HSC Chemistry

HSC Chemistry is one of the most rewarding HSC subjects you can choose. In terms of scaling, Chemistry has consistently been the highest scaled HSC science course, compared to Physics and Biology. Chemistry also provides a very useful foundation for university courses in the health sciences fields (Medicine, Pharmacy and Medical science in particular). With typically around 10,000 students doing Chemistry for their HSC each year, it is also one of the most popular HSC subjects chosen. If you can do well in Chemistry, it will greatly help your UAI and your chances of getting into the university course you desire.

Why choose HSC Chemistry
As mentioned, HSC Chemistry is the highest scaled science course commonly available across practically all schools in NSW. The first reason is that because sciences (HSC Physics in particular) generally scale well, there is an economy of scale in choosing and doing both subjects. For example, if you are a logically oriented student who tends to do well at quantitative / conceptual-based subjects like mathematics, there is a good chance you will enjoy science subjects. The sad thing about the HSC and the way schools structure their subject offerings (for most schools anyway) is that students often do not have much subjects to choose from. Therefore they are left with little choice from which they can select, and most often always end up doing the same subjects (Mathematics + science combination). While this is not a bad thing, this means that if you are a student who is intent on choosing quantitative subjects, you will most likely doing at least 2 out of the 3 subjects. Based on scaling statistics of past years, Chemistry and Physics scale the highest out of the sciences.

Students should also note that Chemistry has traditionally scaled as well as English Advanced. In the past few years, HSC Chemistry had a scaled mean (published by UAC's yearly scaling report, in their Table A3) of around 30/50. This places HSC Chemistry at around the same scaled mean as Economics, English Advanced, and slightly higher than Physics (28-29 out of 50 in recent years). While it is recommended that you choose subjects based on your talents and interests, if you are going to do at least 1 or 2 HSC science subjects, you may as well choose Chemistry as one of your science subjects in order to benefit from the good scaling .

Doing well in HSC Chemistry
HSC Chemistry is a very experience-based course. There are many things which a student will realise at the end of their Preliminary Chemistry course, or even halfway through their HSC year. For example, students find it hard to accept that there is no clearly defined pattern when trying to determine the valency of transition metals. Valencies of common anions and cations need to be rote-memorised, as there is no common thread of logic which can be used to derive them (not within the scope of the HSC subject, that is). Therefore many things come with experience, as time goes on and students slowly familiarise with the piecemeal bits of facts that they need to remember and use throughout HSC Chemistry. We will look at a few key examples of what we mean which makes this course experience-based.

Common valencies
The common valencies of anions and cations need to be remembered quite well. For example, there is no 'reason' that will be given to you throughout your HSC why carbonate ions have a charge of -2. Similarly there is no 'reason' that will be given to you to explain why silver ions have a charge of +1, whereas most other transition metals have an oxidation state of +2. These odd exceptions and facts will come with experience.

Some common valencies you should remember are:
– How to calculate the charge on monatomic ions using the periodic table. For example, Groups I, II and III would have a charge of +1, +2 and +3 respectively, whereas Groups V, VI and VII would have a charge of -3, -2 and -1 respectively.
– Transition metals have an oxidation state of +2 most of the time. Know the exceptions (discussed in next point)
– Common exceptions to transition metals having a +2 oxidatoin state are: Iron (can be iron (II) or iron (III)), copper (can be copper (I) or copper (II)) and silver (almost always +1 only, as silver (I)).
– All the common polyatomic anions (carbonate, sulfate, nitrate are the three that are most commonly referred to throughout the course)

Solubility rules
Solubility rules for HSC Chemistry are important to remember, as most of the time they help you get the state of various salts correct when writing your balanced formulae. For example, in the reaction between magnesium metal and dilute sulfuric acid, how would you know whether the resultant salt, magnesium sulfate, is in aqueous or solid state? You would know this only from remembering some general rules of solubility, that magnesium sulfate would be soluble in water.

