GCSE method cards.

Use a light microscope to observe, draw and label onion cells and cheek cells, measure onion cells with a calibrated eyepiece graticule, and work out the magnification of your drawing.

Change

The cell measured (five different cells)

Measure

Length of each cell in µm

Keep the same

Objective; Direction measured; The slide

1. Put a drop of water on the slide
2. Take a thin layer of onion
3. Make sure the layer is flat
4. Stain the onion
5. Lower the coverslip
6. Soak up the extra liquid
7. Choose your animal cells
8. Focus at high power
9. and 2 more steps

Technique that earns marks: At ×400 the objective nearly touches the coverslip. Focus with the fine focus knob only. The coarse knob moves the stage a long way for each turn. At high power it drives the slide into the objective.

Hazard: Scalpel (or knife) used to cut the onion. Cut on a white tile, away from your body, with your fingers behind the blade.

Equation: magnification = size of image ÷ size of real object

Classic slip: Skipping “Make sure the layer is flat”. The layer folds over on itself. Along the crease cells overlap, and cells next to it tear and stretch, so one of the cells you measure is not one whole cell: an anomaly to spot.

Compare how well three antiseptics stop bacteria growing, by measuring the clear zone round a disc of each on an agar plate of E. coli.

Change

Type of antiseptic

Measure

Size of the clear zone round each disc in mm

Keep the same

The bacteria and how much culture is spread; Size of the paper discs; Soaking time and volume of antiseptic on each disc; Incubation temperature and time

1. Disinfect the bench and wash your hands
2. Light the Bunsen and work beside it
3. Label the plate
4. Take a sample of the bacteria
5. Open the plate
6. Spread the bacteria
7. Soak the discs
8. Flame the forceps
9. and 4 more steps

Technique that earns marks: Keep the lid taped on. Look at the zones through the clear base of the plate. The edge of a clear zone is where the cloudy lawn stops. It is easiest to see where the clear agar looks dark.

Hazard: Bacteria, and any other microorganisms that land on the plate and grow. Use aseptic technique; incubate at 25 °C, not 37 °C; tape the lid on and never open the plate after incubation; the plate is sterilised in an autoclave.

Equation: area = π r²

Classic slip: Skipping “Disinfect the bench and wash your hands”. Microorganisms from the bench and your hands can reach the plate. More odd colonies grow on the lawn, and one is more likely to grow inside a clear zone and spoil a reading.

Investigate the effect of the concentration of sucrose solution on the mass of potato cylinders, and use it to estimate the concentration inside the potato cells.

Change

Concentration of sucrose (sugar) solution in mol/dm³

Measure

Percentage change in mass of the potato cylinder in %

Keep the same

Same potato; Size of each cylinder; Volume of solution; Time in the solution; Temperature; Blotting

1. Zero (tare) the balance
2. Choose which potato to use
3. Bore the cylinders
4. Cut the skin off the ends
5. Trim to the same length
6. Label the boiling tubes
7. Add the solutions
8. Choose how long to leave the cylinders in the solutions
9. and 2 more steps

Technique that earns marks: Cover the tubes. Water evaporates from an open tube, which makes the solution more concentrated.

Hazard: Sharp cork borer and scalpel. Bore and cut on a white tile, pushing down onto the tile and away from your fingers; carry the scalpel blade down.

Equation: percentage change = (final mass − initial mass) ÷ initial mass × 100

Classic slip: Skipping “Zero (tare) the balance”. Every mass is out by the same few hundredths of a gram (a zero error, a systematic error). It cancels when you subtract to find the change in mass, but each percentage change is slightly wrong because you divide by a wrong starting mass.

Use qualitative reagents to test foods for starch, reducing sugars, protein and lipids. Then compare each food’s Benedict’s colour with a series of glucose standards to estimate how much reducing sugar it contains.

