Matter, Mixtures and Separation
Chemistry is the study of matter: what it is made of and how it changes. This chapter starts from the particles in every substance, sorts substances into elements, compounds and mixtures, then separates mixtures and tests how pure the result is.
Learning objectives
By the end of this chapter, you can:
- tell elements, compounds and mixtures apart, and read a simple chemical formula
- use the terms solute, solvent, solution, solubility, saturated, miscible and immiscible
- describe the kinetic particle theory, the three states of matter, changes of state and diffusion
- match separation methods to mixtures
- read chromatograms and Rf values
- use melting and boiling points to judge purity
1.1Elements, compounds and mixtures
All matter is made of tiny particles. An atom is the smallest particle of an element. Atoms can be joined together into a molecule. Every substance is an element, a compound or a mixture (Figure 1.1).
- An element can exist as single atoms, such as helium and argon, or as molecules, such as oxygen, O2.
- Each element has a symbol of one or two letters, such as H for hydrogen, O for oxygen and Na for sodium. A chemical formula shows the atoms in one particle. In H2O the small 2 counts the hydrogen atoms, so a water molecule always has two hydrogen atoms and one oxygen atom.
- A molecule can be an element or a compound. H2 is an element because both atoms are hydrogen. CO2 is a compound because it holds carbon and oxygen.
- Air is a mixture of nitrogen, N2, oxygen, O2, carbon dioxide, CO2, and other gases. Its make-up varies: air at the top of a mountain holds less oxygen than air at sea level.
| compound | mixture | |
|---|---|---|
| how it forms | by a chemical reaction | by mixing, with no reaction |
| ratio of its parts | fixed | can vary |
| properties | different from those of its elements | similar to those of its parts |
| melting and boiling | at fixed temperatures | over a range of temperatures |
| energy change | usually a large one when it forms or breaks down | little or none when it is mixed or separated |
| how it is separated | only by chemical methods, such as strong heating or passing electricity through it | by physical methods, such as filtration, distillation and chromatography (1.3 and 1.4) |
1.1.1Solutions
When sugar is stirred into tea it seems to disappear, but it is still there. It has dissolved, and the tea is now a mixture called a solution.
- A substance that dissolves is soluble. One that does not is insoluble. A substance can be soluble in one solvent and insoluble in another.
- A saturated solution is one in which no more solute can dissolve at that temperature. An aqueous solution is one in which the solvent is water.
- Miscible liquids mix completely to form one liquid, such as ethanol and water. Immiscible liquids do not mix, such as oil and water, and form separate layers.
- The density of a substance is its mass in a given volume. In layers of immiscible liquids the least dense liquid floats on top and the densest sinks to the bottom.
- An impurity is any unwanted substance mixed into another. A sample with no impurity is pure.
- Sparkling water consists of water and carbon dioxide. Is it a compound or a mixture? Explain.
- Air contains argon, nitrogen and carbon dioxide. Name one element in air that exists as single atoms, one element that exists as molecules and one compound.
- Salt dissolves in water. Name the solute and the solvent, and say why the solution is called aqueous.
1.2Particles and the states of matter
How the particles are arranged and how fast they move decide whether a substance is a solid, a liquid or a gas (Figure 1.2).
| solid | liquid | gas | |
|---|---|---|---|
| arrangement | very closely packed, in an orderly pattern | closely packed, disorderly | very far apart, disorderly |
| movement | vibrate and rotate about fixed positions | slide past one another | move quickly and randomly in any direction |
| forces of attraction | very strong | strong | very weak |
| volume and shape | definite volume and shape | definite volume, takes the shape of its container | no definite volume or shape, fills its container |
1.2.1Changes of state and energy
Heating transfers thermal energy to a substance. The energy becomes kinetic energy of the particles, so they move faster and the temperature rises. At the melting point and the boiling point, the energy is used to overcome the forces of attraction instead, so the temperature stays constant until the change of state is complete (Figure 1.3).
| change | from, to | energy |
|---|---|---|
| melting | solid, liquid | absorbed |
| boiling and evaporation | liquid, gas | absorbed |
| sublimation | solid, gas | absorbed |
| freezing | liquid, solid | released |
| condensation | gas, liquid | released |
| vapour deposition | gas, solid | released |
- Evaporation happens at the surface of a liquid at any temperature. Boiling happens throughout the liquid, as bubbles of gas, at the boiling point only.
