Unit 2 Synthesis
You'll be able to classify any sample of matter correctly using the full hierarchy.
Forensic scientists can separate the coloured dyes hidden inside a single dot of black ink using paper chromatography, turning one black mark into a rainbow, with the same separation skills you built in this unit.
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Q1 · Without looking at notes, draw the "matter family tree", how does matter split from the top level down through pure substances (elements and compounds) and mixtures?
Q2 · Name one thing you found genuinely surprising or confusing in Unit 2. Why did it surprise you?
● Know
- The full classification hierarchy of matter (pure substance/mixture → element/compound/homogeneous/heterogeneous)
- The three states of matter and their particle arrangements
- How a solution forms (solute, solvent) and what affects solubility
● Understand
- Why elements and compounds are both pure substances but are fundamentally different
- How the particle model explains changes of state and diffusion
- How to choose a separation technique from the properties of a mixture
● Can do
- Classify any sample of matter correctly using the full hierarchy
- Answer exam-style questions across all Unit 2 topics
- Select and justify a separation technique for a given mixture
You started Unit 2 with a piece of matter. A whole unit later, you can now classify exactly what that matter is, a pure substance (element or compound) or a mixture, and say how you would separate it. Here's the full picture.
Every sample of matter fits into this hierarchy:
Text description
- All matter splits into pure substances and mixtures.
- Pure substance: element (e.g. O₂, Fe, Au) or compound (e.g. H₂O, NaCl, CO₂).
- Mixture: homogeneous (e.g. saltwater) or heterogeneous (e.g. sand + water).
Key distinctions:
- Element vs Compound: Both are pure substances, one type of particle throughout. Elements have ONE type of atom; compounds have two or more types chemically bonded. You cannot separate a compound by physical means, you need a chemical reaction.
- Pure substance vs Mixture: Mixtures have variable composition and can be separated physically. Pure substances always have the exact same fixed properties (like a specific melting point).
| Substance | Category | Reason |
|---|---|---|
| O₂ (oxygen gas) | Element | One type of atom (oxygen) only |
| H₂O (water) | Compound | Two types of atoms (H and O) chemically bonded |
| Air | Homogeneous mixture | N₂, O₂, Ar etc. evenly mixed, no chemical bonding |
| Sand + water | Heterogeneous mixture | Visible separate parts, separated by filtering |
| NaCl (table salt) | Compound | Na and Cl chemically bonded, one type of particle (NaCl formula unit) |
The particle model explains the properties of solids, liquids and gases at the particle level:
| State | Particle arrangement | Particle movement | Shape and volume |
|---|---|---|---|
| Solid | Close together, ordered (regular pattern) | Vibrate in place only | Fixed shape and fixed volume |
| Liquid | Close together but disordered | Slide/flow past each other | No fixed shape; fixed volume |
| Gas | Far apart, random | Move quickly in all directions | No fixed shape or volume |
Changes of state (all physical changes, no new substance formed):
- Going up the list, from solid to liquid to gas, the particles are gaining energy and the attractions between them are losing the argument. Melting (solid → liquid) happens when the vibrations get violent enough to break the particles out of their fixed positions, so they can slide instead of only wobble. Boiling and evaporation (liquid → gas) happen when particles gain enough energy to escape the liquid's surface altogether.
- Going down the list the same thing runs backwards, because the particles are losing energy rather than gaining it. Condensation (gas → liquid) is particles slowing down until the attractions can hold them close again, and freezing (liquid → solid) is them slowing further until the attractions lock them into place. That is why every one of these changes is reversible: nothing was made or destroyed, the particles just changed how much energy they were carrying.
- Sublimation (solid → gas, skipping liquid) is the odd one, and it is worth knowing why it is odd. In dry ice and in iodine the attractions between particles are weak enough that the particles escape straight into the gas state before the solid ever gets the chance to collapse into a liquid.
Two other important particle model predictions:
- Diffusion particles spread from high concentration to low concentration. Explains why a drop of food colouring spreads in water without stirring.
