed and react.

Bubble the gas through limewater using a delivery tube.
Carbon dioxide turns the limewater milky, or cloudy.
1. The cloudiness is a white precipitate of calcium carbonate: $CO2(g)+Ca(OH)2(aq)→CaCO3(s)+H2O(l)\text{CO}_2\text{(g)} + \text{Ca(OH)}_2\text{(aq)} \rightarrow \text{CaCO}_3\text{(s)} + \text{H}_2\text{O(l)}$
Carbon dioxide is released when a carbonate reacts with an acid and when fuels containing carbon burn.
A lit splint cannot confirm carbon dioxide, because other gases, such as nitrogen, also put out a flame.

Common Mistake

Do not describe the limewater result as fizzing or bubbling, because the observation that identifies carbon dioxide is the milky colour.

Chlorine bleaches damp litmus paper

Definition

Litmus paper

Paper containing an indicator that is red in acidic conditions and blue in alkaline conditions.

Hold a piece of damp blue litmus paper in the gas, using tongs.
Chlorine turns the paper red and then bleaches it white.
1. The paper must be damp, because chlorine dissolves in the water on the paper to form an acidic solution that bleaches.
Chlorine is a pale green gas with a sharp smell, and it is toxic, so the test is done in a fume cupboard.
Chlorine is produced at the positive electrode during the electrolysis of a solution containing chloride ions.

Common Mistake

Do not call the gas chloride, because chloride is the name of the ion and chlorine is the name of the element and the gas.

How a gas was made suggests its test

A metal reacting with an acid suggests hydrogen, so test the gas with a lit splint.
A carbonate reacting with an acid suggests carbon dioxide, so bubble the gas through limewater.
Hydrogen peroxide decomposing, or pondweed photosynthesising, suggests oxygen, so test the gas with a glowing splint.
The electrolysis of a chloride solution can give chlorine, so test the gas with damp litmus paper.
A negative result rules a gas out, so a gas that leaves limewater clear is not carbon dioxide.

Exam technique

When a question asks you to describe a gas test, give both what you do and what you see, such as putting a glowing splint into the gas and seeing it relight.
Name the observation precisely, because 'relights', 'squeaky pop', 'milky' and 'bleached' identify a gas, while 'goes out' or 'changes colour' do not.

Self review

How is hydrogen identified?
Why does a glowing splint relight in oxygen but not in air?
What forms when carbon dioxide reacts with limewater?
Why must the litmus paper be damp in the test for chlorine?
Which test would you use on the gas given off when marble chips react with hydrochloric acid?

4.2.2 Tests for aqueous ions and flame tests

Carrying out a flame test

Definition

Flame test

A test that identifies some metal ions from the colour they give to a blue Bunsen burner flame.

Clean a nichrome wire loop by dipping it in hydrochloric acid and holding it in a blue Bunsen flame until it gives no colour.
1. Cleaning removes traces of other ions, which would add their colours to the flame.
Dip the clean loop into the solid sample or a solution of it.
Hold the loop at the edge of a blue flame and note the colour produced.
1. A blue flame is used because the yellow safety flame would hide the colour of the ion.
Clean the wire again, or use a fresh loop, before testing the next sample.

Practical

Aim: identify the metal ion in a solid from the colour it gives a flame.
Apparatus: a Bunsen burner, a heatproof mat, a nichrome wire loop with a handle, dilute hydrochloric acid in a small beaker or watch glass, the solids to test, known samples of lithium, sodium, potassium, calcium and copper(II) compounds, and wooden splints.

Method

Light the Bunsen burner and open the air hole to give a roaring blue flame.
Dip the wire loop in dilute hydrochloric acid and hold it in the flame, repeating until the flame shows no colour.
Dip the clean loop in the acid again, then into a small amount of the solid so that some sticks to it.
Hold the loop at the edge of the flame and record the colour.
Clean the loop again, or use a new one, before each sample.
Compare each colour with the flames from the known samples.

