9.8 - Planning an Investigation
What a planning question looks like
In a planning question, you're asked to design a suitable experiment:
A student wants to find out how the concentration of hydrochloric acid affects the rate of its reaction with magnesium. Plan an investigation the student could carry out.
A good plan covers each essential part of an investigation rather than describing one step in excessive detail:
- The quantity you change (with a range of values)
- The quantity you measure (and how it's measured)
- The quantities you keep the same
- Apparatus and method
- Repeats
- Safety (a hazard and a precaution to manage it)
The ingredients of a plan
| Ingredient | What it means | Worked example (acid concentration and rate) |
|---|---|---|
| The quantity you change | A range of values with even steps | 0.5, 1.0, 1.5, 2.0, 2.5 mol/l acid, diluted from a stock solution |
| The quantity you measure | What is measured, and how | Volume of hydrogen gas, read from a measuring cylinder inverted over water at fixed times |
| Quantities kept the same | Named specifically with values | Mass of magnesium, volume of acid, temperature |
| Apparatus | Matched to each measurement | Measuring cylinder, delivery tube, balance, stopwatch |
| Method | Step-by-step instructions | Add a fixed mass of magnesium to a fixed volume of acid, seal at once, record gas volume at fixed times; repeat per concentration |
| Repeats | Repeat at each value | Two or three runs per concentration to check the volumes agree |
| How results answer the aim | What the comparison shows | Compare gas volumes across concentrations to see if a higher concentration increases rate |
| Safety | A hazard with a precaution | Dilute acid is an irritant (wear eye protection); hydrogen gas is flammable (keep away from naked flames) |
Name constants specifically
State the specific quantity being kept constant, not a vague label:
- "Keep the volume of acid at 50 cm3" scores; "keep the acid the same" does not.
- "Keep the concentration of the electrolyte constant" scores; "keep the electrolyte the same" does not.
Choosing the right apparatus
A good plan names the specific piece of equipment needed for each measurement and matches its precision:
| Job to do | Apparatus | Example |
|---|---|---|
| Measure out a volume of solution roughly | Measuring cylinder | Measuring 50 cm3 of dilute hydrochloric acid |
| Measure out a fixed, precise volume | Pipette | Measuring exactly 10 cm3 of sodium hydroxide for a titration |
| Add a variable volume precisely drop by drop | Burette | Adding acid in a titration until the indicator changes colour |
| Measure mass | Balance | Weighing magnesium ribbon or marble chips |
| Measure temperature | Thermometer | Measuring temperature rise in a calorimetry experiment |
| Time an event | Stopwatch | Timing how long it takes to collect a fixed volume of gas |
| Collect an insoluble gas | Measuring cylinder inverted over water with a delivery tube (or gas syringe) | Collecting hydrogen gas produced in a reaction |
| Heat a substance | Bunsen burner with tripod and gauze (or hotplate for flammable liquids) | Heating a reaction mixture |
| Separate an insoluble solid from a liquid | Filter funnel and filter paper | Separating a precipitate from a solution |
Improvements to methods
- Replace a measuring cylinder with a pipette or burette when higher precision is needed.
- Replace an open container with an inverted measuring cylinder over water or a gas syringe if a gas was lost.
Common exam questions
Question 1
A student plans to investigate how the concentration of hydrochloric acid affects the rate of its reaction with magnesium. Name the quantity to change, one quantity to keep the same, and how the results would be measured.
The quantity to change is the concentration of the acid, given a range of values such as 0.5 to 2.5 mol/l. A quantity to keep the same is the mass of magnesium ribbon used each time. The results are measured as the volume of hydrogen gas produced, read from a measuring cylinder inverted over water at fixed time intervals.
Question 2
Name the kinds of thing a good chemistry plan should include.
The quantity to change, given a range of values; a quantity you can measure and how you would measure it; the quantities kept the same, named specifically; the apparatus and method; repeats at each value; and a line on safety.
Question 3
Why is "use different concentrations" a weak way to describe the quantity you change in a plan?
Because it gives no range; a good answer states several specific values in even steps, such as 0.5, 1.0, 1.5, 2.0, and 2.5 mol/l.
Question 4
A student investigating the voltage of a simple cell writes "keep the electrolyte the same" as a constant. Why does this not score, and what would?
"The electrolyte" alone does not say which quantity about it was held constant; the credited answers name a specific quantity, such as the volume, type, or concentration of the electrolyte, or the depth to which the electrodes are immersed.
Question 5
Name a piece of apparatus a student could use to add exactly 10 cm3 of a solution to a flask.
A pipette. A measuring cylinder only measures a volume roughly, so it does not give the precision a fixed, known volume needs.
Question 6
Why would a plan repeat the experiment at each acid concentration rather than running it once?
Because a single reading at one concentration gives nothing to check it against; running it two or three times and checking the volumes agree closely is what shows a reading can be trusted.
Question 7
A plan says "hydrochloric acid is corrosive" as its safety point. What is missing?
A way of reducing the hazard; the hazard alone scores nothing, so it needs something like "wear eye protection when handling the acid".