- Expansion when heated particles move faster and push further apart, so materials expand. Hot air balloons work because warm air is less dense.
When a solute dissolves evenly in a solvent, the result is a solution (a homogeneous mixture). For example, salt (solute) dissolves in water (solvent) to make salt water (solution).
- A dilute solution has only a little dissolved solute; a concentrated solution has a lot.
- A saturated solution holds the maximum solute that will dissolve at that temperature, no more will dissolve.
- Solubility usually increases with temperature: warm water dissolves more sugar or salt than cold water.
Because the parts of a mixture keep their own properties, we can separate them physically. Choose the technique that matches the property that differs:
| Technique | Separates | Property used |
|---|---|---|
| Filtration (filtering) | Insoluble solid from a liquid (sand from water) | Particle size |
| Evaporation | A dissolved solid from its solvent (salt from salt water) | Boiling point of the solvent |
| Distillation | Two liquids, or a solvent you want to keep | Different boiling points |
| Chromatography | Dissolved coloured substances (dyes in ink) | Different solubility / attraction to paper |
No new substances are made when you separate a mixture, it is a physical process, so the parts can be recovered unchanged.
One last thread ties the unit together. Almost every claim in these thirteen lessons is one somebody had to test, and Lesson 11 gave you the rules for testing fairly: change one thing on purpose (the independent variable), measure one thing to see what happened (the dependent variable), and hold everything else still (the controlled variables). Those three labels are not just for the fair-test lesson. "Hot water dissolves more sugar" is a claim about an independent variable, temperature, and a dependent variable, mass dissolved, and it only means anything because everything else was kept the same. Whenever this unit told you that one factor affects another, that is the shape of the evidence behind it.
A forms when a dissolves in a . When no more can dissolve at that temperature, the solution is . Heating the solvent usually lets it dissolve solute.
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1Start with the test that splits the field fastestStir a spoonful into warm water. Does it dissolve?Do not start with the most impressive test, start with the one that rules out the most. Salt and sugar are both soluble and sand is not, so this single observation eliminates a third of the possibilities in about thirty seconds. If anything is left sitting on the bottom, filter it out and you have a sand sample in the residue.
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2Find a property where the two survivors differHeat a dry sample gently in an evaporating basin.Salt and sugar behave identically in water, so no amount of dissolving will separate them and you need a different property. Their melting points are nowhere near each other: sugar melts at about 186 °C and then browns and chars, while salt does not melt until about 801 °C, so a Bunsen flame leaves it completely unchanged. Browning means sugar; nothing happening means salt.
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3Confirm rather than assumeEvaporate the solution from step 1 and look at what crystallises out.One test that agrees with your guess is not proof, because a single result can have more than one cause. Getting the powder back unchanged also confirms something worth stating: dissolving was a physical change, not a chemical one, so nothing new was ever made and the sample you are identifying is still the sample you started with.
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4Say what the evidence supports, and what it does notDissolves, does not char, crystallises back as cubes: table salt.A good answer names the limits of its own evidence. These tests separate the three candidates you were given; they do not prove the powder is not some fourth soluble white solid nobody mentioned. And never taste an unknown substance to identify it, however tempting the shortcut looks. Measuring a physical property is what a laboratory does instead, though note which property: Lesson 3's displacement method is no use on a soluble powder, because the sample would dissolve into the very water whose rise you were measuring.
A student is given an unlabelled white powder. She adds it to water and it dissolves completely, leaving a clear solution. She then evaporates the water and gets the powder back. Predict: is the powder an element, a compound, or a mixture? Explain your reasoning using Unit 2 concepts.
How close was your prediction?
Good reasoning, you used evidence from the observation, not just guessing.
Key idea: scientific reasoning uses observations as evidence. One test doesn't tell the whole story, scientists need multiple tests to classify an unknown substance.
For each of the following, write its full classification (element / compound / homogeneous mixture / heterogeneous mixture) and explain your reasoning in one sentence.
