Read a volume–time graph, or plot three points and calculate a mean rate.
The chemistry
A gas-volume experiment records the gas collected as time passes. Put time on the horizontal axis and collected volume on the vertical axis.
Mean rate over an interval is the change in volume divided by the elapsed time. A tangent gives a rate at a particular time; this first activity asks for a mean rate.
A curve that levels off shows that no further gas is being collected. A faster reaction need not produce a greater final amount. These example data leave out random measurement errors and delays in collecting the gas.
Worked example
A model reaction collects 60 cm³ of gas in 40 s, starting at zero.
Mean rate over that interval = 60 ÷ 40 = 1.50 cm³/s. This value does not describe the rate at every instant.
Model data for calcium carbonate reacting with excess dilute hydrochloric acid in supervised school chemistry. Gas volume is measured at fixed temperature and pressure. This idealised dataset has no measurement noise; it is graph practice, not a replacement for a practical.
Plot the gas-volume data, then calculate the mean rate from 0 to 80 s. Give the rate to 2 decimal places.
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