- A solid below its melting point expands when heated: the particles vibrate harder and sit slightly further apart. It contracts when cooled.
- A gas takes up far more space than the same substance as a liquid because its particles are far apart: liquid nitrogen shrinks about 700 times when it forms from the gas. The space between the particles is why a gas is easy to compress, and a liquid or a solid is not.
1.2.2Diffusion
It happens in liquids and gases because their particles move randomly. A drop of ink spreads through water, and a smell spreads through a room. Dust specks in a beam of light jump about because moving air particles keep hitting them. Diffusion ends in an even mixture, but the particles carry on moving.
- A higher temperature gives the particles more kinetic energy, so diffusion is faster.
- Heavier particles need more energy to move at the same speed, so they diffuse more slowly.
- Questions often give the mass of a particle as its relative molecular mass, Mr. The larger the Mr, the heavier the particle. Chapter 4 shows how Mr is worked out.
Figure 1.4 shows the second point. Ammonia and hydrogen chloride gases diffuse towards each other along a tube and form white ammonium chloride where they meet. The ring forms nearer the hydrogen chloride end, because the lighter ammonia particles travelled further in the same time.
- Use the kinetic particle theory to explain why a liquid takes the shape of its container but a solid does not.
- Dry ice, solid carbon dioxide, sublimes at room temperature. Describe what happens to its particles, and why the gas takes up much more space.
- Why does the smell of cooking spread faster in a warm kitchen?
1.3Separating mixtures
The method depends on a physical property in which the components differ: particle size, solubility, or melting and boiling points.
| Mixture | Method | Property used and how it works |
|---|---|---|
| solid and solid | magnet | one solid is magnetic (iron, cobalt, nickel) |
| sieving | the solids have different particle sizes | |
| suitable solvent | only one solid dissolves, such as salt from sand in water, then filter | |
| sublimation | one solid changes straight to a gas on heating, such as iodine from salt. The gas deposits on a cool surface as the sublimate. | |
| solid and liquid | filtration | an insoluble solid stays on the filter paper as the residue. The liquid passes through as the filtrate. |
| evaporation to dryness | heat until all the solvent has vaporised, leaving the dissolved solid | |
| crystallisation | for a solute that decomposes on heating, such as sugar. Heat to a saturated solution, cool, then filter off the crystals. | |
| simple distillation | the solvent has a lower boiling point than the solute. It is vaporised, condensed and collected as the distillate. | |
| liquid and liquid | separating funnel | for immiscible liquids. They form layers, the least dense on top, and each is run off through the tap. |
| fractional distillation | for miscible liquids with different boiling points, such as ethanol and water, using a fractionating column. It also separates crude oil and liquid air |
1.3.1Distillation in detail
- Add boiling chips for smooth boiling. The thermometer bulb sits at the side arm, so it reads the temperature of the vapour leaving.
- In fractional distillation the lower-boiling liquid leaves the column first (ethanol at 78 °C). Remove the receiver once the thermometer rises above that temperature, or the distillate is contaminated.
- The column's glass beads give a large surface for the higher-boiling vapour to condense on and fall back. Liquids with close boiling points need a longer column.
Worked exampleA mixture holds four substances. W and X are immiscible liquids and X is denser. Y is a solid that dissolves in W but not in X. Z is a solid that dissolves in neither liquid. Outline how to separate all four.
- Name two separation methods in which a component changes state.
- How do you decide which method to use to separate two substances?