- Oxygen gas (O₂)
- Salt water
- Glucose (C₆H₁₂O₆)
- Muesli
- Copper wire (pure)
- Carbon dioxide (CO₂)
A student is given an unknown white powder. Using the skills from Unit 2, describe a series of at least three tests they could perform to help identify it. For each test:
- Name the test or observation
- Describe what they would observe and record
- Explain what the result would tell them about the powder
At the start of this lesson you were given the challenge: You're given an unlabelled white powder, using everything you've learned in Unit 2, how would you start figuring out what it is? Now you have all the tools to answer that properly!
Write a step-by-step investigation plan for identifying the mystery white powder. Then complete the full matter family tree, labelling every branch and adding one real example at each category (element, compound, homogeneous mixture, heterogeneous mixture).
Q1. Classify each of the following as element, compound or mixture, and explain your reasoning: (a) oxygen gas (O₂) (b) salt water (c) glucose (C₆H₁₂O₆). (3 marks)
Q2. Describe what happens at the particle level when water boils. Use the particle model to explain why bubbles of gas form. (3 marks)
Q3. A student is given an unknown white powder. Describe a series of tests (at least three) they could conduct to help identify it. For each test, name what they would observe and what it would tell them. (5 marks)
Answers
▾MCQ 1
C H₂O (water) is a compound because it contains two types of atoms (hydrogen and oxygen) chemically bonded together. O₂ and Au are elements (one type of atom each); air is a mixture.
MCQ 2
B One type of atom that cannot be broken down further = element. A compound also can't be broken down physically, but it has two or more types of atoms. A mixture can always be separated physically.
MCQ 3
D Salt and pepper are two different substances physically mixed together. You can clearly see the white salt grains and the dark pepper flakes, different visible parts = heterogeneous mixture.
MCQ 4
A In a solution, the substance that dissolves (sugar) is the solute, and the liquid it dissolves into (water) is the solvent. Together they form a solution.
MCQ 5
B Sand is insoluble and its grains are large, so filtering through filter paper traps the sand while the water passes through. Distillation or evaporation would needlessly remove the water, chromatography separates dissolved colours, and magnetism only works on magnetic materials.
Short Answer 1
Model answer: (a) O₂ is an element it contains only one type of atom (oxygen) and cannot be broken down into simpler substances. (b) Salt water is a homogeneous mixture it contains water and dissolved salt (two substances) physically combined; it looks even throughout and can be separated by evaporation. (c) Glucose (C₆H₁₂O₆) is a compound it contains carbon, hydrogen and oxygen chemically bonded together; it is a pure substance with one type of particle (glucose molecule).
Short Answer 2
Model answer: When water boils, the liquid water particles gain energy from the heat source. Their speed increases until they have enough energy to overcome the attractions holding them close together. The particles escape from the liquid and spread out into the gas phase (water vapour). Bubbles of gas form because groups of water particles below the surface have enough energy to escape into the gas phase simultaneously, these pockets of gas rise to the surface and appear as bubbles. Boiling is a physical change, the water molecules (H₂O) themselves are unchanged, just further apart.
Short Answer 3
Model answer: Test 1: Add to water and stir, observe whether it dissolves. If it fully dissolves to give a clear solution, this suggests it may be a pure substance or a soluble compound (like salt or sugar). If it doesn't dissolve or leaves visible particles, it may be a mixture or an insoluble substance. Test 2: Never taste an unknown substance, that is one of the first safety rules in any lab, and it is not needed here because the safe tests do the job. Note that density is not the tool for this particular sample: measuring the volume by displacement fails because salt and sugar dissolve in the very water whose rise you would be measuring. Displacement is fine for an insoluble solid, granular ones included, provided you dislodge the air trapped between the grains. Use behaviour in water and behaviour on heating instead. Test 3: Evaporate the solution (if it dissolved), if a white residue forms, the substance is non-volatile (like salt) not sugar (which would caramelise/burn). Test 4: Heat a small sample gently in an evaporating basin. Sugar melts and then browns and chars well below 200 °C; salt does not change at all, because its melting point is about 801 °C. Each test narrows down the possible identity of the powder.