Accuracy and safety

A clean loop and a blue flame stop other colours hiding the colour of the metal ion.
Testing known samples helps to tell apart similar colours, such as the reds from lithium and calcium ions.
If no nichrome wire is available, soak a wooden splint in water, dip it in the solid and hold it in the flame, taking it out before the wood catches fire.
Wear eye protection when using hydrochloric acid, and tie back long hair near the flame.

Five metal ions give characteristic flame colours

Definition

Cation

An ion with a positive charge, such as a metal ion.

Lithium ions, $Li+\text{Li}^{+}$ , give a red flame.
Sodium ions, $Na+\text{Na}^{+}$ , give a yellow-orange flame.
Potassium ions, $K+\text{K}^{+}$ , give a lilac flame.
Calcium ions, $Ca2+\text{Ca}^{2+}$ , give an orange-red flame.
Copper(II) ions, $Cu2+\text{Cu}^{2+}$ , give a green flame.
The flame test identifies only the metal ion, so a second test is needed to identify the negative ion in the compound.

Common Mistake

Do not rely on a flame test for a mixture, because the strong yellow-orange of sodium ions can hide the colours of other ions.
Lithium and calcium give similar red colours, so compare an unclear flame with flames from known samples.

Sodium hydroxide gives coloured metal hydroxide precipitates

Definition

Precipitate

An insoluble solid that forms when two solutions are mixed and react.

Add a few drops of sodium hydroxide solution to a solution of the sample.
The metal ions react with hydroxide ions to form an insoluble metal hydroxide, which appears as a precipitate.
Copper(II) ions give a blue precipitate of copper(II) hydroxide: $CuSO4(aq)+2NaOH(aq)→Cu(OH)2(s)+Na2SO4(aq)\text{CuSO}_4\text{(aq)} + 2\text{NaOH(aq)} \rightarrow \text{Cu(OH)}_2\text{(s)} + \text{Na}_2\text{SO}_4\text{(aq)}$
Iron(II) ions give a green precipitate of iron(II) hydroxide.
1. The green precipitate slowly turns orange-brown at its surface as it reacts with oxygen in the air.
Iron(III) ions give an orange-brown precipitate of iron(III) hydroxide.

Example

A pale green solution gives a green precipitate with sodium hydroxide solution, so it contains iron(II) ions.

Tests for carbonate, halide and sulfate ions

Definition

Anion

An ion with a negative charge, such as a halide, sulfate or carbonate ion.

Definition

Halide ion

The negative ion formed when a halogen atom gains one electron, such as chloride, bromide or iodide.

Carbonate ions, $CO32−\text{CO}_3^{2-}$ $CO_{3}^{2 -}$ : add dilute acid and bubble any gas given off through limewater.
1. Fizzing, with the limewater turning milky, shows that carbon dioxide formed, so carbonate ions are present.
Halide ions: add dilute nitric acid, then silver nitrate solution.
1. Chloride ions, $Cl−\text{Cl}^{-}$ , give a white precipitate of silver chloride.
2. Bromide ions, $Br−\text{Br}^{-}$ , give a cream precipitate of silver bromide.
3. Iodide ions, $I−\text{I}^{-}$ , give a yellow precipitate of silver iodide.
Sulfate ions, $SO42−\text{SO}_4^{2-}$ $SO_{4}^{2 -}$ : add dilute hydrochloric acid, then barium chloride solution.
1. A white precipitate of barium sulfate shows that sulfate ions are present: $Na2SO4(aq)+BaCl2(aq)→BaSO4(s)+2NaCl(aq)\text{Na}_2\text{SO}_4\text{(aq)} + \text{BaCl}_2\text{(aq)} \rightarrow \text{BaSO}_4\text{(s)} + 2\text{NaCl(aq)}$

Common Mistake

Do not acidify with hydrochloric acid before adding silver nitrate, because its chloride ions would give a white precipitate whatever the sample contains.
The acid goes in first to remove carbonate ions, which would otherwise form a precipitate of their own and give a false result.

