Matter, Mixtures and Separation

Singapore Junior Chemistry Olympiad

By the end of this chapter, you can answer

1

Is air an element, a compound or a mixture, and how can you tell?

2

Why does the temperature of ice stay the same while it melts?

3

Why does a smell spread faster in a warm room, and why does a heavier gas travel less far?

4

How do you get salt, sand and water back from a muddy salt solution?

5

A food colouring looks purple. How can you find out which dyes were mixed to make it?

6

One solid melts at exactly 0 °C. Another melts anywhere between −5 °C and 5 °C. Which one is pure?

What do you think?

1Sea water looks like one liquid. Is it a single substance?

No. It is water with salt and other minerals dissolved in it, so it is a mixture.

2Wet clothes dry on a line. Where does the water go?

It evaporates: it turns into a gas and spreads into the air.

3Why can you smell perfume from across a room?

Its particles move about and spread through the air. This is diffusion.

1

Elements, 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.

An element cannot be broken down into two or more simpler substances by chemical methods. It exists as single atoms, such as argon, or as molecules, such as oxygen, O₂.
A compound contains two or more elements chemically combined in a fixed ratio, such as water, H₂O.
A mixture contains two or more elements or compounds that are not chemically combined.

Positions and sizes are drawn for illustration.

1

Elements, Compounds and Mixtures

Each element has a symbol of one or two letters: H for hydrogen, O for oxygen, 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. Air at the top of a mountain holds less oxygen than air at sea level.

1

Elements, Compounds and Mixtures

	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

Elements, Compounds and Mixtures

1.1Solutions

DefinitionA solvent dissolves a substance, the solute. Together they form a solution. Solubility is how much solute can dissolve in a solvent at a given temperature.

A substance that dissolves is soluble. One that does not is insoluble.
A saturated solution holds as much solute as can dissolve at that temperature. An aqueous solution has water as its solvent.
Miscible liquids mix completely (ethanol and water). Immiscible liquids form layers (oil and water). The least dense liquid floats on top.
An impurity is any unwanted substance mixed into another.

Your turnSalt dissolves in water. Name the solute and the solvent. Why is the solution called aqueous?

Salt is the solute and water is the solvent.

It is aqueous because its solvent is water.

1

Elements, Compounds and Mixtures

Your turn(a) Sparkling water is water and carbon dioxide. Is it a compound or a mixture? (b) Name one element in air that exists as single atoms, one that exists as molecules, and one compound in air.

(a) A mixture: they are not chemically combined, and the amount of carbon dioxide can vary.

(b) Argon, single atoms. Nitrogen, N₂, molecules. Carbon dioxide, CO₂, a compound.

Quick checkWhich of these is a mixture?

Awater

Bcarbon dioxide

Cair

Doxygen

NoteC: air is nitrogen, oxygen, carbon dioxide and other gases, not chemically combined, in a ratio that varies.

2

Particles and the States of Matter

The kinetic particle theory says matter is made of tiny particles in constant, random motion.

	arrangement	movement	attraction
solid	closely packed, orderly	vibrate in place	very strong
liquid	closely packed, disorderly	slide past one another	strong
gas	far apart	fast and random	very weak

AnalogyA packed classroom in a lesson (solid), a crowded canteen (liquid) and an empty hall at break (gas).

2

Changes of State and Energy

Heating a solid raises its temperature. At the melting point the energy breaks the forces of attraction, so the temperature stays constant.
The liquid warms again, then boils. Boiling happens throughout the liquid, evaporation only at the surface.
The gas warms. Melting, boiling and sublimation absorb energy. Freezing and condensation release it.

2

Changes of State and Energy

Your turnDry ice, solid carbon dioxide, sublimes at room temperature. Describe what happens to its particles, and why the gas takes up much more space.

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 closely packed solid.

Quick checkWhat happens to the temperature of a pure liquid while it boils?

Ait rises steadily

Bit falls

Cit stays constant

Dit rises, then falls

NoteC: the energy goes into overcoming the forces of attraction, not into raising the temperature.

2

Diffusion

DefinitionDiffusion is the net movement of particles from a region of higher concentration to a region of lower concentration.

Ammonia particles are lighter, so they move faster and travel further.
They meet nearer the HCl end and form a white ring of ammonium chloride. A higher temperature makes diffusion faster.

2

Diffusion

Your turnWhy does the smell of cooking spread faster in a warm kitchen?

A warmer kitchen gives the particles more kinetic energy, so they move faster and diffuse faster.

Quick checkWhich gas diffuses fastest at the same temperature? Relative molecular masses: chlorine 71, carbon dioxide 44, helium 4, oxygen 32.

Achlorine

Bcarbon dioxide

Chelium

Doxygen

NoteC: helium has the smallest M_r, so its particles are the lightest and move fastest.

3

Separating 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.

3

Separating Mixtures

3.1Sublimation

Dry ice goes straight from solid to gas, and so does iodine. Salt does not.

Warm a mixture of iodine and salt. Only the iodine sublimes: it changes from solid to gas with no liquid between.
The gas meets a cool surface and changes back to a solid, the sublimate. The salt is left behind.

3

Separating Mixtures

Mixture	Method	Property used and how it works
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.

NoteEvaporation to dryness separates the solid but it may not be pure. Sea water contains several minerals. Crystallisation gives a pure solid. Separating and purifying are not the same thing.

3

Separating Mixtures

3.1Filtration

Separates an insoluble solid from a liquid. The liquid that passes through is the filtrate. The solid left on the paper is the residue.

3.1Evaporation to dryness

Separates a dissolved solid from its solvent. Heat until all the solvent has boiled off and only the solid is left.