1.4Chromatography
A purple food colouring can be a mixture of a pure blue dye and a pure red dye. Filtration cannot separate dissolved substances, and distillation only removes the liquid. Chromatography separates the dyes by a property they do not share: how well they dissolve in a given solvent.
- Place a drop of the mixture on the start line, drawn in pencil near one edge of the paper.
- Dip that edge in the solvent, keeping the spot above the liquid.
- The solvent rises and carries the substances with it. A more soluble substance moves faster and ends higher on the paper.
The finished paper is the chromatogram. The highest point the solvent reached is the solvent front.
To identify the dyes, run the pure dyes in the same solvent (Figure 1.6). A spot that lines up with a pure dye has moved the same distance, so it is that dye. A pure substance gives one spot. More than one spot means a mixture.
1.4.1The retention factor
Runs of different lengths put the same substance at different heights, so heights cannot be compared directly. Compare each spot with its own solvent front instead.
- Rf has no unit and lies between 0 and 1.
- A very soluble substance has an Rf close to 1. One that hardly dissolves stays near the start line, close to 0.
- Rf is constant only for the same solvent and temperature. Never compare Rf values found in different solvents.
In Figure 1.7 the longer run carries the spot higher, but the solvent front rises in proportion. Both runs give Rf = 0.5.
1.4.2Colourless substances
Amino acids and sugars are colourless. Their spots are made visible with a locating agent or by viewing the paper under ultraviolet light. Some need both.
Chromatography is used to detect pesticides and poisons in food, banned substances in an athlete's urine or blood, and DNA fragments in forensic samples.
- Why is the start line drawn in pencil and not in ink?
- The same substance is run in the same solvent at the same temperature for 10 minutes and for 20 minutes. Will the Rf values differ? Explain.
- Why can an Rf found using ethanol not be compared with one found using water?
1.5Judging purity
Impurities can make a medicine harmful, and some products must reach a certain purity to work. Chromatography is one test. Melting and boiling points are another.
Salt raises the boiling point of water and lowers its freezing point from 0 °C. This is why salt is spread on icy roads.
Worked exampleThe melting points of water (H2O), tridecane (C13H28) and tetradecane (C14H30) are 0 °C, −5 °C and 5 °C. A solid X melts at 0 °C. A solid Y melts over −5 °C to 5 °C.
(a) State whether X and Y are pure or mixtures.
(b) Identify the solids.
- Liquid P boils over 130 °C to 140 °C. What does this tell you about its purity?
- Name two ways to test whether a solid is pure.
Answers to Check your understanding
1.1
- A mixture. The water and carbon dioxide are not chemically combined, and the amount of carbon dioxide can vary.
- Argon exists as single atoms. Nitrogen, N2, exists as molecules. Carbon dioxide, CO2, is a compound.
- Salt is the solute and water is the solvent. The solution is aqueous because its solvent is water.
1.2
- In a liquid the particles are closely packed but slide past one another, so the liquid flows into the shape of the container. In a solid they are held in fixed positions by strong attractions and can only vibrate, so it keeps its own shape.
- The particles gain energy and overcome the forces of attraction, so they move far apart and move quickly and randomly. In the gas the particles are far apart, so it takes up much more space than the solid, where they are closely packed.
- A warmer kitchen gives the particles more kinetic energy, so they move faster and diffuse faster.
1.3
- Any two of: evaporation, crystallisation, simple distillation, fractional distillation, sublimation.
- By the physical properties in which the components differ, such as particle size, solubility, and melting and boiling points.
1.4
- Graphite does not dissolve in the solvent, so the line stays put. A dissolving ink would run up the paper and its colours would confuse the result.
- No. The spot and the solvent front both move further in the longer run, in the same proportion, so the ratio stays the same.
- The Rf of a substance changes with the solvent, so values from different solvents must not be compared.
1.5
- A pure substance boils at one temperature. Liquid P boils over a range, so it is a mixture.
- Chromatography (a pure substance gives one spot) and melting or boiling point (a pure substance has a specific value).