Oftentimes, the secret to getting yourself from a Band 5 to a Band 6 in an HSC Chemistry extended response isn’t at all in what you’re saying but how you’re saying it – and never is this more obvious than in a Chemistry 6-or-7 mark question. You’ll generally get 2, one in the core and one in the option and frequently they have very little to do with actual chemistry.
However, it is important to learn how to absolutely smash out good responses because this is what what separates the “good” people from the “great” people. The best part is – it’s actually pretty easy!
Step 1 – Read the question thoroughly
It sounds obvious, but you need to pick out the verbs and other directive terms that are going to define what direction you take. It’s these subtle differences that put you over the line. We’ll work through a question here as we go – a REAL one, from a REAL HSC (in fact, MY real HSC!)
2014 HSC Question 32(e) (7 marks)
Explain how the differences in the structure and composition of soaps and detergents determine their uses and their impacts on the environment.
First – the verb. According to the Board of Studies, ‘Explain’ means “relate cause and effect; make the relationships between things evident; provide why and/or how”. Thus, it’s perfectly acceptable to use “cause” and “effect” subheadings to help make your answer clearer.
And now for other directive terms: “Differences in structure and composition of soaps and detergents” – so most of the differences come from the composition, but the question is telling you to address structure as well.
“Determine their uses and impact on the environment” – there are TWO things here: the ‘uses’, and the ‘impact’. That means you have to address both with equal weight. First, what about is it about the chemistry of that class of surfactant? This is the cause. And then, what does that make it useful for? That’s the effect!
Step 2 – Plan your answer properly
NEVER just vomit information, you have to attack the syllabus points. Take a look at the sample answers below to see what the difference is.
We’ll also plan out our answer to 32(e) – there are four classes of surfactant, and for each we need to address uses and environmental concerns with reference to the molecular-level chemistry.
So, a good, easy answer structure that will tick all of the boxes might be:
Surfactant one: (e.g. Cationic surfactants)
USES/ENVIRONMENTAL IMPACT (pick one to go with here – we’ll write out a use for now)
Cause: The cationic amine group can weakly interrupt cell membranes, rupturing and killing bacteria and certain cells. This is a trait related to its chemical composition.
Effect: Cationic surfactants are effective topical disinfectants, since they can kill invading bacteria without damaging the skin cells in the treated area.
Rinse, lather, repeat – you’ve just built yourself an easy response that aims square for 7/7!
Step 3 – Consider if diagrams, tables or flow charts will help convey the information efficiently.
Remember that you have to keep your answer to space provided as best you can! The more you run over the allotted space the more risk you run of being penalised. Representing information visually can display your understanding far more thoroughly than the same amount of writing – hop on over to draw.io right now and try your hand to constructing a good flow chart.
In our example, it’s a great idea to include structural diagrams of the different classes of surfactant. This proves that know the structure yourself.
There’s a good example of what your diagrams might look like – labelled with their identity, and also labelling the non-polar hydrophobic tail, and the hydrophilic head. I do recommend using skeleton structures though, and don’t think you have to memorise the names!
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Step 4 – Try and picture what the marking criteria will be – and play to them.
This can get a bit dangerous, because it’s very possible you can guess wrong. But that’s what practice questions were for! You’ll be far more familiar with patterns in marking criteria than what you might realise.
What might be in our tester question? Looking at it again, it’s pretty obvious there’ll be marks for understanding the structure and composition of different soaps and detergents. In the same vein, there’ll be marks for understanding their uses, and environmental impacts (and remember – an impact isn’t always negative!). Since it’s an explain question, they’re be marks for spelling out the cause-effect relationship between the two. And there’s always marks for being clear and concise. There’s our guesses – what are the real criteria?
What do you know? Four for four! Having the idea of how to approach the question will go a long way towards getting into those top two brackets.
So now you know how – time to try it out! I’ll leave the surfactant question for now – it’s from an option, so you may not need that material covered. And plus, this way you can try writing an answer for it on your own!
EXAMPLE – 2014 HSC Question 31
With reference to the underlying chemistry and with relevant equations, assess the impacts on society of TWO uses of ethanol.
Let’s be careful here and go through our simple 4-step process. No need for you to rush!
- Key terms – underlying chemistry, assess (make a judgement of), impacts on society, TWO, uses of ethanol.
- Draw out the syllabus points – ethanol’s uses, the chemistry of ethanol, social impact. Select two uses – e.g. solvent and fuel. Establish what part of chemistry makes it useful in that field. Assess the impact on society.
- A diagram of ethanol’s polar and non-polar ends would be useful for conveying its use as a solvent but also could consume a lot of space – be discerning! Same be said for a flow chart of ethanol as a carbon-neutral fuel. In this instance, I would recommend no – not enough space.
- What’s likely to appear on the marking criteria?