Change

Concentration of glucose (the standards) in %

Measure

Colour of the Benedict’s solution after heating

Keep the same

Volume of solution tested in cm³; Volume of Benedict’s solution in cm³; Temperature of the water bath in °C; Time in the water bath in minutes

1. Make a solution of each solid food
2. Choose how to heat the Benedict’s tubes
3. Pipettes for the solutions
4. How much reagent to add
5. Judge each colour against a white tile
6. Let the Sudan III tubes stand
7. How long to leave the tubes in the water bath

Technique that earns marks: Judge each colour against a white tile. A pale colour is hard to judge against the dark bench. White behind the tube shows its true colour.

Hazard: Biuret reagent (contains sodium hydroxide, corrosive, and copper sulfate). Wear goggles; add it with a dropping pipette; wash any splash off at once with plenty of water.

Equation: No equation: learn each test and its positive result.

Classic slip: Skipping “Let the Sudan III tubes stand”. The stained fat is still spread through the liquid as tiny red droplets, so no layer has formed yet: hard to judge.

Investigate the effect of pH on the rate at which amylase breaks down starch, and find the optimum pH of amylase.

Change

pH of the buffer solution

Measure

Time taken for the starch to be broken down in s

Keep the same

Temperature; Volume of starch solution; Volume of amylase solution; Volume of buffer; Concentration of starch and of amylase; Drops of iodine in each well; Time between tests

1. Put iodine in the spotting tiles
2. Measure the solutions
3. Choose how often to test a drop
4. Warm them in the water bath first
5. Mix the amylase in
6. Rinse the rod between drops
7. When do you stop?

Technique that earns marks: Mix the amylase in. The amylase must be spread evenly through the starch, so every drop you test is the same mixture.

Hazard: Iodine solution. Wear safety goggles, use the dropping bottle, and wipe up spills straight away.

Equation: rate = 1 ÷ time

Classic slip: Skipping “Warm them in the water bath first”. The solutions start at room temperature and take a minute or two to warm up, so the amylase works slowly at first. The fast runs (pH 6 and 7) take about 1.5 to 2 times as long; slow ones about 1.2 times. The curve is flatter and the temperature is not controlled (a systematic error).

Investigate the effect of light intensity on the rate of photosynthesis in pondweed, by changing the distance between the pondweed and a lamp.

Change

Light intensity, changed by the distance from the lamp to the pondweed in cm

Measure

Rate of photosynthesis: the volume of oxygen collected in a given time (or the number of bubbles in a given time) in cm³ in 5 minutes

Keep the same

Temperature of the solution; Carbon dioxide concentration; The pondweed; Time to adjust; Time for each reading; Other light

1. Cut the end of each stem
2. Choose how to measure the rate
3. Fill with the solution
4. Put the pondweed in
5. Put the thermometer in
6. Choose which lamp to use
7. Measure the distance
8. Switch off the room lights
9. and 1 more steps

Technique that earns marks: Let the pondweed adjust. The pondweed takes a few minutes to settle to the rate that suits its new light intensity.

Hazard: Water near the lamp’s mains plug and lead. Keep the lead and plug away from the water, dry your hands before touching the switch, and mop up spills at once.

Equation: light intensity ∝ 1 ÷ distance²

Classic slip: Skipping “Cut the end of each stem”. The squashed cut end lets only about half the gas out; the rest stays in the leaves. Every count is about half what it should be (a systematic error). Under a funnel some of the gas still escapes from the leaves and is collected, so volumes are about a quarter too low.

Plan and carry out an investigation into the effect of a factor on your reaction time, using the ruler-drop test.

Change

The factor you chose

Measure

Reaction time in s

Keep the same

Starting position of the ruler; Gap between thumb and finger; Arm position; Same catcher and same dropper; No warning; Same hand; Same conditions for the catcher; Caffeine: the drink and the wait

1. Your factor
2. Rest your arm
3. Choose the gap between thumb and finger
4. Choose where the ruler starts
5. Choose whether to practise before you start
6. When the ruler is dropped
7. Where to read the ruler

Technique that earns marks: Your partner waits a different, random time before each drop and gives no warning. If you know when it will drop, you can time your catch instead of reacting to what you see.

Hazard: A dropped metre rule. The person dropping wears safety goggles; hold the ruler over the table, not over a gap.

