In chemistry, the word “pure” has a very precise meaning that is different from how we use it in everyday life. Understanding this difference is the first step in chemical analysis.

Defining Purity 

There are two ways to think about the word ‘pure’:

1. In Everyday Life: When we talk about “pure orange juice,” we mean that nothing extra has been added to it. It’s just the juice from the orange, although it’s still a mixture of water, sugars, acids, and vitamins.

2. In Chemistry: The scientific definition is much stricter. A pure substance is a single element or a single compound that is not mixed with any other substance. For example, pure water contains only H₂O molecules, and pure table salt contains only NaCl.

Exam Tip: Be careful with your wording in an exam. If a question asks for the chemical definition of a pure substance, you must state that it is a single element or compound. Simply saying “it has nothing else in it” might not be specific enough to get the mark.

Identifying Purity with Melting Points 

We can test the purity of a substance by measuring its melting and boiling points.

• A pure substance will melt and boil at a specific, fixed temperature. This is known as a sharp melting point. For example, pure water always melts at exactly 0°C and boils at exactly 100°C (at standard pressure).

• An impure substance, or mixture, will melt and boil over a range of temperatures. For instance, salty water will start to boil at a temperature above 100°C.

Exam Tip: Exam questions often provide data showing the melting temperature of different samples. If a sample melts at a single, sharp temperature (e.g., exactly 122°C), it is pure. If it melts over a range (e.g., 118-121°C), it is impure.

Identifying Purity with Chromatography 

Chromatography is a powerful technique used to separate mixtures, and it can also be used to check for purity.

• When a pure substance is analysed using chromatography, it will produce only one single spot on the chromatogram. This is because there is only one substance present to be separated.

• An impure substance (a mixture) will separate into two or more spots, as it contains multiple different substances that travel at different speeds.

Exam Tip: You must be able to interpret a simple chromatogram. If a substance produces only one spot, it is pure. If it produces several spots, it is a mixture. This is a very common and straightforward question.

Many of the products we use every day, from paint to medicine, are not single pure substances. Instead, they are carefully designed mixtures called formulations.

What is a Formulation?

A formulation is a mixture that has been designed as a useful product.

They are not just random mixtures; they are complex recipes where each chemical ingredient has a specific job to do. The components are mixed in carefully measured quantities to ensure the final product has the exact properties needed for its function. Importantly, the substances in a formulation do not react with each other.

Exam Tip: The three key ideas to remember for the definition of a formulation are:

1. It is a mixture (not a single compound).

2. It’s designed for a specific purpose.

3. The components are mixed in carefully measured quantities. Including these points will help you secure full marks in an exam question.

Examples of Formulations 

Formulations are all around us. Here are some common examples:

• Paint: A paint formulation contains several key ingredients.

  • Pigment: Gives the paint its colour.

  • Solvent: Dissolves the other components and allows the paint to be spread easily.

  • Binder (Resin): Forms a film that holds the pigment in place after the paint dries.

  • Additives: Chemicals added to change properties, like making the paint thicker or mould-resistant.

• Medicines: A pill or tablet is a formulation containing the active drug itself, plus other substances like fillers to add bulk, sweeteners to improve the taste, and coatings to help it dissolve in the right part of the body.

• Fuels: Petrol is a formulation of different hydrocarbons and additives designed to make a car engine run efficiently and cleanly.

• Cleaning Agents: A laundry detergent is a complex formulation of surfactants, water softeners, enzymes, and fragrances, all working together to clean clothes effectively.

• Fertilisers: NPK fertilisers are formulations containing specific, carefully measured percentages of nitrogen, phosphorus, and potassium compounds to help crops grow.

• Foods and Cosmetics: Many processed foods, sun creams, and deodorants are also carefully designed formulations.

Exam Tip: You may be given information about the components of a product and asked to explain why it is a formulation. You should identify that it is a mixture, state that each component has a specific job, and note that the components are present in measured amounts to create a useful product.

