On the grid, plot a graph of time for first blue colour to appear on the $y$-axis against mass of sodium chloride on the $x$-axis. Label the axes. Plot the points and draw a smooth curve of best-fit.
Use the graph to predict the time taken for the blue colour to appear if $0.7\text{ g}$ of sodium chloride is used.
Which experiment number is shown twice?
Why is this experiment repeated?
Use your graph to deduce how sodium chloride affects the rate at which the iron rusts.
The student carries out all six experiments again, but this time uses pieces of iron that have been painted. All other variables stay the same. Describe the effect this change has on the time for the blue colour to appear and explain your answer.
Suggest why the times recorded by each student are different.
Suggest why all the graphs have the same shape.