Equation: No equation to learn: convert the catch distance to a time with the table or equation given.

Classic slip: Arm not supported. The hand moves up and down between and during catches, so the readings scatter more.

Investigate the effect of the direction of light on the growth of mustard seedlings, and the effect of gravity on the growth of bean roots, recording lengths and labelled drawings.

Change

Direction of light

Measure

Increase in height of the seedlings, and the direction they grow in mm

Keep the same

Species and age of the seeds; Number of seedlings in each dish; Volume of water; Temperature; Time

1. Check the dark box is light-tight
2. Put cotton wool in three dishes
3. Wet the cotton wool
4. Sow the seeds
5. Label the dishes
6. Set up the slit box
7. Choose how dish C gets its light
8. Thin the seedlings
9. and 2 more steps

Technique that earns marks: Where the beans go. Only gravity should affect the direction the roots grow. Light from one side would affect it too, so the seedlings are kept so that gravity is the only thing that can change the direction.

Hazard: Seeds treated with fungicide. Use untreated seed, never eat seeds, and wash your hands after handling them.

Equation: No equation: shoots grow towards light; roots grow towards gravity.

Classic slip: Skipping “Check the dark box is light-tight”. A pin-prick of light gets in at one corner. Dish B’s seedlings all lean 15–35° towards it and are not as tall or as yellow as true dark-grown seedlings, so dish B is no longer a fair “dark” control (a systematic error).

Estimate the population of daisies in part of the school field using random quadrats, and use a transect to investigate how light intensity affects their distribution away from a tree.

Change

Distance from the tree (and so the light intensity) in m

Measure

Number of daisy plants in each quadrat

Keep the same

Size of quadrat; Counting rule; Time of the light readings; Same person counting

1. Choose where each quadrat goes
2. Lay out the tapes
3. Choose how many quadrats to use
4. Lay the transect line
5. Choose where along the tape to place the quadrats
6. What do you count?
7. Plants on the edge
8. When to take the light readings

Technique that earns marks: Kneel by the quadrat and part the grass with your fingers. A population is a number of organisms, so you count whole plants. A daisy plant is one flat rosette of leaves; it can have one, two or three flowers.

Hazard: Throwing a metal quadrat. Never throw a quadrat. Carry it and place it at the coordinates.

Equation: population = mean number per m² × total area in m²

Classic slip: Too few quadrats in a patchy field. The mean depends on whether the few quadrats happened to land on patches or on bare ground, so the estimate could be far too high or too low.

Investigate the effect of temperature on the rate of decay of fresh milk, by timing how long lipase takes to lower its pH and by following its pH over four days.

Change

Temperature in °C

Measure

Time for the pink colour to disappear in s

Keep the same

Volume of milk; Type of milk; Volume and concentration of sodium carbonate; Volume and concentration of lipase; Drops of phenolphthalein; How the end point is judged; Covering the beakers; time of day the pH is read

1. Calibrate the pH meter
2. Make up the pink tubes
3. Set up the water baths
4. Leave the tubes in the baths first
5. Set up the souring milk
6. Cover the beakers
7. Stir until the pink has gone
8. When do you stop the clock?
9. and 1 more steps

Technique that earns marks: Take each beaker out, rinse the electrode, stand it in the milk, wait for the reading to settle and record it. Put the beaker back. The milk decays all the time. Readings exactly a day apart show how much the pH changes in a day.

Hazard: Sodium carbonate solution. Wear safety goggles; wash any splashes off the skin with water.

Equation: rate = 1 ÷ time

Classic slip: Skipping “Calibrate the pH meter”. Every pH reading is 0.2 to 0.4 too high or too low (a systematic error). The change in pH from one day to the next is still right, because the same error is in both readings.

Make a pure, dry sample of copper(II) sulfate crystals from copper(II) oxide and dilute sulfuric acid, and work out your percentage yield.