Chromatography is a powerful analytical technique used to separate the different substances in a mixture. It works like a race, where different components travel at different speeds, allowing us to see what a mixture is made of.

How Chromatography Works 

The separation in chromatography depends on the interaction between two phases:

• The Stationary Phase: This is the phase that doesn’t move. In paper chromatography, it’s the absorbent paper.

• The Mobile Phase: This is the phase that moves. It’s a solvent that travels up the stationary phase, carrying the dissolved mixture with it.

The separation happens because each chemical in the mixture has a different balance between two competing factors:

1. How soluble it is in the solvent (mobile phase).

2. How strongly it is attracted to the paper (stationary phase).

A substance that is very soluble in the solvent and weakly attracted to the paper will travel a long way up. A substance that is less soluble and strongly attracted to the paper will travel a shorter distance.

Exam Tip: Remember that separation depends on the distribution between the two phases. A substance that spends more time in the mobile phase travels further. A substance that spends more time “stuck” to the stationary phase travels less far.

Paper Chromatography: The Method

This is a required practical, so you need to know the steps:

1. Draw the Origin Line: Use a pencil to draw a straight line about 1-2 cm from the bottom of a piece of chromatography paper. A pencil is used because it is insoluble and won’t move with the solvent.

2. Spot the Sample: Place a small, concentrated spot of the mixture (e.g., ink) onto the centre of the pencil line.

3. Add the Solvent: Pour a small amount of solvent into a beaker.

4. Place the Paper: Suspend the paper in the beaker so that the bottom edge is in the solvent, but the solvent level is below the pencil line. This is crucial, otherwise, the sample spot would just dissolve into the solvent at the bottom.

5. Run the Chromatogram: Place a lid on the beaker to stop the solvent from evaporating and allow the solvent to travel up the paper.

6. Dry: Remove the paper just before the solvent front reaches the very top and mark the position of the solvent front with a pencil. Allow the paper to dry. The finished result is called a chromatogram.

Interpreting the Chromatogram

We can use the finished chromatogram to identify substances and check for purity.

• A pure substance will produce only one single spot.

• A mixture will separate into two or more spots.

Exam Tip: A common question shows a chromatogram of an unknown substance and several known substances. You can identify the unknown by finding which known substance has a spot that has travelled the same distance and is the same colour.

The Rf Value (Retention Factor)

To identify a substance more accurately, we can calculate its Rf value. This is a ratio that is constant for a particular substance in a specific solvent.

The formula is: Rf = Distance travelled by substance / Distance travelled by solvent

To calculate it:

1. Measure the distance from the pencil origin line to the centre of the spot.

2. Measure the distance from the pencil origin line to the solvent front.

3. Divide the distance of the spot by the distance of the solvent front.

The Rf value will always be a number less than 1.

Example Calculation:

• Distance moved by spot = 4 cm

• Distance moved by solvent = 10 cm

• Rf = 4 / 10 = 0.4

Exam Tip: When measuring the distances for the R₟ value calculation, always measure from the pencil origin line, not from the bottom of the paper. Also, measure to the centre of the spot.

Hydrogen gas is often produced when a metal reacts with an acid. To confirm its presence, chemists use a simple and distinctive test that relies on hydrogen’s flammability.

The Test Procedure

1. Collect the unknown gas in a test tube.

2. Take a lit wooden splint.

3. Place the burning splint into the mouth of the test tube.

The Result

If hydrogen gas is present, you will hear a characteristic ‘squeaky pop’ sound.

This sound is a mini-explosion caused by the rapid combustion reaction between hydrogen and the oxygen in the air, which produces water.

The reaction involves the following molecules:

Exam Tip: This is one of the three gas tests you must remember for your GCSE exams. The phrase ‘squeaky pop’ is the specific observation required for the hydrogen gas test, so be sure to learn it.