Some commonly applicable solubility rules you will need for HSC Chemistry:

– All alkali metals (Group I metals) like sodium, potassium, lithium etc are soluble as an ion
– All nitrate salts are soluble
– All chloride salts are soluble
– Most alkali earth metals (Group II) like magnesium, calcium etc are soluble as an ion
– All hydrogen compounds (ie common acids like sulfuric acids, nitric acid, hydrochloric acid) are soluble.
– Only some hydroxides are soluble (be careful here)
– Only some sulfides are soluble
– Only some carbonates are soluble
– Only some phosphates are soluble

The above is actually a very general and basic recall of the complete solubility rules that a good student should remember. Actually this is just from the top of the author's memory from when he did his HSC many years ago, but it highlights the point that solubility rules ought to be remembered well. There will be many situations where you would like to know about the water-solubility of certain salts, in order to get the state correct. You can often find neat and useful summaries of solubility rules at various places online that are sufficient for HSC purposes.

Module-specific experience
HSC Chemistry modules are similar to HSC Physics in that they appear quite piecemeal and separated from each other. A student can have an excellent understanding in one module but have a poor understanding of the next. Therefore it is important to keep a consistent regime of study throughout the HSC year, and gain a comprehensive understanding of each module.

Within each module, a good Chemistry student would need to know about the subtle points in order to have a complete understanding. For example, in the 'Production of Materials' module, it is a good idea to read through a reputable textbook like Chemistry in Contexts or Conquering Chemistry and get a feel of all the various polymers (addition and condensation polymer types) that can be produced from various monomers. A good student would be able to identify the relationship between the monomer used and the polymer it results in, as well as some basic chemical and physical properties that can be predicted from looking at the polymer or even monomer structure. For example, if we see large functional groups, we know there will be chain stiffening, causing hardness, rigidity and tensile strength of the resultant polymer. If we add plasticisers or vulcanise the polymer, we know this will give the polymer flexibility and elastic properties (eg garden hose made from PVC). All these little facts come from experience, from sitting down and reading into a textbook to get the necessary background information needed. Or you may have a great teacher at school or HSC tutoring which might supplant your knowledge with the necessary background information.

Another example, in the next module, 'The Acidic Environment', the content deals almost exclusively with acids and bases, and the reactions that come from dealing with such chemicals. Through doing many questions and figuring why you went wrong each time you did, you should gain a mastery of predicting how buffers react to changes via Le Chatelier's principle. Nearing the final exams, a good student would be able to predict all reactions to changes at a glance. For example, a common enclosed system is a fizzy softdrink. If you pressurise a softdrink can with more carbon dioxide, what happens? Increased gas pressure results in more dissolution of carbon dioxide in order to counteract the pressure change. What if you increase the temperature? Increasing temperature causes the system to react endothermically, which is the release of carbon dioxide gas. Also the specific solubility of carbon dioxide decreases as you increase temperature. Students should be able to identify and relate all these aspects of an enclosed system in order to achieve an excellent mark from HSC Chemistry.

How to ace HSC Chemistry
The short answer is to gain the necessary experience. Do not feel bad when at first the amount of odd facts which do not fit into any pattern seems overwhelming. Do not let that demotivate and demoralise you. Instead, understand that all the necessary knowledge will come with experience. Practice makes perfect, so do more questions and ask more questions. If there's anything you do not understand, ask a teacher or tutor.

It is important to gain a solid grasp of the important fundamentals early on for a subject like Chemistry. What this means is to get a good understanding of the things which you will use again and again throughout your HSC Chemistry course, early on, preferably before year 12 starts. The things mentioned in this article, plus the following, are repeatedly used throughout the entire course:

– Common valencies (discussed above)
– Solubility rules (discussed above)
– Naming salts and covalent compounds
– Identifying the bonding structure of common substances – covalent molecular? ionic lattice? Covalent lattice? Metallic lattice?
– Understand how intermolecular forces work, and how they relate to physical properties (boiling and melting points, ductility, luster, hardness, flexibility, tensile strength etc)
– Naming carbon compounds (including multi-chains containing functional groups, multiple double and triple bonds, with attached halogens)