Complete Practical Exam Guide

I know all you science students have practical exams coming up so I’ve written this post to help you out.

Post written by Joanna Huang (1oth in the state Chemistry 2016). See all articles first and personally get in touch with our state rankers here

Assessment 2 is usually the time schools give out a practical exam to test your scientific method, ability to read instructions and perform investigations in a safe manner.

School practicals are usually worth 15% to 20% of your HSC School mark. Considering that your practical exams are probably out of 40 marks or less, it is quite important to secure all those marks, so have a look at the practical exam tips below!

1. Tables

  • Have a title that summarises what the table shows
  • Have them fully enclosed on all four sides.
  • Use a ruler
  • Have clear headings, with units in brackets. If the numbers to be recorded are very small or very large, use a multiplier in the units to avoid writing lots of zeroes in the data
  • Have the independent variable on the left hand column and the dependent variable on the right hand column.

2. Graphs

The purpose of drawing graphs is to allow you to see mathematical patterns or relationships. It allows you see if the two variables are directly proportional to each other, inversely proportional to each other, etc.

  • Have a heading!
  • Choose a suitable scale. Your graph should take up as much of the graph paper as possible. (At least ¾ of the way across and ¾ of the way up). Also try to increase in increments of 2, 5, 10 etc. rather than numbers harder to work with.
  • Decide if you should have a line of best fit, or curve of best fit.
  • When calculating the gradient, always draw the triangle you used by drawing construction lines in pencil. Also remember to include the units of the gradient.
  • When you are extrapolating, draw it in dotted lines in pencil.
  • When reading off graphs, if a point is marked close to an increment line, don’t assume it’s supposed to lie on the line. And don’t thicken the cross to convince yourself that it lays on the increment line.
  • Mark your points with crosses, rather than dots

3. Models

Because not everyone will be able to do the practical at the same time, some people may be doing another part of the exam.

Sometimes teachers use molecular modelling kits to model specific molecules and ask students to identify what it is. The molecules could range from water, to hydrocarbons, to monomers, to citric acid.

4. Risk Assessment

Remember to identify Hazard, describe its risk and the steps you did to minimise the risk. Your risk assessment could look something like this: (or in table format)

Chemistry Example: Reactivity of Cyclohexane and Cyclohexene with Bromine Water
Hazard: Cyclohexane and Cyclohexene are moderately toxic vapours
Risk: Aspiration of high concentrations into lungs will irritate the respiratory system and may lead to lung damage, nausea, and headaches.
Minimising the risk: Experiment was performed in a well-ventilated area, under a fume cupboard. Windows and fans were also turned on to dissipate the fumes

5. Accuracy

Accuracy refers to how close the value you obtained is to the “true” or accepted value. When asked to evaluate the accuracy of your answer, find the percentage error to the accepted value.

Accuracy also refers to how precise your measurements are, and hence is related to the precision of your instruments. How exact is the quantity measured? For example, if you use a 50mL beaker to measure 2mL, your results will be very inaccurate.

Precision of an instrument is limited to the finest division on its scale. To improve accuracy of the experiment, better and more precise equipment can always be used. For example, using a measuring cylinder instead of a beaker, and using a pH probe rather than indicators.

6. Reliability

In terms of first hand investigations, reliability refers to reproducibility. When I repeated my experiments, did I get consistent (identical or very close to identical) results? If so, then my experiment was reliable. Remember, reliability refers to repetition and consistency.

In terms of second hand sources, reliability refers to how trustworthy the source is. A question could ask you to explain how you decided if a source was reliable. In this type of question, there are few points to cover:

  • The domain of the site (if it’s a webpage): Does the URL include a domain such as .edu, .gov etc.? If so, then it is trustworthy and more reliable than a private webpage.
  • The author (if it’s a newspaper, journal, textbook or information site): If the author is a renowned and respectable professor in the field, then the source is quite reliable. If you obtained your information from Wikipedia or a blog post, you have to be careful about what you read and crosscheck with other sources.
  • Crosschecking: Comparing the source to other webpages, books and articles can also help assess the reliability of a source. See if they all say the same thing about the topic.

7. Validity

Teachers love to ask about validity because an invalid experiment implies poor scientific method and inaccurate results to follow.

So is my experiment valid? My experiment is valid if I’m measuring what I intend to measure. The point of a valid experiment is to fairly test my hypothesis. Hence for my experiment to be valid:

  • All variables apart from the independent and dependent variable must be kept constant
  • All systematic errors must be minimized
  • Random errors reduced by taking the mean of multiple measurements.

Quick Chemistry Example: The Combustion of Alkanols

The Aim: To perform an experiment to determine the heat of combustion of the Alkanols provided.

(Rough Overview)
In the experiment, 250mL of water was placed in a beaker, on top of a tripod, and heated using an alcohol burner. Temperature was taken using a thermometer.

Results were collected and then graphed: Molar heat of combustion Vs. Molecular mass. A linear graph was obtained.

Similar to when you would have performed the experiment at school, the results would have been quite inaccurate. In this experiment, accuracy could be improved by better equipment.

  • Using two foam cups to contain the water, and a foam covering on top so minimise heat loss to the environment
  • Using a data logger attached to a graphing device to measure the exact max temperature etc.

The experiment was reliable as different groups in the classroom, performing the same experiment with the same Alkanols got the same results.

Validity could be improved by better equipment, closing windows an fans to reduce air currents spreading the heat, and also heating up the Alkanol to undergo complete combustion rather than incomplete combustion to maximise energy and heat transfer.

But was the experiment valid?

No, if the aim was to obtain exact results of the heat of combustion of Alkanols. Yes, if the aim of the experiment was to show the trend of what happens to the heat of combustion when molecular weight increases.

Hence your judgement of validly should relate to your aim. In this case, the experiment was not valid.

8. Calculations

  • Only round off at the end. Rounding off early will lead to the wrong answer
  • Always remember to put your final answer in the appropriate number of significant figures
  • Remember to also always include units in your answer
Post written by Joanna Huang (10th in the state Chemistry 2016)

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