Launch practical
About 50 minutes
- Set up the bench with the 10 items of apparatus.
- Take the readings yourself, into the results table you draw first.
- Process your results and plot the graph.
- Answer exam questions on your own results, which you mark against the mark scheme.
Runs in your browser: a 3D bench with 2D close-ups of each instrument for exact readings. Nothing to install and no account.
What you measure, and how
One run takes 19 readings. You take every one yourself, the way you would in class.
- On the 3D bench (Fig. 1) you set out the apparatus and read each instrument where it stands.
- In the 2D close-up (Fig. 2) the same instrument is drawn large, from the same run, for an exact reading. It is also how the practical runs on a device that cannot show 3D.
- In your table you write what you read, to the instrument’s resolution, and the notebook checks it against the instrument.
Your eye does the reading. Each volume starts with your eye off level, so the scale lines up with the wrong mark. You move your eye, on the bench or in the close-up, until it is level. Nothing is right by default.
What it shows
ρ = m ÷ V
Density is the mass of each cubic centimetre of a material: density = mass ÷ volume. It depends on what the object is made of, not on how big it is, so it can tell a gold crown from a gold-plated fake.
The graph to expect. Mass is directly proportional to volume: twice the solution has twice the mass, so the points lie on a straight line through the origin.
On the AQA 2027 Physics Equations Sheet. Volume of an irregular object comes from displacement.
Apparatus
| Item | What it is for | Resolution |
|---|---|---|
| Digital balance | Measures mass. | 0.1 g |
| Metal block (cuboid) | The regular object. | — |
| 30 cm ruler | Measures the block’s sides. | 1 mm |
| Vernier callipers | Measures the block’s sides more precisely. | 0.1 mm |
| Model crown | The irregular object: is it gold? | — |
| Thin cotton thread and thick string | To lower the crown into the water. | — |
| Eureka (displacement) can on a brick | Overflows by the crown’s volume. | — |
| Measuring cylinders: 25 cm³, 100 cm³ (×2) and 250 cm³ | Measure volumes of water and solution. | 0.5 cm³, 1 cm³, 2 cm³ |
| Sugar solution (about 150 cm³) | The liquid. | — |
| Plastic tray and paper towels | Catch spills. | — |
Method
In our own words, as the Lab runs it. Your school's method sheet may differ in detail.
Zero the balance
With nothing on the pan, press TARE (or ZERO) so the balance reads 0.0 g.
Why: A balance that does not read zero with nothing on it adds the same error to every mass (a zero error).
Choose how to measure the block
Pick the instrument for the length, width and height of the block.
Do: Vernier callipers (0.1 mm).
Why: The sides are only 2–5 cm long, so a 1 mm uncertainty is 2–5 % of each one, and the volume multiplies three of them.
Choose how to find the crown’s volume
The crown is irregular, so its volume has to be found by displacement.
Do: Eureka can, catching the overflow.
Why: An object pushes aside its own volume of water when it is completely under the surface.
Set up the eureka can
Stand the can on a brick so a measuring cylinder fits under the spout, then fill it until water runs out of the spout into a beaker.
Why: The can must be full to the spout so that every drop the crown pushes aside runs out.
Choose a measuring cylinder for the overflow
Choose the cylinder that will give the most precise reading of about 10 cm³ of water.
Do: 25 cm³ (0.5 cm³ divisions).
Why: The smaller the divisions, the more precisely you can read a small volume. The cylinder must still hold all the water.
Tie on the crown
Tie the crown to a thread so you can lower it gently into the water and lift it out again.
Do: Thin cotton thread.
Why: Whatever goes under the water with the crown displaces water too.
Put the water work in a traySafety
Stand the brick, can and cylinders in a plastic tray, away from the balance.
Why: Spilt water makes the floor slippery, and water must not get into the balance.
Take the readings
Block: find its mass on the balance, then measure its three sides; volume = length × width × height. Crown: find its mass, then lower it on its thread into the water and measure the volume it displaces, three trials. Solution: put the empty measuring cylinder on the balance and record its mass, then add about 20, 40, 60, 80 and 100 cm³, recording the volume and the mass each time; mass of solution = reading − the empty cylinder’s mass.