Oxygen is a highly reactive gas that is essential for combustion (burning). This property is used to create a simple, reliable test to identify it.

The Test Procedure

1. Collect the unknown gas in a test tube.

2. Light a wooden splint and then gently blow it out so that it is still glowing red at the tip.

3. Immediately insert the glowing splint into the mouth of the test tube.

The Result

If the gas is oxygen, the glowing splint will relight and burst back into a flame.

This happens because the high concentration of oxygen in the test tube provides the fuel (the hot splint) with enough reactant to start the combustion process again. The reaction involves the following key molecule:

Exam Tip: The specific wording for this test is crucial. You must state that you use a “glowing splint”and that the positive result is that it “relights.” Saying you use a “lit splint” or that it “catches fire” might not be precise enough to earn the marks.

Carbon dioxide is produced in many reactions, such as the combustion of fuels and the reaction between an acid and a carbonate. The standard chemical test for this gas uses a solution called limewater.

The Test Procedure

1. The reagent used is limewater, which is the common name for a dilute aqueous solution of calcium hydroxide (Ca(OH)₂).

2. The unknown gas is either bubbled through the limewater or the test tube is sealed and shaken with the limewater.

The Result

If the gas is carbon dioxide, the limewater will turn milky or cloudy.

This cloudiness is a precipitate of calcium carbonate (CaCO₃), which is insoluble in water.

Exam Tip: You must remember the chemical name for limewater is calcium hydroxide solution. The positive result is that it turns “milky” or “cloudy.” Simply saying “it turns white” is less precise and may not get you the mark.

Chlorine is a reactive halogen gas with a characteristic sharp, bleaching smell. Its powerful bleaching properties are used as the basis for its chemical test.

The Test Procedure

1. The reagent used is a piece of damp litmus paper.

2. Place the damp litmus paper into the test tube containing the unknown gas.

The Result

If the gas is chlorine, the damp litmus paper will be bleached and turn white.

Chlorine is a powerful bleaching agent that chemically destroys the dyes in the litmus paper. The key reactant in this test is the chlorine molecule itself.

Exam Tip: It’s important to specify that the litmus paper must be damp. The water is needed for the chlorine to dissolve and react to show its bleaching effect. The final observation is that the paper turns white, not just colourless.

When an ionic compound is heated, the metal ions within it can produce a distinctive colour. Chemists use this property to identify which metal ions are present in a sample.

Traditional Flame Tests 

A flame test is a quick and simple qualitative test used to identify metal ions.

Procedure:

1. Clean a wire loop by dipping it in acid and then heating it in a roaring blue Bunsen flame until it gives no colour.

2. Dip the clean loop into the solid sample of the compound.

3. Place the loop back into the hot, blue Bunsen flame.

4. Observe and record the colour of the flame produced.

Results: You need to learn the characteristic flame colours for these five metal ions:

Disadvantages of Flame Tests:

• Subjectivity: It can be hard to distinguish between similar colours (e.g., crimson and orange-red).

• Masking Effect: If a mixture of ions is present, the colour of one ion can hide the colour of another. The bright yellow flame of sodium is particularly good at masking other colours.

• Low Sensitivity: Very small amounts of an ion may not produce a visible colour.

Exam Tip: The masking effect of sodium’s yellow flame is a very common exam point. Remember that this makes flame tests unreliable for mixtures.

Flame Emission Spectroscopy (FES) 

Flame emission spectroscopy is a modern, instrumental method that overcomes the problems of traditional flame tests.

How it Works:

1. A sample is placed into a flame, and the light it produces is passed through a spectroscope.

2. The spectroscope separates the light into a line spectrum, which shows the specific wavelengths of light emitted.

3. Every metal ion has its own unique line spectrum, which acts like a “fingerprint”.

4. By comparing the spectrum from the unknown sample to the spectra of known metals, we can accurately identify the ions present.

