Unit 2 Synthesis
In 2016, researchers at the University of Melbourne used 2.3 million X-ray measurements to map a single protein molecule atom by atom — applying every concept from this unit in one experiment.
Printable Worksheets
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Q1 · Without looking at notes, draw the "matter family tree" — how does matter split from the top level all the way down to atoms?
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
- The three sub-atomic particles and the meaning of atomic number and mass number
● 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 read atomic number and mass number from a periodic table entry
● Can do
- Classify any sample of matter correctly using the full hierarchy
- Answer exam-style questions across all Unit 2 topics
- Count atoms in a chemical formula using subscripts
You started Unit 2 with a piece of matter. Twenty lessons in, you can now trace exactly what that matter is made of — right down to the atom. Here's the full picture.
Every sample of matter fits into this hierarchy:
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):
- Solid → Liquid: melting (particles gain energy, vibrations break free of fixed positions)
- Liquid → Gas: boiling/evaporation (particles gain enough energy to escape surface)
- Gas → Liquid: condensation (particles lose energy)
- Liquid → Solid: freezing (particles lose energy, lock into place)
- Solid → Gas directly: sublimation (e.g. dry ice, iodine)
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.
Every atom is made of three types of sub-atomic particle:
| Particle | Location | Charge | Relative mass |
|---|---|---|---|
| Proton | Nucleus | Positive (+1) | 1 |
| Neutron | Nucleus | Neutral (0) | 1 |
| Electron | Shells around nucleus | Negative (−1) | ~0 (negligible) |
Reading a periodic table entry for an element:
- Atomic number = number of protons in the nucleus. In a neutral atom, this also equals the number of electrons.
- Mass number = number of protons + number of neutrons. So: neutrons = mass number − atomic number.
Example: Carbon has atomic number 6 and mass number 12. So carbon has 6 protons, 6 electrons and 6 neutrons (12 − 6 = 6).
The periodic table is arranged so that:
- Periods (rows) = number of electron shells the atom has.
- Groups (columns) = number of electrons in the outermost shell (and similar chemical properties).
Reading chemical formulas: subscripts tell you how many atoms of each element. CO₂ has 1 carbon atom and 2 oxygen atoms. Ca(OH)₂ has 1 calcium, 2 oxygen and 2 hydrogen atoms = 5 atoms total.
The of an element tells you the number of in the nucleus. In a neutral atom, it also equals the number of . The mass number equals protons plus .
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
C — Ca(OH)₂: 1 calcium (Ca) + 2 oxygen (O) + 2 hydrogen (H) = 5 atoms total. The subscript 2 outside the bracket multiplies everything inside it: OH × 2 = O₂H₂.
MCQ 5
C — A colour change is one of the five signs of a chemical change. The iodine reacts with starch to form a new blue-black coloured substance (iodine-starch complex). This is not reversible under normal conditions — a new substance has formed.
Short Answer 1
Model answer: (a) O₂ is an element — it contains only one type of atom (oxygen), and oxygen is listed on the periodic table. (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 atoms 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: Taste a tiny amount (only if known safe) or check if it is sweet/salty/tasteless — can help distinguish sugar (sweet), salt (salty) or another unknown. 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: Add iodine solution — a blue-black colour change would confirm starch is present. Each test narrows down the possible identity of the powder.