- Connecting ethanol’s chemistry to its uses
- Assessing the extent of its impact on society
- Correct and accurate equations
- Fluency and chemical literacy
For reference, here’s the actual criteria!
Bam. Our guesses were pretty much straight on. So let’s try this again.
Ethanol is a compound with many uses that are influenced by its chemistry. Two of most common uses are as a solvent and as a fuel.
Ethanol is useful as a solvent as the C2H5 group is non-polar, and this allows it to dissolve other non-polar substances by forming dispersion forces more powerful than those between the target molecules. However, the polar OH group can form both polar and hydrogen bonds, allowing ethanol to dissolve polar and ionic compounds, even if they have strong hydrogen bonding. In particular, this makes it miscible with water in all proportions. This means a small portion of ethanol can be used to dissolve a non-polar medications and then this can be diluted to different dosages easily with water, among many other similar uses. This gives us an easy and cost-effective way to treat children, elderly, or disabled people who cannot take tablets. This has been a hugely positive benefit to society.
Ethanol has also seen limited use as fuel, since organic substances can all undergo complete combustion to release energy. It is produced by fermentation of glucose from sugar cane, then combusted, according to the following formulae:
Photosynthesis: 6CO2(g)+ 6H2O(l) → C6H12O6(aq) + 6O2 (g)
Fermentation: C6H12O6(aq) → 2 C2H5OH(l) + 2 CO2(g)
Combustion: 2C2H5OH(l) + 6O2(g) → 4CO2(g) + 6H2O(g)
It can be seen that overall, no substances are produced or consumed – it is a closed chemical loop. This makes ethanol a carbon-neutral fuel, making it more environmentally friendly than present fuels such as octane. However, since it is miscible with water, ethanol has been known to damage combustion engines and also has a lower energy density than present fuels. These two factors mean it has not seen widespread use and thus has had limited impact on society. However, by improving environmental conditions and reducing fuel costs, the impact is has had has been positive, and stands to expand with further research.
In summation, ethanol’s uses as a solvent and a fuel have both proved positive for society – more so as a solvent but advances in technology could allow the impact of ethanol as a fuel to expand.
Now that is a nice answer.
It’s a shame that it didn’t lend itself to diagrams, and also a shame that the formatting of the chemical states in the equations isn’t consistent, nor are they technically complete – reaction conditions should be written above the arrows (this is why I like handwriting these things).
But other than that, it covers everything. It demonstrates a very deep understanding of the connection between the fundamental chemistry and its uses, awareness of the social dimension of the question and uses clear, concise language. I don’t mind admitting this answer is hundreds of times better than the one I submitted when this question appeared in my actual exams – we all learn from our mistakes! This is an easy 6, and a strong candidate for a 7! (I don’t like saying it’s a surefire 7 because that last mark has less to do with the quality of your answer and more to do with the quality of a typical answer. The Chemistry exams are marked in such a way that there’s a roughly bell-curved-shaped distribution of marks, so the difference between 6 and 7 isn’t about being good – it’s about being better than everyone else.)
For comparison, here’s an answer I might have written without considering any of the above points.
Ethanol is used as both a solvent and a fuel. It is a useful solvent because it has a polar and a non-polar end, and this allows to dissolve non-polar substances, polar substances and ionic lattices. Comparatively, other common solvents can only dissolve either non-polar substances or polar/ionic substances. This makes it useful for dissolving multiple different compounds at once, hence why it is frequently used in perfumes which contain a substances with mixed polarities. While it has a low energy density as a fuel, it is carbon neutral and hence more environmentally friendly than octane and other traditional fuels. However, since it is miscible with water it often causes damage to combustion engines, and hence has not seen widespread use as a fuel.
Okay, so what’s wrong with this answer? Or, rather, what’s not wrong with it? It almost completely ignores the social impact of both uses and uses an incredibly poor example of ethanol as a solvent. Of all the impacts that’s had on society, perfume would have to be the least meaningful.
On top of that, it skims over the relevant chemistry and has no equations, despite the carbon-neutral statement absolutely SCREAMING for THREE. Three whole equations!
This answer would be lucky to score a 3/7 – with the equations! Without them, it’s almost destined for a 1/7. But, likewise, that bell-curve marking could pull it up to a 2/7 if there are some other, really, REALLY, poor responses.
But why was the first answer so much better? Because of practice, attention to detail and hard work.
Matt Saunders is a huge nerd who first got into writing through fanfiction. He’d known science was the path for him since a young age, and after discovering a particular love of bad chemistry jokes (and chemistry too), he’s gone onto to study Forensic Chemistry at UTS. His HSC in 2014 was defined in equal parts by schoolwork and stagecraft, which left him, weirdly enough, with a love of Maths strong enough to inspire him to tutor any level, along with 7-10 Science and HSC Chemistry.