Elements compounds and mixtures

Elements, Compounds, and Mixtures

Every substance in chemistry falls into one of three categories: element, compound, or mixture.

An element is a substance made of only one type of atom, so it cannot be broken down into anything simpler by chemical means. The periodic table lists all known elements — examples include iron (Fe), oxygen (O₂), and carbon (C).

A compound is a substance made of two or more different elements chemically bonded together in a fixed ratio. Because the elements are chemically joined, a compound can only be separated into its elements by a chemical reaction, not by physical methods.

Water (H₂O) is a compound because it always contains hydrogen and oxygen in a 2 : 1 atom ratio. Compounds have properties that differ completely from the elements they contain — sodium (Na) is a reactive metal and chlorine (Cl₂) is a toxic gas, yet sodium chloride (NaCl) is safe table salt.

A mixture contains two or more substances that are not chemically bonded together. Because no chemical bonds form between them, the substances in a mixture keep their own individual properties and can be separated by physical methods such as filtration or distillation.

Air is a mixture of gases including nitrogen, oxygen, and argon. Unlike compounds, mixtures can exist in any proportion — a cup of tea can have more or less sugar stirred in.

MisconceptionStudents often state that compounds can be separated by physical methods. Compounds require a chemical reaction to break them apart because the atoms are chemically bonded together. Mixtures are separated by physical methods because no chemical bonds exist between the different substances.

Exam TipIf the question says "chemically bonded," the answer is compound, not mixture.

Pure Substances and Mixtures — Melting and Boiling Points

A pure substance has a fixed, sharp melting point and a fixed, sharp boiling point. During melting or boiling, the temperature remains constant until the entire change of state is complete. Pure water, for example, melts at exactly 0 °C and boils at exactly 100 °C.

A mixture, however, melts or boils over a range of temperatures rather than at a single fixed point. This happens because the different substances in the mixture have different intermolecular forces, so they change state at different temperatures. As a result, the temperature rises gradually during the change of state instead of staying constant.

This difference provides a practical test for purity. If a substance melts sharply at the expected temperature, it is pure. If melting begins below the expected temperature or occurs over a range, the substance contains impurities — meaning it is a mixture.

Examiner InsightEdexcel frequently asks students to interpret heating curves. A flat horizontal section on a heating curve indicates a pure substance changing state. A sloped section during the expected change of state indicates a mixture or impure substance.

Exam TipLook for the word "range" — if a substance melts over a range, it is impure.

Simple Distillation

Simple distillation separates a solvent from a solution, or a liquid from a dissolved solid. The method works because the liquid has a lower boiling point than the dissolved solid, so it evaporates first and can be collected separately.

The solution is heated in a flask until the liquid boils and turns to vapour. The vapour travels into a condenser [a tube surrounded by a cold-water jacket], where it cools and condenses back into a liquid. This liquid, called the distillate, is collected in a separate container.

Simple distillation is suitable when there is only one liquid to recover, or when the boiling points of components differ by a large amount. For example, pure water can be obtained from salt water by simple distillation — the water boils at 100 °C, leaving the salt behind in the flask.

Fractional Distillation

Fractional distillation separates a mixture of two or more miscible liquids [liquids that dissolve in each other] with different boiling points. A fractionating column provides the key advantage over simple distillation — it gives the vapours repeated opportunities to condense and re-evaporate, which improves separation.

The mixture is heated in a flask. Vapours of all the liquids rise into the fractionating column, which is packed with glass beads or has internal surfaces. The liquid with the lowest boiling point remains as vapour all the way to the top, passes into the condenser, and is collected first. Liquids with higher boiling points condense at lower levels in the column and drip back into the flask. The thermometer reading rises in steps — each step corresponds to the boiling point of the next liquid being collected.

By recording the temperature and changing the collection vessel at each step, each liquid is collected separately. For example, ethanol (boiling point 78 °C) can be separated from water (boiling point 100 °C) by fractional distillation — the ethanol distils over first.

MisconceptionStudents sometimes state that fractional distillation works because the liquids have "different densities." The correct reason is that the liquids have different boiling points. The liquid with the lowest boiling point evaporates first and reaches the top of the column as vapour.

Exam TipAlways link separation to boiling point differences, not density.

Filtration

Filtration separates an insoluble solid from a liquid. The technique relies on the filter paper having tiny pores that allow the liquid to pass through but trap the solid particles.

The mixture is poured through filter paper held in a funnel. The liquid that passes through is called the filtrate. The solid that remains on the filter paper is called the residue. Filtration is used, for example, to separate sand from water, or to remove an insoluble precipitate from a reaction mixture.

Crystallisation

Crystallisation separates a dissolved solid (solute) from a solution by evaporating the solvent so that crystals form. The method works because the solute is left behind as the solvent evaporates.

The solution is gently heated in an evaporating basin to remove most of the solvent. Heating continues until the solution becomes saturated [contains the maximum amount of dissolved solute at that temperature]. The point of saturation can be tested by dipping a cool glass rod into the hot solution — if small crystals form on the rod, enough solvent has been removed.

The basin is then left to cool slowly so that regular, well-formed crystals grow. Slow cooling produces larger, more regular crystals because the solute particles have time to arrange in an orderly pattern. The crystals are then filtered and dried.

