Density required practical

Aim: To find the density of regularly shaped solids, irregularly shaped solids and liquids by measuring their mass and volume.

Variables

• Independent: The object or liquid being tested
• Dependent: Density (calculated from mass and volume)
• Control: Temperature of the liquid; Using the same balance and measuring equipment for all samples; Water level filled exactly to the spout of the displacement can

Equipment

• Top-pan balance
• Ruler, vernier callipers or micrometer
• Regular solid blocks, e.g. metal cubes or cuboids
• Irregular objects, e.g. a stone or a lump of modelling clay
• Displacement (eureka) can
• Measuring cylinder
• Thread
• Water and another liquid, e.g. salt water or oil

Method

1. Regular solid: measure its mass on a balance.
2. Measure its length, width and height with a ruler or callipers and calculate volume = length × width × height.
3. Irregular solid: measure its mass, then fill a displacement can with water until it drips from the spout and wait for the dripping to stop.
4. Place a measuring cylinder under the spout, tie thread to the object and lower it gently into the can until fully under water.
5. Read the volume of water collected in the measuring cylinder: this equals the volume of the object.
6. Liquid: place an empty measuring cylinder on the balance and zero (tare) it.
7. Pour in a known volume of liquid, for example 50 cm³, and record the mass.
8. Calculate the density of each sample and repeat measurements to find a mean.

Safety

• Wipe up spilt water straight away to prevent slipping.
• Lower heavy objects gently on thread so they do not crack the glassware or splash.
• Take care carrying glass measuring cylinders and report breakages.

Results and calculations

Record mass, dimensions or volume and density for each sample in a table. Use density = mass ÷ volume (ρ = m ÷ V); in g/cm³ for grams and cm³, or kg/m³ for SI units (1 g/cm³ = 1000 kg/m³).

Common mistakes and improvements

• Not filling the displacement can right up to the spout, or dropping the object in so water splashes out: fill until it drips, wait, then lower the object slowly on a thread.
• Reading the measuring cylinder from above or below the surface: read the bottom of the meniscus at eye level.
• Using a ruler for small blocks gives large percentage uncertainty: use vernier callipers or a micrometer for greater resolution.
• Forgetting to tare the balance with the empty cylinder so the mass includes the container: zero the balance first, or subtract the empty mass.

Exam tips

• Always show the equation, substitute values with units, and give the unit of your answer.
• Be ready to convert units, for example cm³ to m³ (divide by 1 000 000) or g to kg.
• Explain why the displacement method works: the object pushes out a volume of water equal to its own volume.
• If asked how to improve accuracy, suggest repeating and averaging, using a more precise balance or reading the meniscus correctly.
• Know that a 1 cm³ measuring cylinder resolution may be too coarse for small objects; a narrower cylinder gives better resolution.