5. The intensity of the lines can also be used to measure the concentration of the ions.

Exam Tip: You must be able to interpret a simple line spectrum. A question might show you the spectrum for a mixture and the spectra for several known metals. You need to match the lines in the mixture to the lines in the known spectra to identify which metals are present.

Advantages of Instrumental Methods (like FES)

Instrumental methods are generally better than simple chemical tests for several reasons:

• More Accurate: They provide a more certain and objective result.

• More Sensitive: They can detect even tiny amounts of a substance.

• Faster: They produce results very quickly.

• Analyse Mixtures: They can identify several different ions in the same sample, which is very difficult to do with a simple flame test.

The main disadvantages are that the equipment is very expensive, and it requires a specially trained person to operate it.

Another important set of chemical tests allows us to identify positive metal ions (cations) in a solution. This method involves adding sodium hydroxide solution (NaOH) and observing the insoluble precipitate that forms.

The Principle of the Test

When sodium hydroxide solution is added to a solution containing dissolved metal ions, a precipitation reaction occurs. The metal ions react with the hydroxide ions (OH⁻) from the sodium hydroxide to form a solid, insoluble metal hydroxide.

The color of this precipitate, and whether it dissolves when you add more sodium hydroxide, helps to identify the metal ion.

General word equation: Metal Salt + Sodium Hydroxide ⟶ Metal Hydroxide (precipitate) + Sodium Salt

For example, for copper(II) ions: Cu²⁺(aq) + 2OH⁻(aq) ⟶ Cu(OH)₂(s)

Exam Tip: You must be able to write ionic equations for the formation of these precipitates. Remember to balance the charges. For example, an Fe³⁺ ion will need three OH⁻ ions to form Fe(OH)₃.

The Procedure

The way you add the sodium hydroxide solution is very important.

1. Add a few drops of sodium hydroxide solution to the sample solution and observe the color of any precipitate that forms.

2. Then, add sodium hydroxide solution in excess (add a lot more) and observe if the precipitate dissolves.

Results: Coloured Precipitates

Some transition metal ions produce vividly colored precipitates. These precipitates do not dissolve when excess sodium hydroxide solution is added.

Exam Tip: You must learn these three colors. A good way to remember them is that copper sulfate solution is blue, so its precipitate is also blue. Iron(II) is green, and Iron(III) is the color of rust (which is hydrated iron(III) oxide).

Results: White Precipitates

Other metal ions form white precipitates. For these, it is essential to see what happens when you add excess sodium hydroxide.

Exam Tip: The key distinguishing feature here is for the aluminium ion. Its white precipitate is the only one that redissolves in excess sodium hydroxide. To tell the difference between calcium and magnesium, you would need to do a different test, such as a flame test (calcium gives an orange-red flame).

Carbonates are compounds containing the carbonate ion (CO₃²⁻). To identify them, we use a test that relies on their characteristic reaction with acid to produce a specific gas.

The Principle of the Test

The test for carbonate ions involves adding a dilute acid, such as hydrochloric acid (HCl), to the sample. If carbonate ions are present, a chemical reaction occurs that produces carbon dioxide gas.

The overall reaction is: Carbonate + Acid ⟶ Salt + Water + Carbon Dioxide

The key ionic equation, showing the essential reaction, is: CO₃²⁻(aq) + 2H⁺(aq) ⟶ H₂O(l) + CO₂(g)

The Procedure

1. Place a small amount of the substance you are testing into a test tube.

2. Add a few drops of dilute acid (e.g., hydrochloric acid). If carbonate ions are present, you will see fizzing or effervescence as carbon dioxide gas is produced.

3. To confirm the gas is carbon dioxide, you must perform the limewater test. Use a delivery tube to bubble the gas produced through a second test tube containing limewater (an aqueous solution of calcium hydroxide).

Positive Result

A positive test for carbonate ions is confirmed when the gas produced is bubbled through limewater and the limewater turns milky or cloudy.

