Density

• • Density is defined as:The mass per unit volume of a material
• Objects made from low density materials typically have a low mass.

  • Similarly sized objects made from high density materials have a high mass
  • For example, a bag full of feathers is far lighter compared to a similar bag full of metal
  • Or another example, a balloon is less dense than a small bar of lead despite occupying a larger volume
• Calculating Density:

  
• Calculating Density:

  • Where: 
  • ρ = density (kg/m3)
  • m = mass (kg)
  • V = volume (m3)
• • Gases, for examples, are less dense than solids because the molecules are more spread out (same mass, over a larger volume).
• Gases are less dense than solids:

  
• Determining Density, 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.
• Experiment 1: Measuring the Density of Regularly Shaped Objects.

  Method:

  1. Place the object on a digital balance and note down its mass
  2. 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
  3. Repeat these measurements and take an average of these readings before calculating the density
• Calculating the volume of an object depends on its shape:

  
• Measuring the Density of Regularly Shaped Objects.

  Method:

  • 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.
• 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.
• Experiment 2: Measuring the Density of Irregularly Shaped Objects.

  Method:

  1. Place the object on a digital balance and note down its mass
  2. Fill the eureka can with water up to a point just below the spout
  3. Place an empty measuring cylinder below its spout
  4. Carefully lower the object into the eureka can
  5. Measure the volume of the displaced water in the measuring cylinder
  6. Repeat these measurements and take an average before calculating the density
• 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.
• Experiment 3: Measuring Density of Liquids.

  Method:

  1. Place an empty measuring cylinder on a digital balance and note down the mass
  2. Fill the cylinder with the liquid and note down the volume
  3. Note down the new reading on the digital balance
  4. Repeat these measurements and take an average before calculating the density
• Experiment 3: Measuring Density of Liquids.

  Method:

  • 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.
• 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