Section 1 of 7Density and equipment
| Item | Purpose |
|---|---|
| Density: ρ = m ÷ V | ρ in kg/m3, m in kg and V in m3 |
| 30 cm ruler | Measure straight lengths up to 30 cm |
| Vernier caliper | Measure objects up to about 15 cm more precisely |
| Micrometer | Measure very small thicknesses of a few centimetres or less |
| Digital balance | Measure the mass of solids and liquids |
| Displacement (eureka) can | Collect a volume of water equal to the volume of an irregular object |
| Measuring cylinder | Measure the volume of a liquid or of displaced water |
Section 2 of 7Method 1: regular solid
1
Place the object on the digital balance, which has been zeroed, and record its mass.
2
Measure each dimension (length, width, height or radius) with a ruler, vernier caliper or micrometer, choosing the one that suits the size.
3
Repeat each measurement and calculate an average.
4
Work out the volume from the dimensions, for example length × width × height, converting centimetres to metres by dividing by 100.
5
Calculate the density using ρ = m ÷ V.
Section 3 of 7Method 2: irregular solid
1
Record the mass of the object on the zeroed balance.
2
Fill the displacement can with water until it just starts to run out of the spout, then let it stop dripping.
3
Place a dry measuring cylinder under the spout.
4
Lower the object gently into the can so that no water splashes out, and collect all the displaced water.
5
Read the volume of water in the cylinder, which equals the volume of the object, and use ρ = m ÷ V.
Section 4 of 7Method 3: a liquid
1
Place an empty measuring cylinder on the zeroed balance and record its mass.
2
Pour in some of the liquid and record the volume at the bottom of the meniscus, with your eye level with the surface.
3
Weigh the cylinder with the liquid.
4
Subtract the empty mass from the full mass to find the mass of the liquid.
5
Repeat, take an average and divide mass by volume, converting grams to kilograms by dividing by 1000.
Section 5 of 7Evaluating the investigation
Systematic errors
- Systematic errors An unzeroed balance gives every mass reading the same offset, so always zero it before use.
- When measuring a liquid, zero the balance with the empty cylinder on it or subtract the empty mass.
Random errors
- Random errors Length readings and the displaced-water reading vary from trial to trial, so repeat and average them.
- Lower an object carefully into the can, because splashing gives a wrong volume.
Safety
- Safety Handle the glassware carefully, and keep water away from the electric balance to avoid shock and damage.
- Stand up so that any accident or spill can be dealt with quickly.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Density | The mass of a substance per unit volume, found using ρ = m ÷ V. |
| Displacement | Pushing aside water equal to the volume of an object placed in it. |
| Meniscus | The curved surface of a liquid in a measuring cylinder; read the bottom of it. |
| Systematic error | An error that shifts all readings by the same amount, such as an unzeroed balance. |
| Random error | Unpredictable variation between repeated readings. |
Section 7 of 7Exam technique
1
Forgetting to convert units so that density is not in kg/m3 (1 cm3 = 0.000 001 m3).
2
Reading the top of the meniscus instead of the bottom.
3
Dropping the object in the eureka can from a height so that water splashes.
4
Calculating the liquid density with the mass of the cylinder included.