Change

None: this is a preparation, not an investigation

Measure

Mass of dry crystals in g

Keep the same

Volume and concentration of the acid; Copper(II) oxide in excess; How far the solution is evaporated

1. Set up the heating
2. Set up the filter funnel
3. Get the water bath ready
4. Warm the acid
5. Add the copper(II) oxide
6. Decide when to stop adding
7. Filter the mixture
8. Concentrate the solution
9. and 2 more steps

Technique that earns marks: Warm the acid. The copper(II) oxide reacts much faster with warm acid. Do not boil it: hot acid spits.

Hazard: Hot dilute sulfuric acid (1.0 mol/dm³, irritant). Wear goggles; heat the acid only until it is almost boiling, then turn the Bunsen off.

Equation: metal oxide + acid → salt + water

Classic slip: Skipping “Dry the crystals”. The crystals are still wet with solution: your first weighing is 5–15 % too heavy (more for tiny crystals), which would make the yield too high.

Find the volume of dilute sulfuric acid that exactly neutralises 25.0 cm³ of 0.100 mol/dm³ sodium hydroxide solution, then (Higher tier) the acid’s concentration in mol/dm³ and g/dm³.

Change

None: a titration measures one quantity, the titre

Measure

Titre: the volume of acid needed to neutralise the alkali in cm³

Keep the same

Volume of alkali in cm³; Concentration of the alkali in mol/dm³; Indicator; End-point colour

1. Rinse the burette with the acid
2. Rinse the pipette with the alkali
3. Rinse the conical flask
4. Fill the jet below the tap
5. Take the funnel out
6. Choose how to measure the alkali
7. Choose the indicator
8. Stand the flask on a white tile
9. and 3 more steps

Technique that earns marks: Read the burette at eye level. If your eye is above or below the meniscus it lines up with the wrong mark (parallax).

Hazard: Sodium hydroxide solution, 0.100 mol/dm³ (irritant). Wear goggles throughout, and use a pipette filler: never pipette by mouth.

Equation: concentration = amount ÷ volume

Classic slip: Skipping “Rinse the burette with the acid”. The water left inside dilutes the acid, so more of it is needed to neutralise the alkali. Your titres come out high (most at the start, less after each refill) and the acid’s concentration calculates too low (a systematic error).

Find out what forms at each electrode when four salt solutions are electrolysed with graphite (inert) electrodes, test the gases, and use the results to test your hypothesis.

Change

The salt in the solution (its positive and negative ions)

Measure

The product at each electrode

Keep the same

Potential difference in V; Time in min; Concentration of the solution in mol/dm³; Volume of solution in cm³; The electrodes

1. Test the leads
2. Clean the rods between solutions
3. Choose how to collect the gases
4. Test the gas at the positive electrode
5. Read the upturned scale at eye level
6. Keep the hydrogen tube upside down

Technique that earns marks: Read the upturned scale at eye level. Looking from above, the surface seems to line up with a mark higher up the cylinder. On an upturned scale that is a smaller number, so you read too low.

Hazard: Chlorine gas from the chloride solutions (toxic). Keep the lid on, ventilate the room, use 4 V for no more than 5 minutes, and never sniff the gas.

Equation: At the negative electrode: Cu²⁺ + 2e⁻ → Cu; At the positive electrode: 2Cl⁻ → Cl₂ + 2e⁻

Classic slip: Skipping “Test the leads”. If one of your leads is broken, which happens in about one kit in seven, nothing happens when you switch on. The ammeter reads 0.00 A, and you have to find the fault before you can start.

Investigate how the volume of sodium hydroxide solution added to hydrochloric acid affects the temperature of the mixture, and use the graph to find the volume that neutralises the acid.

Change

Volume of sodium hydroxide solution added in cm³

Measure

Highest temperature of the mixture in °C

Keep the same

Volume of hydrochloric acid; Concentration of the acid and the alkali; Starting temperature of the solutions; Container, lid and stirring

1. Check the thermometer
2. Let both solutions reach room temperature
3. Choose what to do the reaction in
4. Choose a measuring cylinder for the 5 cm³ portions
5. Fit the lid after each addition
6. Choose a thermometer
7. Stir after each addition
8. Wait for the highest temperature

Technique that earns marks: Stir after each addition. Until it is stirred, the cold alkali and the warm mixture are in layers, and the thermometer reads only the part around it.