Repeats: Repeat the displacement three times (with a eureka can, refill it to the spout each time) and take a mean. A mean reduces the effect of random errors (reading the meniscus) but not systematic ones (thick string, a zero error, drips).
Make safe and clear upSafety
Empty the water you used for the crown, and the measuring cylinders, into the sink. Dry the crown, the block and the bench, and mop up any spills. Switch off the balance and put the metal block back in the kit tray. Rinse the sugar solution out of the cylinder so it does not go sticky.
While you take readings
Wait for the can to stop dripping
After filling the can, wait until the spout stops dripping before you put the measuring cylinder under it.
Why: Drips still coming out of a just-filled can are not displaced by the crown.
Read the meniscus at eye level
Crouch so your eye is level with the bottom of the meniscus for every volume reading.
Why: Looking down at the scale from above, the meniscus lines up with a higher mark (parallax).
Hazards and control measures
| Hazard | What could happen | Control measure |
|---|---|---|
| Heavy metal block dropped | Bruised or broken toes or fingers | Keep the block on the bench away from the edge; lower objects gently. |
| Water spilt on the floor | Slips and falls | Do the water work in a tray and mop spills straight away. |
| Glass measuring cylinder knocked over | Cuts from broken glass | Stand cylinders away from the bench edge; use plastic cylinders for the overflow. |
| Water near the electronic balance | Electric shock risk and a damaged balance | Keep the can away from the balance and dry objects before weighing them. |
Variables
- Independent: you change
- Volume of solution in the cylinder in cm³
- Dependent: you measure
- Mass of solution in g
- Control: you keep the same
- The solution. Use the same, well-stirred sugar solution throughout.
- Temperature. Use everything at room temperature.
Results from a specimen run
| Object | Mass in g | Length in cm | Width in cm | Height in cm | Volume in cm³ | Density in g/cm³ |
|---|---|---|---|---|---|---|
| Metal block | 135.0 | 4.00 | 2.80 | 1.54 | 17.25 | 7.83 |
| Crown | 88.5 | — | — | — | 10.33 | 8.56 |
| Trial | Volume of water collected in cm³ |
|---|---|
| 1 | 10.5 |
| 2 | 10.0 |
| 3 | 10.5 |
| Mean | 10.33 |
| Pour | Volume of solution in cm³ | Mass of cylinder and solution in g | Mass of solution in g |
|---|---|---|---|
| Empty | 0 | 98.7 | 0.0 |
| 1 | 19 | 119.2 | 20.5 |
| 2 | 40 | 142.7 | 44.0 |
| 3 | 57 | 160.9 | 62.2 |
| 4 | 80 | 185.7 | 87.0 |
| 5 | 98 | 204.7 | 106.0 |
Processing the data
- Volume of the block
- 17.2 cm³
- Density of the block
- 7.83 g/cm³
- What is the block made of?
- Iron
- Volume of the crown
- 10.3 cm³
- Density of the crown
- 8.56 g/cm³
- Is the crown gold?
- Gold-plated brass (8.5 g/cm³)
- Shape of the graph
- Mass is directly proportional to volume: a straight line through the origin.
- Gradient (the density of the solution)
- 1.09 g/cm³
- In kg/m³
- 1090 kg/m³
| Quantity | Specimen run | Accepted |
|---|---|---|
| Block (iron) | 7.83 g/cm³ | 7.90 g/cm³ |
| Crown (gold-plated brass) | 8.56 g/cm³ | 8.50 g/cm³ |
| Sugar solution (gradient of your graph) | 1.09 g/cm³ | 1.09 g/cm³ |
Across 120 simulated careful groups, density of the solution (ρ) came out between 1.04 and 1.14 g/cm³ (median 1.10). That is the simulation's model, not a promise of what a real class gets.
Common mistakes and the fix
Mistakes to check in this practical, and what to do instead. In a write-up or an exam answer, the mark goes to naming the step and saying what it does to the readings.
Method: Leave it
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.
The fix: Zero the balance. A balance that does not read zero with nothing on it adds the same error to every mass (a zero error).
