Worked Solutions
Module 1: Properties and Structure of Matter — Worked Solutions (Preliminary Chemistry)
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Worked examples for Preliminary Chemistry Module 1: Properties and Structure of Matter. Each shows where the marks are awarded, the key idea, and the full solution explained by your choice of tutor — Stella, Ella or Cassie.
How to use these
Try each question first, then check your working. Use the tutor tabs to read the full solution in the style that suits you: Stella is direct and challenging, Ella is warm and explains the why, and Cassie is concise and analytical.
Example 1 — Separating a mixture
Question
A student is given a mixture of sand, sodium chloride and iron filings. Describe a sequence of physical techniques that could be used to separate this mixture into its three pure components, and justify the property exploited at each step.
Solution
Each step exploits one physical property. Work through them in order.
Step 1 — magnetism. Iron filings are magnetic; sand and salt are not. Pass a magnet over the mixture to remove the iron filings.
Step 2 — solubility. Sodium chloride dissolves in water; sand does not. Add water and stir, then filter. The sand stays on the filter paper (residue); the salt passes through dissolved in the filtrate.
Step 3 — boiling point / volatility. Evaporate the filtrate. Water boils off and the solid sodium chloride is left behind.
Three components, three properties: magnetism, solubility, then volatility. Name the property at every step — that's where the marks are.
The trick with separations is to ask: what's different about each substance that I can use? We separate physically, so we never change what the substances are — we just sort them.
First, the iron filings are magnetic while sand and salt are not. So we run a magnet over the mixture and lift the iron out cleanly.
Next, only the sodium chloride dissolves in water — sand is insoluble. So we add water: the salt goes into solution, the sand stays as solid. We then filter, and the sand is trapped on the filter paper while the salty water (the filtrate) passes through.
Finally, the salt is dissolved in the water, so we use the difference in boiling point. We evaporate the water away (it has a much lower boiling point) and the solid sodium chloride crystallises out.
The reason this works is that each technique targets a property unique to one component, so we peel them off one at a time.
Three components → three physical properties.
- Magnetism: magnet removes iron filings.
- Solubility: add water, filter → sand = residue, salt = filtrate.
- Boiling point: evaporate filtrate → solid $\text{NaCl}$ remains, water boils off.
Order matters: magnetism, then dissolve/filter, then evaporate.
Where the marks go
- 1 mark: Uses magnetism to separate the iron filings
- 1 mark: Uses dissolving and filtration to separate sand from salt
- 1 mark: Uses evaporation to recover solid sodium chloride and justifies the property exploited
Key idea
Physical separation exploits differences in physical properties — magnetism, solubility and boiling point — without changing the chemical identity of the substances.
Example 2 — Electron configuration
Question
Write the full electron configuration (using the Schrödinger/subshell model) of a neutral sulfur atom ($Z = 16$), and use it to explain why sulfur is placed in Group 16 of the periodic table.
Solution
Fill the subshells in order of increasing energy until you've placed all 16 electrons.
$1s^2\,2s^2\,2p^6\,3s^2\,3p^4$ — count them: $2+2+6+2+4 = 16$. Correct.
Group placement comes from the valence shell, which is the highest principal energy level, $n = 3$ here. That shell holds $3s^2\,3p^4$, so 6 valence electrons.
Six valence electrons → Group 16. Don't count inner-shell electrons toward the group number; only the outer shell matters.
We build the configuration by filling subshells from lowest energy upward — this is the Aufbau idea. Sulfur has 16 electrons to place.
Start at $1s$ (holds 2), then $2s$ (2), then $2p$ (6), then $3s$ (2). That's 12 so far, leaving 4, which go into $3p$. So the configuration is $1s^2\,2s^2\,2p^6\,3s^2\,3p^4$.
Now, why Group 16? The group number for a main-group element comes from the number of valence electrons — the electrons in the outermost shell, here $n = 3$. We have $3s^2\,3p^4$, which is $2 + 4 = 6$ electrons.
Six outer-shell electrons means sulfur sits in Group 16. The reason this matters is that those valence electrons are the ones involved in bonding, so they dictate the element's chemistry — and elements in the same group share similar valence arrangements.
Fill subshells for 16 electrons:
- $1s^2\,2s^2\,2p^6\,3s^2\,3p^4$ (total $= 16$)
Valence shell ($n = 3$):
- $3s^2\,3p^4$ → 6 valence electrons
6 valence electrons → Group 16.
Where the marks go
- 1 mark: Correct full electron configuration $1s^2\,2s^2\,2p^6\,3s^2\,3p^4$
- 1 mark: Identifies the valence shell electrons as $3s^2\,3p^4$ (6 electrons)
- 1 mark: Links 6 valence electrons to placement in Group 16
Key idea
Electrons fill subshells from lowest energy upward; the number of valence (outermost shell) electrons determines a main-group element's group number.