Investigation - Measuring density

A guide to carrying out an experiment to investigate the relationship between the mass and volume of liquids and regular solids

Purpose: to investigate experimentally the relationship between the mass and volume of liquids and regular solids, and analyse and interpret the data gathered.

The main variables in a science experiment are the independent variable, the dependent variable and the control variables.

The Independent Variable is what we change or control in the experiment.

The Dependent Variable is what we are testing and will be measured in the experiment.

The Control Variables are what we keep the same during the experiment to make sure it’s a fair test.

Variables

In this experiment the:

  • Independent Variable is the volume of the object.
  • Dependent Variable is the mass of the object.

Remember - these variables are controlled (or kept the same) because to make it a fair test, only 1 variable can be changed, which in this case is the volume of the object.

Equation

Density = \frac{mass}{volume}

Prediction

As the volume of the material increases, the mass will also increase.

Justification for the prediction

The greater the volume of the object the greater the number of atoms present.

This will result in the object having greater mass.

Regular objects

Apparatus

Six regular objects of the same material but different volumes, a half-metre rule, a top pan balance.

Method

  1. Select the smallest object. Measure the length, breadth and height using a half-metre rule. Record the results in cm in a suitable table.
  2. Repeat each of these measurements of length, breadth and height and calculate the average.
  3. Using the average values of length, breadth and height, calculate the volume of the object using: Volume = length x breadth x height. Record the volume in cm3 in the table.
  4. Place the object on the top pan balance. Record the mass in g in the table.
  5. Repeat the procedure for the other five objects.

Results

Mass/gLength/cm (1)Length/cm (2)Average length/cmBreadth/cm (1)Breadth/cm (2)Average breadth/cmHeight/cm (1)Height/cm (2)Average height/cmVolume/cm3

Graph

Plot a graph of mass in g on the y-axis against volume in cm3 on the x-axis.

Draw a line of best fit through the points.

The gradient of the graph = \frac{mass}{volume} = density

Calculate the gradient of the graph and hence the density of the object.

Gradient of graph equals density

Conclusion

We can see from the graph that as the volume of the object increases its mass also increases.

This agrees with our prediction.

In fact, since the line of best fit is a straight line through the origin, we can be even more precise.

We can say that the volume of the object is directly proportional to its mass.

As the volume increases the mass of the object increases in direct proportion.

The gradient of the graph equals the density of the material.

Cause of error

The main cause of error in this experiment is the measurement of length, breadth and height.

This can be kept to a minimum by repeating each measurement and calculating the average.

Liquids

Apparatus

A measuring cylinder, a top pan balance, tap water.

Method

This experiment is very similar to the one for regular solids but there is a different way of measuring the mass and volume of the water.

  1. Place an empty, dry measuring cylinder on the top pan balance. Read the mass and record in a suitable table in g.
  2. Remove the cylinder from the top pan balance. Pour 50cm3 of water into it and place it on the balance again. Read the mass and record in the table.
  3. To calculate the mass of the water, subtract the mass of the empty cylinder from the mass of the cylinder plus water. Record the mass in the table.
  4. Read the volume of water from the measuring cylinder. Record the volume of water in cm3.
  5. Repeat the procedure adding 50cm3 each time up to 300cm3 for 6 results.

Safety

Water should not be poured into the measuring cylinder when it is on the top pan balance.

Water spilled on the electric balance could cause electric shock.

Always remove the measuring cylinder from the balance before adding water.