Worked examples: Introduction to Chemistry

Work through each example by covering the answer, attempting it yourself, then checking your method against the solution. Every example flags the slip that most often costs marks. These are original questions in SPM style, not past-year papers.

Example 1, Identifying variables

Question. A student investigates how the length of a magnesium ribbon affects the time taken for it to react completely with dilute acid. State the manipulated, responding and controlled variables.

Solution. The manipulated variable is the length of the magnesium ribbon (what is deliberately changed). The responding variable is the time taken for the ribbon to react completely (what is measured). Controlled variables include the concentration and volume of acid and the temperature, kept constant for a fair test.

Common slip: naming “the reaction” as the responding variable instead of a measurable quantity. The responding variable must be something you can measure, here the time.

Example 2, Writing a hypothesis

Question. Write a suitable hypothesis for the investigation in Example 1.

Solution. “The longer the magnesium ribbon, the longer the time taken to react completely.” A good hypothesis states the relationship between the manipulated and responding variables and is testable.

Common slip: writing a hypothesis that only names one variable, or that is a fact rather than a testable prediction.

Example 3, Choosing apparatus

Question. A titration requires exactly 25.0 cm³ of a solution to be delivered into a conical flask. Name the most suitable apparatus and justify the choice.

Solution. A pipette (or a burette), because it delivers an accurate, known volume. A measuring cylinder is not suitable because it gives only an approximate volume, which would make the titration result unreliable.

Common slip: choosing a measuring cylinder for an accurate volume, it is not precise enough for titration.

Example 4, Safety with a reason

Question. State one safety precaution when heating a liquid in a test tube, and give a reason.

Solution. Point the mouth of the test tube away from yourself and others, so that if the liquid spurts out suddenly, no one is scalded. (Another acceptable answer: heat gently and move the tube through the flame to avoid sudden boiling.)

Common slip: giving the precaution without the reason. The reason carries the mark.

Example 5, Hazard symbols

Question. A bottle carries a corrosive hazard symbol. State what this tells you and one precaution you should take.

Solution. The symbol tells you the chemical can attack and damage skin, eyes or materials on contact. A suitable precaution is to wear safety goggles and gloves and handle it carefully to avoid splashes.

Common slip: confusing “corrosive” with “toxic”, corrosive means it damages on contact, toxic means it is poisonous if taken in.

Example 6, Chemistry in daily life

Question. Give one example of how chemistry is used in medicine and name a related career.

Solution. Chemistry is used to develop and manufacture medicines such as painkillers and antibiotics that treat illness. A related career is a pharmacist, who prepares and dispenses medicines and advises on their safe use.

Common slip: naming a career with no link to chemistry, or giving an example without saying what the chemistry actually does.

Using these examples

Notice that every answer either names a measurable quantity, justifies a choice, or pairs a rule with a reason, the three habits this chapter rewards. Once the method feels automatic, move on to the practice questions and mark yourself the same way. A one-to-one teacher can check that your variable and safety answers are complete, which is exactly where marks are gained or lost in the SPM Chemistry written papers.

Example 7, Improving a fair test

Question. In an experiment to compare how quickly two antacids neutralise acid, a student uses a different volume of acid for each antacid. Explain why the results are unreliable and how to improve the experiment.

Solution. The results are unreliable because the volume of acid is a variable that has not been controlled, so any difference in the result could be caused by the different volumes rather than by the antacids. To improve the experiment, use the same volume and concentration of acid for both antacids, keeping every variable constant except the antacid being tested.

Common slip: suggesting a change that introduces a new uncontrolled variable, instead of fixing the one identified.

Example 8, Recording data correctly

Question. A student records the temperature of a reaction as “thirty-one”. State two improvements to how this reading is recorded.

Solution. First, include the unit (°C), because a number without a unit is meaningless in science. Second, record it to the precision of the instrument, for example 31.0 °C if the thermometer allows, so the reading is consistent and comparable. Clear, complete recording is expected throughout the practical papers.

Common slip: omitting the unit, a value with no unit cannot score, even if the number is right.

From examples to marks

These eight examples cover the whole chapter, and together they train the same core habits: identify measurable variables, control everything else, choose apparatus to match the accuracy needed, justify every safety rule, and record data with units and appropriate precision. Rehearse them until the reasoning is automatic, then test yourself with the practice questions for this chapter.