This cloudiness is caused by the formation of a white precipitate of calcium carbonate.

Exam Tip: You must link the initial observation (fizzing when acid is added) to the confirmatory test (limewater turning cloudy). Both parts are needed for a complete answer. Simply stating that the substance fizzed is not enough to prove a carbonate was present.

Halide ions are negative ions formed from halogen atoms (Group 7). To identify which halide ion is present in a solution, chemists use a precipitation reaction with silver nitrate solution.

The Procedure

The test must be carried out in a specific order:

1. Acidify the Sample: First, add a few drops of dilute nitric acid (HNO₃) to the sample solution. This is done to remove any other ions (like carbonate ions) that might also form a precipitate with silver nitrate, which would give a false positive result.

2. Add Silver Nitrate: Next, add a few drops of silver nitrate solution (AgNO₃).

3. Observe: A precipitate will form if halide ions are present. The color of the precipitate identifies the specific halide ion.

Exam Tip: You must use nitric acid. Using hydrochloric acid (HCl) would add chloride ions (Cl⁻) to the sample, making the test for chloride ions impossible and invalidating the results.

Results

The silver ions (Ag⁺) from the silver nitrate solution react with the halide ions to form an insoluble silver halide precipitate. You must learn the colors of these three precipitates.

The key ions and the precipitates they form are shown below.

🧠 Exam Tip: The colors can be difficult to distinguish, especially white and cream. An exam question will usually describe the colors clearly for you. You just need to match the ion to the correct color: Chloride → White, Bromide → Cream, Iodide → Yellow.

Sulfate ions (SO₄²⁻) are negative ions found in compounds like copper sulfate and magnesium sulfate. The chemical test to identify them involves creating a specific white precipitate.

The Principle of the Test

The test for sulfate ions relies on the fact that when sulfate ions react with barium ions (Ba²⁺), they form barium sulfate (BaSO₄), which is a dense white precipitate. The reaction must be carried out in acidic conditions to prevent other ions from interfering.

The key ionic equation for this precipitation reaction is: Ba²⁺(aq) + SO₄²⁻(aq) ⟶ BaSO₄(s)

The Procedure

The test must be performed in the correct order to be valid.

1. Acidify the Sample: First, add a few drops of dilute hydrochloric acid (HCl) to the solution being tested. This is a crucial step to remove any carbonate ions (CO₃²⁻) that might be present. Carbonate ions would also form a white precipitate with barium ions (barium carbonate), giving a false positive result.

2. Add Barium Chloride: Next, add a few drops of barium chloride solution (BaCl₂).

Exam Tip: You must remember to add the acid first. A common exam question asks why the acid is added. The answer is: to remove carbonate ions, which would interfere with the test.

Positive Result

If sulfate ions are present in the sample, a white precipitate of barium sulfate will form upon adding the barium chloride solution.

The key molecules and ions involved in this test are shown below.

Exam Tip: Be very specific with your observation. The positive result for a sulfate ion is a “white precipitate.” Don’t just say the solution “turns white” or “goes cloudy.” Using the correct scientific term will ensure you get the marks.

While simple chemical tests are useful, modern science often relies on advanced machines for analysis. These instrumental methods provide faster, more accurate, and more detailed information about the chemical composition of a substance.

Flame Emission Spectroscopy (FES) 

A key example of an instrumental method is Flame Emission Spectroscopy (FES). It is a powerful technique used to identify metal ions and measure their concentration in a sample, and it is far more reliable than a traditional flame test.

How FES Works

The process is based on the unique light that different metal ions emit when heated.

1. Sample Introduction: A solution containing the sample is sprayed into a hot flame.

2. Excitation: The heat energy from the flame excites the electrons in the metal ions, causing them to jump to higher energy levels.

3. Emission: These electrons are unstable at higher energy levels, so they immediately fall back to their original positions. As they fall, they release the extra energy as light of specific wavelengths.