3

Separating Mixtures

3.1Simple distillation

Separates a pure solvent from a solution, e.g. water from salt water. It is collected as the distillate.
Add boiling chips for smooth boiling. The thermometer bulb sits at the side arm and reads the temperature of the vapour leaving.
In the condenser, cooling water enters at the lower end, so it flows against the vapour and the condenser stays full of water.

3

Separating Mixtures

3.2Separating funnel

Oil and water do not mix. Shaken together, they settle again.

Immiscible liquids form layers. The least dense liquid floats on top.
Open the tap to run off the lower layer into a separate beaker. Close the tap when the boundary reaches it.

3

Separating Mixtures

Mixture	Method	How it works
liquid and liquid	separating funnel	for immiscible liquids. They form layers, the least dense on top, and each is run off through the tap.
liquid and liquid	fractional distillation	for miscible liquids with different boiling points, using a fractionating column. It also separates crude oil and liquid air

The lower-boiling liquid leaves the column first (ethanol at 78 °C). Remove the receiver once the thermometer rises above that temperature.
The column's glass beads give a large surface for the higher-boiling vapour to condense on. Close boiling points need a longer column.

Your turn(a) Name two separation methods in which a component changes state. (b) How do you decide which method to use?

(a) Any two of evaporation, crystallisation, simple distillation, fractional distillation, sublimation.

(b) By the physical properties in which the components differ, such as particle size, solubility, and melting and boiling points.

3

Separating Mixtures

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.

1. Filter the mixture. Z stays on the paper as the residue.

Z is the only insoluble solid.

2. Pour the filtrate into a separating funnel and run off X, the lower layer.

W and X are immiscible, so they form layers.

3. Evaporate or crystallise the solution of Y in W to recover Y.

Y is dissolved in W.

4

Chromatography

A purple colouring can be a mixture of a pure blue dye and a pure red dye. Chromatography separates them by how well they dissolve in a solvent.

DefinitionChromatography separates a mixture of substances that have different solubilities in a given solvent.

Drop of mixture on the start line, drawn in pencil.
Dip the edge in the solvent, spot above the liquid. The solvent rises. A more soluble substance moves faster and ends higher.
Run the pure dyes in the same solvent. A spot that lines up has moved the same distance, so it is that dye.

4

Chromatography

NoteThe start line is pencil because graphite does not dissolve. An ink that dissolves would run up the paper and confuse the result.

The finished paper is the chromatogram. The highest point the solvent reached is the solvent front.

A pure substance gives one spot. More than one spot means a mixture.

Your turnWhy is the start line drawn in pencil and not in ink?

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.

4

Chromatography

4.1The retention factor

Runs of different lengths put the same substance at different heights. Compare each spot with its own solvent front.

DefinitionThe retention factor, R_f, is the distance moved by the substance divided by the distance moved by the solvent front, both from the start line.

R_f has no unit and lies between 0 and 1.
R_f is constant only for the same solvent and temperature.

4

Chromatography

Your turn(a) The same substance is run in the same solvent at the same temperature for 10 minutes and 20 minutes. Do the R_f values differ? (b) Why can an R_f found in ethanol not be compared with one found in water?

(a) No. The spot and the solvent front both move further in the longer run, in the same proportion, so the ratio stays the same.

(b) The R_f of a substance changes with the solvent.

Quick checkWhich change alters the R_f value of a substance?

Arunning for longer

Busing a different solvent

Ca larger spot

Da longer paper

NoteB: R_f is fixed for a given solvent and temperature, however long the run.

4

Chromatography

4.2Colourless substances

Amino acids and sugars are colourless. Make their spots visible with a locating agent or by viewing the paper under ultraviolet light. Some need both.

DefinitionA locating agent is a chemical that reacts with a colourless substance to form a coloured spot.

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.

5

Judging Purity

Impurities can make a medicine harmful, and some products must reach a certain purity to work. Melting and boiling points give a test.

DefinitionA pure substance has a specific melting point and boiling point under fixed conditions. A mixture melts or boils over a range, and more impurity changes the point further.

Salt raises the boiling point of water and lowers its freezing point from 0 °C. This is why salt is spread on icy roads.

5

Judging Purity

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.

1. X melts at one temperature. Y melts over a range.

A pure substance has a specific melting point. A mixture has a range.

(a) X is pure. Y is a mixture.

From step 1.

2. 0 °C is the melting point of water.

A pure substance is identified by its melting point.

3. −5 °C and 5 °C are the melting points of tridecane and tetradecane.

Y melts between the two.

(b) X is ice. Y could be a mixture of tridecane and tetradecane.

"Could", because the range alone does not prove it.

5

Judging Purity

Your turn(a) Liquid P boils over 130 °C to 140 °C. What does this tell you? (b) Name two ways to test whether a solid is pure.

(a) A pure substance boils at one temperature. P boils over a range, so it is a mixture.

(b) Chromatography (a pure substance gives one spot) and melting or boiling point (a pure substance has a specific value).

Quick checkWhich separation method gives a pure solid from a solution of a substance that decomposes on heating?

Aevaporation to dryness

Bcrystallisation

Csimple distillation

Dsieving

NoteB: crystallisation is gentler. Evaporation to dryness would decompose the solute.

Exit ticket

Quick checkA substance melts over a range from −180 °C to −160 °C. What is it most likely to be?

Aan element

Ba compound

Ca mixture

Da pure gas

NoteC: a pure substance, element or compound, melts at one temperature. A mixture melts over a range.

Quick checkA spot has an R_f of 0.40. The solvent front moved 10.0 cm. How far did the spot move?

A2.5 cm

B4.0 cm

C6.0 cm

D25 cm

NoteB: distance moved by the spot = R_f × distance moved by the solvent front = 0.40 × 10.0 cm.