Hazard: Sodium hydroxide solution, 2.0 mol/dm³ (corrosive). Wear goggles throughout; pour at bench level; wash any splash off at once with plenty of water.

Equation: acid + alkali → salt + water

Classic slip: Skipping “Check the thermometer”. If your thermometer reads high, every temperature is too high by the same amount (a systematic error). Temperature changes are still right, because the error cancels when you subtract.

Investigate how the concentration of a reactant affects the rate of a reaction: by the volume of hydrogen given off by magnesium and acid, and by the time for sulfur to hide a cross.

Change

Concentration: of sodium thiosulfate in g/dm³ (cross method); of hydrochloric acid in mol/dm³ (gas method)

Measure

Time for the cross to disappear in s (cross method); volume of hydrogen in cm³, every 10 s (gas method)

Keep the same

Cross method: volume and concentration of the hydrochloric acid; Cross method: total volume of the mixture; Cross method: the cross and the person judging it; Gas method: volume of hydrochloric acid; Gas method: length (mass) of magnesium; Both methods: temperature

1. Check the gas syringe
2. Check the bung and tubing are airtight
3. Clamp the gas syringe and connect it
4. Measure out the acid
5. Clean the magnesium ribbon
6. Set up the cross
7. Keep the total volume the same
8. Put the bung in straight away
9. and 3 more steps

Technique that earns marks: Put the bung in straight away. Any hydrogen made before the bung is in escapes into the room and is never measured.

Hazard: Sulfur dioxide given off by the thiosulfate reaction (toxic). Work in a well-ventilated room, do not lean over the flask, and pour each mixture away with plenty of water as soon as it is timed.

Equation: mean rate of reaction = quantity of product formed ÷ time taken

Classic slip: Skipping “Check the gas syringe”. The plunger sticks now and then: about one reading in three is 1–3 cm³ too low before it jumps back (a random error that scatters your curve).

Separate four food colourings (A–D) and an unknown mixture (U) by paper chromatography, calculate Rf values and find out which colourings are in U.

Change

The colouring (A, B, C, D or U)

Measure

Distance moved by each spot in mm

Keep the same

The paper and the solvent; Depth of solvent and height of the start line; Size of the spots; Temperature

1. Check the paper hangs clear of the beaker
2. Draw the start line
3. Spot the colourings
4. Choose the solvent
5. Pour the solvent into the beaker
6. Get a lid ready
7. Choose how long to leave it
8. Mark the solvent front straight away

Technique that earns marks: Mark the solvent front straight away. The wet edge fades within a minute as the solvent evaporates; the solvent distance is measured to this line.

Hazard: Capillary tubes break easily. Handle them gently, do not use chipped ones and put broken glass in the broken-glass bin.

Equation: Rf = distance moved by substance ÷ distance moved by solvent

Classic slip: Skipping “Check the paper hangs clear of the beaker”. The paper touches the glass on one side. The solvent front bends up on that side, so spots near it (lane U) move further than they should and their Rf values come out too high (a systematic error).

See the result of each test on known solutions. Then use flame tests, sodium hydroxide, and the carbonate, halide and sulfate tests to identify the ions in three unknown single ionic compounds.

Change

None: this is identification, not an investigation

Measure

The observation in each test (flame colour, precipitate colour, fizzing)

Keep the same

Volume of each sample in cm depth; Concentration of the solutions in mol/dm³; The order of reagents; The flame

1. Choose the Bunsen flame for the flame tests
2. Choose the acid for the halide test
3. Choose the acid for the sulfate test
4. Label every tube before you pour
5. Clean the wire between flame tests
6. Move the gas into the limewater
7. Add the sodium hydroxide solution

Technique that earns marks: Clean the wire between flame tests. Anything left on the loop from the last solution burns with its own colour in the next test. Sodium’s bright yellow hides the other colours.

Hazard: Bunsen flame and the red-hot wire. Heatproof mat, hair tied back, hold the wire by its handle and let it cool on the mat.