Method: 30 cm ruler (1 mm)
Resolution 1 mm: each side only to the nearest millimetre, so the volume (and the density) can be several per cent out, and the spread between students is about 10 times bigger.
The fix: Choose how to measure the block. The sides are only 2–5 cm long, so a 1 mm uncertainty is 2–5 % of each one, and the volume multiplies three of them.
Method: Straight into a 250 cm³ measuring cylinder
Volume = final reading − initial reading on a cylinder with 2 cm³ divisions. Simple, but for a crown of about 10 cm³ each reading is only to ±1 cm³ (half a division), and the volume is the difference of two readings: a large percentage.
The fix: Choose how to find the crown’s volume. An object pushes aside its own volume of water when it is completely under the surface.
Mass of the block: Typed 88.5 g
The fix: That is the mass of the crown. This is the metal block: read the display with the block on the pan.
Length of the block: Typed 2.8 cm
The fix: That is the block’s width. This reading is its length, measured along another edge.
Exam answer: Leaving out: density = mass ÷ volume, ρ = m ÷ V (or any correct rearrangement, in words or symbols)
The fix: Density = mass ÷ volume, ρ = m ÷ V. Find it on the full AQA 2027 Physics Equations Sheet, then apply it.
Exam answer: Answer 16.2 g/cm³ (no marks)
The fix: You added the three sides. The volume of a cuboid is length × width × height: 4.00 × 2.80 × 1.54 = 17.248 cm³, so ρ = 135.0 ÷ 17.248 = 7.83 g/cm³.
Worked exam-style questions
Written for Study Hack in the style of exam questions, on the specimen run above. They are not taken from any exam board's papers. Try each one before you open the worked answer.
- Question 1Density equation · Knowledge[1 mark]
Write down the equation that links density (ρ), mass (m) and volume (V).
Worked answer and how the marks are given
A full-mark answer
density = mass ÷ volume
ρ = m ÷ VMark scheme
- density = mass ÷ volume, ρ = m ÷ V (or any correct rearrangement, in words or symbols)
Where marks are usually lost
- Missing: density = mass ÷ volume, ρ = m ÷ V (or any correct rearrangement, in words or symbols). Density = mass ÷ volume, ρ = m ÷ V. Find it on the full AQA 2027 Physics Equations Sheet, then apply it.
- Question 2Density of a regular object · Applying knowledge[3 marks]
Table 1 shows your measurements of the metal block.
Table 1 Mass in g Length in cm Width in cm Height in cm 135.0 4.00 2.80 1.54 Calculate the density of the block.
Density = g/cm³
Worked answer and how the marks are given
A full-mark answer
V = 4.00 × 2.80 × 1.54 = 17.248 cm³
ρ = 135.0 ÷ 17.248
ρ = 7.83 g/cm³Mark scheme
- Volume = 4.00 × 2.80 × 1.54 = 17.248 cm³ (method)
- Substitution: ρ = 135.0 ÷ 17.248 (method)
- ρ = 7.83 g/cm³
- Allow answers from 7.749 to 7.905 g/cm³ (the marker's tolerance; correct rounding of the student's own working is also accepted).
- Allow the unit written as g/cm3, g cm-3, gcm-3, g/cc, g per cm3, grams per cubic centimetre and 1 other spellings.
- A correct answer with no working scores full marks.
Where marks are usually lost
- Answer 16.2 g/cm³ (no marks). You added the three sides. The volume of a cuboid is length × width × height: 4.00 × 2.80 × 1.54 = 17.248 cm³, so ρ = 135.0 ÷ 17.248 = 7.83 g/cm³.
- Answer 0.128 g/cm³ (gains 1 mark). Your volume is right but you divided the wrong way round. Density = mass ÷ volume = 135.0 ÷ 17.248 = 7.83 g/cm³.
- Answer 0.00783 g/cm³ (gains 1 mark). Keep the sides in centimetres so the volume is in cm³ and the density in g/cm³: ρ = 135.0 ÷ 17.248 = 7.83 g/cm³.
- Question 3Describing the method · Knowledge[6 marks]
A student wants to find the density of a small rock. The rock has an irregular shape.