Combining the results for both ions identifies a compound

Test separate portions of the sample for each ion, so that one reagent does not interfere with another test.
Use a flame test or sodium hydroxide solution to identify the positive ion.
Use the carbonate, halide and sulfate tests to identify the negative ion.
Put the two ions together, using their charges to name the compound and write its formula.
Give the ion names, such as bromide rather than bromine, because the sample contains ions and not the element.

Example

Identifying a compound from two ion tests

A white solid gives a red flame, and a solution of it gives a cream precipitate with dilute nitric acid and silver nitrate solution; identify the solid.
The red flame shows lithium ions, $Li+\text{Li}^{+}$ .
The cream precipitate with acidified silver nitrate shows bromide ions, $Br−\text{Br}^{-}$ .
The charges cancel in a $1 : 1$ ratio, so the solid is lithium bromide, $LiBr\text{LiBr}$ .

Self review

How is a nichrome wire cleaned before a flame test?
What flame colour do potassium ions give?
What colour is the precipitate that iron(III) ions give with sodium hydroxide solution?
Why is dilute nitric acid, rather than hydrochloric acid, added before silver nitrate?
A solution gives a blue precipitate with sodium hydroxide and a white precipitate with acidified barium chloride; what is the compound?

4.2.3 Instrumental methods of analysis

Instrumental methods use machines to detect and measure substances

Definition

Instrumental method

A method of analysis that uses a machine to detect and measure a property of a substance.

A chemical test, such as a flame test, depends on a person seeing and judging a colour, a precipitate or a sound.
An instrumental method uses a machine to measure a property of the sample, and a computer usually records the result.
Flame emission spectroscopy heats a sample in a flame and measures the light given out, which identifies the metal ions present.
Gas chromatography separates the substances in a mixture and records the time each one takes to pass through the instrument.
The result is a chart or table of measurements, which is then compared with results for known substances.

Example

Hospital laboratories analyse blood samples with instruments, and water companies use them to check drinking water for traces of metal ions.

Instrumental methods are sensitive, accurate and rapid

Definition

Sensitivity

How small an amount of a substance a method of analysis can detect.

They are sensitive, so they detect substances present in very small amounts, even in a tiny sample.
1. A flame test needs enough of the metal ion to colour the flame, whereas flame emission spectroscopy detects traces far too small to see.
They are accurate, giving measurements close to the true value, so they identify substances reliably and can show how much is present.
1. The result does not depend on a person's judgement of a colour, so a yellow-orange flame cannot be mistaken for an orange-red one.
They are rapid, so results come quickly and a machine can analyse many samples one after another, day and night.
The main drawback is that the instruments are expensive to buy and need trained people to run them.

Common Mistake

Do not give 'easier' or 'safer' as advantages of instrumental methods, because the accepted advantages are sensitivity, accuracy and speed.

Identifying substances by matching a reference chart

Definition

Flame emission spectroscopy

An instrumental method that heats a sample in a flame and splits the light given out into a line spectrum, which identifies the metal ions present.

Definition

Reference data

Results recorded for known substances, under the same conditions as an unknown sample, which are compared with the unknown to identify it.

In flame emission spectroscopy, the light from the flame is split into a line spectrum, a set of bright lines at particular wavelengths.
Each metal ion gives its own pattern of lines, like a fingerprint.
To identify an unknown sample, compare its lines with reference spectra for known metal ions, drawn on the same scale.
A metal ion is present only if every line in its reference spectrum appears in the spectrum of the sample.
1. Lines left over after one match belong to another ion, so one spectrum can identify each ion in a mixture.

Example

Identifying substances by matching a reference table

Definition

Retention time

The time a substance takes to pass through a gas chromatography instrument, from injection to detection.

In gas chromatography, each substance leaves the instrument after its own retention time, measured under fixed conditions.
The instrument records a peak for each substance, so the number of peaks shows how many substances the sample contains.
The retention times from the sample are compared with a reference table for known substances, measured under the same conditions.
A substance is identified when its retention time matches a reference value.
A value that matches no reference belongs to a substance that cannot be identified from the data given.

