PAG 1: Determining Density
- Methods for regular and irregular objects are described, and a method for the density of liquids
- The equipment list for the first two investigations are similar:
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
Aim of the Experiment
- The aim of this experiment is to determine the densities of regular objects by using measurements of their dimensions
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
Aim of the Experiment
- The aim of this experiment is to determine the densities of irregular objects using a displacement technique
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
Aim of the Experiment
- The aim of this experiment is to determine the density of a liquid by finding a difference in its mass
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