Equation: Ba²⁺ + SO₄²⁻ → BaSO₄

Classic slip: Skipping “Label every tube before you pour”. What you see in each tube is the same, but if two tubes swap places the observation goes against the wrong solution, and you name the wrong ion.

Analyse water from four sources (pH and dissolved solids), then purify sea water by distillation and test the sea water and the distilled water for sodium and chloride ions.

Change

The source of the water sample

Measure

pH (no unit); mass of dissolved solids in g/dm³; the ion tests before and after distilling (observations)

Keep the same

Volume of each sample evaporated; The balance; Heating to constant mass; The indicator

1. Zero the balance
2. Clean the flame-test wire
3. Choose how to measure the pH
4. Choose how to evaporate the water
5. Heat to constant mass
6. Choose how much mineral water to evaporate
7. Connect the condenser’s cooling water
8. Position the thermometer
9. and 1 more steps

Technique that earns marks: Heat the flask until the water boils, then turn the flame down. Bubbles bursting at the surface throw out tiny droplets of salt water. Boiling hard throws them up into the side arm.

Hazard: Solid spitting from an evaporating basin heated to dryness. Evaporate on a water bath and dry over a low flame; wear goggles.

Equation: No equation: learn the tests for pure water.

Classic slip: Skipping “Zero the balance”. If the empty pan does not read 0.00 g, every mass in your table is out by that amount (a zero error). Your mass of solid is still right: the same error is in both masses, so it cancels when you subtract.

Determine the specific heat capacity of a metal block by heating it electrically, measuring the energy transferred to it and its temperature rise.

Change

Energy transferred to the block in J

Measure

Temperature of the block in °C

Keep the same

Mass of the block; Material of the block; Power of the heater; Insulation

1. Choose your block
2. Measure the mass of the block
3. Insert the immersion heater
4. Add water to the thermometer hole
5. Insert the thermometer
6. Wrap the block in insulation
7. Connect the circuit

Technique that earns marks: Add water to the thermometer hole. Air is a poor thermal conductor. Water fills the gap, giving good thermal contact, so the thermometer reads the temperature of the block.

Hazard: Hot heater and block. Do not touch the heater or block while hot; let them cool before packing away.

Equation: ΔE = m c Δθ

Classic slip: Reading the thermometer. Scatter of about ±0.5 °C about the line.

Investigate how well different materials insulate a beaker of hot water, and how the thickness (the number of layers) affects it.

Change

Type of insulating material (Part 1), or number of layers (Part 2)

Measure

Temperature of the water in °C

Keep the same

Volume of water in ml; Starting temperature in °C; Lid; Size and type of beaker; Thickness of material (Part 1) or type of material (Part 2); Room temperature in °C

1. Check the thermometers
2. Choose your investigation
3. Choose your material
4. Choose your thermometers
5. Measure the water
6. Pack the insulation
7. Put on the lids
8. When will you start the stopwatch?

Technique that earns marks: Each beaker gets its own stopwatch. A hotter beaker cools faster. If the two beakers (and every group) start at different temperatures, the comparison is not fair.

Hazard: Near-boiling water poured from the kettle. Stand the beaker on a heatproof mat and pour slowly; never hold the beaker while pouring.

Equation: ΔE = m c Δθ

Classic slip: Skipping “Check the thermometers”. If one thermometer reads 2–3 °C high, every reading from that beaker is 2–3 °C too high (a systematic error). Its temperature drop is hardly changed, but its curve sits higher than it should.

Find out how the resistance of a wire depends on its length, then measure the resistance of two resistors alone, in series and in parallel.

Change

Length of wire between the clips in cm

Measure

Resistance of the wire in Ω

Keep the same

Material of the wire; Diameter (thickness) of the wire; Temperature of the wire

1. Put the fixed clip on the 0 cm mark
2. Connect the supply + to the ammeter (lead 1)
3. Connect the ammeter to the clip at 0 cm (lead 2)
4. Connect the sliding clip to supply − (lead 3)
5. Connect voltmeter V to the clip at 0 cm (lead 4)
6. Connect voltmeter COM to the sliding clip (lead 5)
7. Set the power pack
8. Switch off between readings
9. and 1 more steps

Technique that earns marks: Unplug lead 3 (your switch) as soon as you have read both meters, and plug it in again only for the next reading. Current heats the wire. Keeping it on only while you read keeps the wire close to room temperature, so its temperature stays the same (a control variable) and it does not burn you.