Examiner InsightExaminers often ask why the solution should not be heated to dryness. Heating to dryness causes the crystals to form too quickly, producing small and irregular crystals. Some substances may also decompose or spit if heated too strongly.

Exam TipState "the solution is left to cool slowly to form large, regular crystals."

Paper Chromatography

Paper chromatography separates the components of a mixture based on their different solubilities in a solvent. Substances that are more soluble in the solvent travel further up the paper, so they separate from less soluble substances.

A baseline [a pencil line drawn near the bottom of the chromatography paper] is drawn in pencil because pencil is insoluble and will not dissolve in the solvent. Small spots of the mixture are placed on the baseline. The paper is then lowered into a shallow layer of solvent in a beaker, with the solvent level below the baseline so the spots do not dissolve directly into the solvent. A lid is placed on the beaker to prevent the solvent evaporating.

As the solvent rises up the paper by capillary action, it carries the dissolved substances with it. Substances with higher solubility in the solvent are carried further up the paper. When the solvent has risen near the top, the paper is removed and the position of the solvent front [the furthest point reached by the solvent] is marked immediately in pencil before it dries.

The finished paper is called a chromatogram. Each separated component appears as a distinct spot. A pure substance produces only one spot. A mixture produces two or more spots. If a spot from an unknown substance has the same Rꜰ value and position as a spot from a known substance run in the same solvent under the same conditions, they are the same substance.

MisconceptionStudents often draw the baseline in pen instead of pencil. Ink from a pen dissolves in the solvent and moves up the paper, interfering with the results. The baseline must be drawn in pencil because pencil is insoluble in the solvent.

Exam TipAlways state "the baseline is drawn in pencil because it is insoluble in the solvent."

Rꜰ Values

The Rꜰ value identifies each component on a chromatogram by expressing how far it has travelled relative to the solvent front. Rꜰ stands for "retardation factor."

Key Equations

Rꜰ value:

$${R}_{f}=\frac{\text{distance moved by substance from baseline}}{\text{distance moved by solvent front from baseline}}$$

Variables:

• Distance moved by substance: measured from the baseline to the centre of the spot, in cm or mm
• Distance moved by solvent front: measured from the baseline to the solvent front line, in cm or mm

SI unit: no unit (Rꜰ is a ratio)

The Rꜰ value is always between 0 and 1 because the substance can never travel further than the solvent. Each substance has a unique Rꜰ value in a given solvent under given conditions. To identify an unknown component, compare its Rꜰ value with Rꜰ values of known substances measured under the same conditions and in the same solvent.

Worked Example

A spot on a chromatogram has moved 4.2 cm from the baseline. The solvent front has moved 6.0 cm from the baseline. Calculate the Rꜰ value.

Equation used

$${R}_{f}=\frac{\text{distance moved by substance}}{\text{distance moved by solvent front}}$$

Given

$$\text{distance moved by substance}=4.2\text{ cm}$$

$$\text{distance moved by solvent front}=6.0\text{ cm}$$

Working

$${R}_{f}=\frac{4.2}{6.0}$$

Answer

$${R}_{f}=0.70$$

Examiner InsightRꜰ values must be compared using the same solvent and conditions. An Rꜰ value calculated in one solvent cannot be compared with one from a different solvent, because solubility differs between solvents.

Exam TipState "under the same conditions" and "using the same solvent" when comparing Rꜰ values.

PRACTICAL: Investigating Paper Chromatography Using Inks/Food Colourings

Aim: To separate and identify the coloured components in different inks or food colourings using paper chromatography, demonstrating that some dyes are mixtures of different pigments.

Method

1. Draw a pencil baseline approximately 2 cm from the bottom of a strip of chromatography paper.

2. Place small, concentrated spots of different inks or food colourings on the baseline, spacing them evenly and labelling each in pencil below the line.

3. Pour a shallow layer of solvent (water or ethanol) into a beaker so the solvent level is below the baseline.

4. Lower the chromatography paper into the beaker so the bottom edge dips into the solvent, ensuring the spots remain above the solvent level.

5. Place a lid on the beaker and leave the solvent to rise up the paper.

6. When the solvent front is near the top of the paper, remove the paper and immediately mark the solvent front with a pencil.

7. Allow the paper to dry. Measure the distance from the baseline to the centre of each spot and from the baseline to the solvent front.

8. Calculate the Rꜰ value for each spot.

Variables

Independent variable (IV): the type of ink or food colouring tested

Dependent variable (DV): the number of spots produced and the Rꜰ value of each spot, measured by ruler from baseline in cm

Control variables (CV): the same solvent used for all samples; the same type of chromatography paper; the same temperature and conditions throughout the experiment

Expected results

Some inks or food colourings produce a single spot, indicating they contain one pure dye. Others produce two or more spots at different heights, indicating they are mixtures of different coloured dyes. Each dye has a different solubility in the solvent, so they travel different distances.

Precaution

The spots on the baseline must be small and concentrated — large spots spread and overlap, making individual components difficult to distinguish. If the solvent level is above the baseline, the dyes dissolve directly into the solvent and no separation occurs.