Describe a method the student could use to determine the density of the rock.
Worked answer and how the marks are given
A full-mark answer
1. Measure the mass of the rock on a balance.
2. Fill a displacement (eureka) can with water until it runs out of the spout, and wait until it stops dripping.
3. Put an empty measuring cylinder under the spout.
4. Tie the rock to thin thread and lower it carefully into the can until it is completely under water.
5. Collect all the water that comes out of the spout in the measuring cylinder.
6. Read the volume of water collected at eye level, at the bottom of the meniscus. This is the volume of the rock.
7. Repeat, refilling the can each time, and calculate a mean volume.
8. Calculate the density using density = mass ÷ volume.Mark scheme
Indicative content:
- Measure the mass using a balance (scales)
- Fill a eureka (displacement) can until water just runs out of the spout
- Or: put some water in a measuring cylinder and note its volume first
- Place the rock in the water (lowered on thread) so it is fully submerged
- Collect the displaced water in a measuring cylinder
- Read the final volume; volume of rock = final − initial (or = volume of water displaced)
- Density = mass ÷ volume
- Read the meniscus at eye level; wait for dripping to stop; repeat and take a mean
Levels of response Level Marks What the answer does 3 5–6 A coherent method that would give valid results: all the important steps are there, in a sensible order. 2 3–4 A method that might not give valid results: most of the important steps are there, but some are missing or out of order. 1 1–2 A method that would not give valid results: some relevant steps are given, but they are not linked into a workable method. In Study Hack a six-mark answer is not given an automatic level: you tick the points your answer makes, then choose the level that fits it.
- No relevant content: 0 marks.
Evaluate
| Source of error | Type | Effect | Improvement |
|---|---|---|---|
| Resolution of the ruler (1 mm) on sides only 2–5 cm long | Random | Each side can be up to half a millimetre out either way, so the volume, and the density, can be several per cent out. | Measure the sides with Vernier callipers (0.1 mm). |
| Balance not zeroed (zero error) | Systematic | Every mass is out by the same amount (too big if the empty pan reads above zero), so every density is out the same way. | Tare the balance with nothing on it before weighing, or subtract the zero error from every reading. |
| The thread or string under the water | Systematic | Adds its own volume to every displacement, so the crown’s density is too low. | Use the thinnest thread that will hold the crown. |
| Drips from the freshly filled can collected with the displaced water | Systematic | Every volume is too big, so the crown’s density is too low. | Wait until the can stops dripping before putting the measuring cylinder under the spout. |
| Water left in the spout and on the cylinder wall | Systematic | A little of the displaced water never reaches the scale, so the volume is slightly low and the density slightly high. | Wait until the spout stops dripping before reading, and tap the spout gently. |
| Reading the meniscus (eye position and the size of the divisions) | Random | Readings scatter by up to a division either way; from above, they also read high. | Read the bottom of the meniscus at eye level, using the smallest cylinder that holds the water. |
| Air bubbles clinging to the crown | Systematic | Bubbles always add to the water pushed aside, so every volume comes out a little high and the crown’s density a little low. How much they add also changes from trial to trial, so the volumes vary too. | Lower the crown slowly and tap it to free bubbles. |
Repeatable. When you repeat the displacement with the same crown, can and cylinder, your volumes agree closely.
Reproducible. Another student, or a different method of finding the volume by displacement, gets a similar density for the same crown.
Other exam boards
How this practical lines up with other GCSE boards' required or core practicals. Codes only; check your own specification for the exact activity.
| Board | Practical | Match | What is different |
|---|---|---|---|
| AQA | Required practical 5 | This page | — |
| Edexcel | 14.3 | Same activity | — |
Print it
- Blank write-up: the nine-step lab book with the correct table headings, ready to print for a real lesson.
- Specimen lab book: this page's run written up in full, with its exam answers ticked against the mark scheme.
- Printable method card: the practical on one index card. Practise with the cards in Study Hack.
- Practical workbook: a full simulated data set to process on paper, with exam questions and answers.
- Teacher and technician pack: apparatus lists, technician notes and marking points.