Hazard: A large current in a short length of thin wire. Use a low pd, switch off between readings and do not touch the wire while current flows

Equation: V = I R

Classic slip: Skipping “Put the fixed clip on the 0 cm mark”. The clip stays where you saw it, past the zero, so every length of wire is longer than you record by the same amount (a zero error). Every resistance is a little too big: the line moves up but its gradient does not change.

Measure how the current through a resistor, a filament lamp and a diode depends on the potential difference across it, in both directions, and plot their I–V characteristics.

Change

Potential difference across the component in V

Measure

Current through the component in A

Keep the same

The component (one at a time); Temperature of the resistor; The meters and their ranges

1. Connect the supply + to the ammeter (lead 1)
2. Connect the ammeter to the component (lead 2)
3. Connect the component back to supply − (lead 3)
4. Choose where to connect the voltmeter
5. Connect voltmeter V to the component (lead 4)
6. Connect voltmeter COM to the other end (lead 5)
7. Choose how to change the lamp’s pd
8. Choose the ammeter range for the diode
9. and 1 more steps

Technique that earns marks: Reverse the supply for negative readings. The I–V characteristic has to show what the component does with the pd the other way round. Swapping the supply reverses the pd across the component itself.

Hazard: The filament lamp gets very hot (24 W). Do not touch the lamp while it is on, and let it cool before taking it out of the holder

Equation: V = I R

Classic slip: The ammeter’s resolution at small currents. Where the current is only a few milliamps (the diode just switching on, the resistor at 2 V) the last digit is a big fraction of the reading, so those points scatter.

Determine the density of a regular metal block, an irregular object (is the crown really gold?) and a liquid.

Change

Volume of solution in the cylinder in cm³

Measure

Mass of solution in g

Keep the same

The solution; Temperature

1. Zero the balance
2. Choose how to measure the block
3. Choose how to find the crown’s volume
4. Set up the eureka can
5. Choose a measuring cylinder for the overflow
6. Tie on the crown
7. Wait for the can to stop dripping
8. Read the meniscus at eye level

Technique that earns marks: Wait for the can to stop dripping. Drips still coming out of a just-filled can are not displaced by the crown.

Hazard: Heavy metal block dropped. Keep the block on the bench away from the edge; lower objects gently.

Equation: ρ = m ÷ V

Classic slip: Skipping “Zero the balance”. Every mass you read includes the zero error, so the block and the crown are off by the same amount every time and so are their densities (a systematic error). Masses found by subtraction, like the solution’s, are not affected.

Investigate the relationship between force and extension for a spring, and use it to find the spring constant.

Change

Force (weight) on the spring in N

Measure

Extension of the spring in cm

Keep the same

The spring; Point on the spring that is measured

1. Hang the spring
2. Fix the pointer
3. Clamp the metre rule
4. Check the rule is vertical
5. Line up zero with the pointer
6. Read again while unloading
7. Read at eye level
8. Wait for the spring to stop bouncing

Technique that earns marks: After the last mass, decide whether to remove the masses one at a time and read the pointer each time. Loading and unloading give two readings at each force, so you can work out a mean; and if the spring returns to its original length it has not passed its elastic limit.

Hazard: Clamp stand toppling with masses hanging from it. Clamp the base to the bench with a G-clamp or weight it with a heavy mass; do not overhang the bench edge.

Equation: F = k e

Classic slip: Skipping “Line up zero with the pointer”. The unloaded pointer sits between marks. You subtract that first reading from every other one, so any error in it shifts every extension by the same amount (a zero error).

Investigate how the acceleration of a trolley depends on the resultant force on it (mass constant) and on its mass (force constant).

