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Required Practical: Determining Density (AQA GCSE Physics: Combined Science)
Revision Note
Required Practical 5: Determining Density
Equipment List
- Resolution of measuring equipment:
- 30 cm ruler = 1 mm
- Vernier calipers = 0.01 mm
- Micrometer = 0.001 mm
- Digital balance = 0.01 g
Experiment 1: Measuring the Density of Regularly Shaped Objects
- The aim of this experiment is to determine the densities of regular objects by using measurements of their dimensions
- Independent variable = Type of shape / volume
- Dependent variable = Mass of the object
Method
- Place the object on a digital balance and note down its mass
- Use either the ruler, Vernier calipers or micrometer to measure the object’s dimensions (width, height, length, radius) – the apparatus will depend on the size of the object
- Repeat these measurements and take an average of these readings before calculating the density
- An example of a results table might look like this:
Analysis of Results
- Calculate the volume of the object depending on whether it is a cube, sphere, cylinder (or other regular shape)
Calculating the volume of an object depends on its shape
- Remember to convert from centimetres (cm) to metres (m) by dividing by 100
1 cm = 0.01 m
50 cm = 0.5 m
- Using the mass and volume, the density of each can be calculated using the equation:
- Where:
- ρ = density in kilogram per metres cubed (kg/m3)
- m = mass in kilograms (kg)
- V = volume in metres cubed (m3)
Experiment 2: Measuring the Density of Irregularly Shaped Objects
- The aim of this experiment is to determine the densities of irregular objects using a displacement technique
- Independent variable = Different irregular shapes / mass
- Dependent variable = Volume of displaced water
Method
Apparatus for measuring the density of irregular objects
- Place the object on a digital balance and note down its mass
- Fill the eureka can with water up to a point just below the spout
- Place an empty measuring cylinder below its spout
- Carefully lower the object into the eureka can
- Measure the volume of the displaced water in the measuring cylinder
- Repeat these measurements and take an average before calculating the density
- An example of a results table might look like this:
Analysis of Results
- The volume of the water displaced is equal to the volume of the object
- Once the mass and volume of the shape are known, the density can be calculated using:
Experiment 3: Measuring Density of Liquids
- The aim of this experiment is to determine the density of a liquid by finding a difference in its mass
- Independent variable = Volume of water added
- Dependent variable = Mass of cylinder
Method
Apparatus for determining the density of a liquid
- Place an empty measuring cylinder on a digital balance and note down the mass
- Fill the cylinder with the liquid and note down the volume
- Note down the new reading on the digital balance
- Repeat these measurements and take an average before calculating the density
- An example of a results table might look like this:
Analysis of Results
- Find the mass of the liquid by subtracting the final reading from the original reading
Mass of liquid = Mass of cylinder with water – mass of cylinder
- Remember to convert between grams (g) and kilograms (kg) by dividing by 1000
1 g = 0.001 kg
78 g = 0.078 kg
- Once the mass and volume of the liquid are known, the density can be calculated using the equation:
Evaluating the Experiments
Systematic Errors:
- Ensure the digital balance is set to zero before taking measurements of mass
- This includes when measuring the density of the liquid – remove the measuring cylinder and zero the balance before adding the liquid
Random Errors:
- A main cause of error in this experiment is in the measurements of length
- Ensure to take repeat readings and calculate an average to keep this error to a minimum
- Place the irregular object in the displacement can carefully, as dropping it from a height might cause water to splash which will lead to an incorrect volume reading
Safety Considerations
- There is a lot of glassware in this experiment, ensure this is handled carefully
- Water should not be poured into the measuring cylinder when it is on the electric balance
- This could lead to electric shock
- Make sure to stand up during the whole experiment, to react quickly to any spills
Examiner Tip
There is a lot of information to take in here! When writing about experiments, a good sequence is as follows:
- If you need to use an equation to calculate something, start off by giving it as this will give you some hints about what you need to mention later
- List the apparatus that you need
- State what measurements you need to make (your equation will give you some hints) and how you will measure them
- Finally, state that you will repeat each measurement several times and take averages
In my experience of teaching this lesson at GCSE, the most common point for errors to occur is in the displacement of water from the eureka can to the measuring cylinder. Here are my biggest tips:
- Make sure the eureka can is completely full before adding the object - you want to make sure that all the displaced water exits the can, if the can is not full, then the water level can rise before it begins to spill out
- Make sure that all the displaced water goes into the measuring cylinder - if you spill some then you are not getting an accurate measurement
- If you notice one of these errors during the investigation, then stop and redo that step
Remember that anomalous results should not be included in any averages you calculate. It can be easy to spot an anomalous result on a graph, but it is much more difficult to spot one from a group of data. So if you notice one during your investigation, you can correct for it by rerunning that part of the investigation.
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