Example

Identifying alcohols in a sample from retention times

Under the same conditions, ethanol has a retention time of $2.1$ minutes, propan-1-ol $3.4$ minutes and butan-1-ol $5.0$ minutes; a sample gives peaks at $2.1$ and $5.0$ minutes, so identify the substances in it.
Count the peaks: two peaks show two substances.
The peak at $2.1$ minutes matches ethanol, and the peak at $5.0$ minutes matches butan-1-ol.
No peak appears at $3.4$ minutes, so the sample contains ethanol and butan-1-ol but no propan-1-ol.

Self review

Give three advantages of instrumental methods over chemical tests.
What does it mean for an instrumental method to be sensitive?
How is a metal ion identified from a line spectrum?
Why must reference data be recorded under the same conditions as the sample?
A gas chromatogram shows three peaks; what does this tell you about the sample?

Experiment	Reactants	Gas made
A	magnesium and dilute hydrochloric acid	hydrogen
B	calcium carbonate and dilute hydrochloric acid	carbon dioxide
C	hydrogen peroxide solution and manganese(IV) oxide	oxygen
D	electrolysis of sodium chloride solution	chlorine

Experiment	Reactants	Gas made
A	magnesium and dilute hydrochloric acid	hydrogen
B	calcium carbonate and dilute hydrochloric acid	carbon dioxide
C	hydrogen peroxide solution and manganese(IV) oxide	oxygen
D	electrolysis of sodium chloride solution	chlorine

4.2 Identifying the products of chemical reactions

4.2.1 Tests to identify gases

Hydrogen burns with a squeaky pop

Oxygen relights a glowing splint

Collecting a gas to test it

Method

Accuracy and safety

Carbon dioxide turns limewater milky

Chlorine bleaches damp litmus paper

How a gas was made suggests its test

4.2.2 Tests for aqueous ions and flame tests

Carrying out a flame test

Method

Accuracy and safety

Five metal ions give characteristic flame colours

Sodium hydroxide gives coloured metal hydroxide precipitates

Tests for carbonate, halide and sulfate ions

Combining the results for both ions identifies a compound

Identifying a compound from two ion tests

4.2.3 Instrumental methods of analysis

Instrumental methods use machines to detect and measure substances

Instrumental methods are sensitive, accurate and rapid

Identifying substances by matching a reference chart

Identifying substances by matching a reference table

Identifying alcohols in a sample from retention times

Review 4.2 Identifying the products of chemical reactions by teaching Genie

Recap your knowledge with an interactive lesson

Put it into practice with exam-style questions

Remember key concepts with flashcards

4.2 Identifying the products of chemical reactions Revision Guide

4.2 Identifying the products of chemical reactions

4.2.1 Tests to identify gases

Hydrogen burns with a squeaky pop

Oxygen relights a glowing splint

Collecting a gas to test it

Method

Accuracy and safety

Carbon dioxide turns limewater milky

Chlorine bleaches damp litmus paper

How a gas was made suggests its test

4.2.2 Tests for aqueous ions and flame tests

Carrying out a flame test

Method

Accuracy and safety

Five metal ions give characteristic flame colours

Sodium hydroxide gives coloured metal hydroxide precipitates

Tests for carbonate, halide and sulfate ions

Combining the results for both ions identifies a compound

Identifying a compound from two ion tests

4.2.3 Instrumental methods of analysis

Instrumental methods use machines to detect and measure substances

Instrumental methods are sensitive, accurate and rapid

Identifying substances by matching a reference chart

Identifying substances by matching a reference table

Identifying alcohols in a sample from retention times

Review 4.2 Identifying the products of chemical reactions by teaching Genie

Recap your knowledge with an interactive lesson

Put it into practice with exam-style questions

Remember key concepts with flashcards

4.2 Identifying the products of chemical reactions Revision Guide