Change

Accelerating force (weight of the hanging masses: the resultant force on the trolley, hanger and masses together) in N

Measure

Acceleration of the trolley in m/s²

Keep the same

Total mass of the trolley, hanger and masses in kg; Slope of the runway; Positions of the light gates and the start line

1. Measure the card
2. Clamp the pulley and hang the masses
3. Compensate for friction
4. Choose how to measure the acceleration
5. Place the second light gate
6. Load the trolley
7. Release from rest

Technique that earns marks: Release from rest. The method assumes the trolley starts from rest at the same place each time.

Hazard: Masses falling from the hanger when it reaches the floor. Put a box of sand under the hanger and keep feet clear of it.

Equation: F = m a

Classic slip: Skipping “Measure the card”. Hand-cut cards are 9.7–10.3 cm long. If yours is not exactly 10 cm, every acceleration is too big or too small by the same percentage (up to 3 %), so the gradient and the mass from it are out too (a systematic error).

Measure the frequency, wavelength and speed of water waves in a ripple tank and of waves on a stretched string, and decide which apparatus and methods measure them best.

Change

Frequency of the vibration generator in Hz

Measure

Wavelength on the string in m

Keep the same

Tension in the string; The string and its length; Depth of water in the ripple tank

1. Level the ripple tank
2. Put the white card under the tank
3. Choose the water depth
4. Start the waves
5. Put a scale in the tank
6. Choose how to hold the pattern still
7. Choose how many waves to measure across
8. Choose how to count the waves
9. and 3 more steps

Technique that earns marks: Fine-tune for the steadiest loops. The loops start to show a little before the exact frequency at which the string resonates.

Hazard: Water spilt from the ripple tank, near the mains lamp and power units. Mop up spills straight away and keep the lamp and power units on the bench, away from the water.

Equation: v = f λ

Classic slip: Skipping “Level the ripple tank”. The water is up to about 1 mm deeper at one end. The waves speed up and slow down along the tank, so the wavelength changes from place to place and your three distances disagree more (a random error).

Investigate the reflection of light by a plane mirror and the refraction of light by glass and Perspex blocks.

Change

Angle of incidence in degrees

Measure

Angle of refraction (and angle of reflection) in degrees

Keep the same

Point where the ray meets the block; The incident lines; Position of the block

1. Make a narrow ray
2. Draw the surface line
3. Draw the normal
4. Draw the incident rays
5. Draw round the block
6. Darken the room
7. Mark the crosses
8. Measure the angles

Technique that earns marks: Mark the crosses. Each ray is drawn by joining crosses. Any error in a cross tilts the line, and how much depends on how far apart the points are.

Hazard: Hot ray box lamp. Switch the ray box off when it is not in use, move it by its base and let it cool before packing it away.

Equation: angle of incidence = angle of reflection

Classic slip: Skipping “Make a narrow ray”. Without the lens the ray is 3–5 mm wide. It is harder to judge where its centre is when you mark each cross, so your angles scatter more (a random error two to three times bigger).

Investigate how the amount of infrared radiation a surface emits depends on the nature of the surface: with a Leslie cube, then with three flasks cooling.

Change

Type of surface

Measure

Detector reading (amount of infrared emitted) in °C or mV

Keep the same

Distance between each face and the detector; Temperature of the surfaces; Angle of the detector; Volume and starting temperature of the water (flasks)

1. Clean the faces of the cube
2. Boil the kettle
3. Fix the distance to the detector
4. Plan the order of your readings
5. Measure the water for the flasks
6. Start the flasks at the same temperature
7. Point the detector straight at the face
8. Wait for the reading to settle
9. and 1 more steps

Technique that earns marks: Point the detector straight at the face. At an angle the detector catches less of the face’s infrared and more of the room’s.

Hazard: Very hot water from the kettle. Pour the water only where the cube and flasks stand, and never carry hot water across the lab

Equation: A matt black surface emits infrared fastest; a shiny silver one slowest.

Classic slip: Skipping “Clean the faces of the cube”. Fingerprints on the shiny silver face make it a slightly better emitter, so its readings come out a little too high (a systematic error on that face).