4. Analysis: This emitted light is passed through a spectroscope, which separates it into its different wavelengths and produces a line spectrum.

Each metal ion has a unique “fingerprint” line spectrum. By comparing the spectrum of the unknown sample to a database of known spectra, scientists can identify exactly which metal ions are present.

Furthermore, the intensity of the lines in the spectrum is proportional to the concentration of the ion in the sample. A more concentrated sample will produce brighter, more intense lines.

Exam Tip: A common exam question will show you the line spectrum for a mixture and the spectra for several individual metals. To identify the metals in the mixture, you simply need to match the pattern of lines. If all the lines for a known metal are also present in the mixture’s spectrum, then that metal is in the mixture.

Advantages of Instrumental Methods

Compared to traditional chemical tests (like flame tests or precipitation tests), instrumental methods like FES have significant advantages:

• Accuracy: They are much more accurate and less subjective than judging a colour by eye.

• Sensitivity: They are extremely sensitive and can detect even tiny traces of a substance in a sample.

• Speed: They are very rapid, producing results quickly.

• Analysis of Mixtures: They can easily identify several different substances in a single mixture, whereas in a flame test, one ion’s colour can mask another’s.

Disadvantages of Instrumental Methods

Despite their power, these methods have some drawbacks:

• Cost: The equipment is very expensive to buy and maintain.

• Training: A highly skilled and specially trained person is needed to operate the machinery.

• Data Comparison: The results are often only useful when compared against a database of known reference data.

Exam Tip: You must be able to state the advantages and disadvantages of instrumental methods. For advantages, think S.A.M. (Sensitive, Accurate, Mixtures). For disadvantages, think about the cost and the need for a specialist operator.

Flame Emission Spectroscopy (FES) is a powerful instrumental method used by scientists to analyze samples containing metal ions. It’s a significant upgrade from the simple flame test because it is more accurate, sensitive, and can provide more detailed information.

How Does FES Work? 

The technique is based on a simple principle: when metal ions are heated, they emit light of specific colors. FES analyzes this light in detail.

1. Heating the Sample: A solution containing the metal ions is sprayed into a very hot flame.

2. Excitation: The heat energy from the flame excites the electrons within the metal ions, causing them to jump to a higher, less stable energy level.

3. Emission: The electrons immediately fall back to their original, lower energy level. As they do, they release the energy they absorbed as light of specific wavelengths.

4. Analysis: This light is passed through a spectroscope, which separates the different wavelengths and produces a line spectrum.

Interpreting the Results 

The line spectrum is like a unique barcode or fingerprint for a metal ion.

• Identification: Each metal ion has its own unique pattern of lines at specific wavelengths on the spectrum. By comparing the line spectrum of an unknown sample to a database of reference spectra, we can identify exactly which metal ions are present. For example, a sample producing a specific pattern of lines could be identified as containing both sodium (Na⁺) and potassium (K⁺) ions.

• Quantification (Concentration): The intensity (brightness) of the lines is proportional to the concentration of the ion in the sample. A more concentrated sample will produce more intense lines.

Exam Tip: You must be able to interpret a simple line spectrum. An exam question might show you the spectrum for a mixture and the spectra for several known metals. To identify the metals in the mixture, you simply need to match the pattern of lines. If all the lines for a known metal are also present in the mixture’s spectrum, then that metal is in the mixture.

Advantages of FES Over Traditional Flame Tests

Instrumental methods like FES have several key advantages:

• Can Analyze Mixtures: This is a major benefit. FES can easily identify multiple different ions in the same sample, whereas in a simple flame test, the strong yellow color from sodium can easily mask the colors of other ions.

• More Sensitive: FES can detect even tiny amounts of a substance.

• More Accurate: The results are objective and machine-read, removing human error in judging colors.

• Faster: The analysis is very rapid.

The main disadvantages are that the equipment is very expensive and requires specialist training to operate.