The volume of a gas produced during a reaction (see Unit 12) can be measured using: • a gas syringe – a in Figure 2.06 • an upturned measuring cylinder (or upturned burette) full of water at the start of the experiment – b in Figure 2.06 a
2.05 Density mass (g) volume (cm3) • Gases with densities that are less than air can be collected by the downward displacement of air – Figure 2.07 a. • Density (g/cm3) =
• Gases with densities that are greater than air can be collected by the upward displacement of air – Figure 2.07 b.
gas collecting
a
b
Unit 2 Experimental chemistry
2.04 Measuring gas volumes
gas
gas syringe
gas jar
reaction mixture
b
gas gas collecting
• If two liquids do not mix, the less dense one fl oats on the one that is denser.
TIP
upturned measuring cylinder
Figure 2.07
Figure 2.06
Progress check 2.05 What piece of apparatus should you use to measure out 23.4 cm3 of solution accurately? [1]
It is better not to use the words ‘lightweight’ or ‘light’ when referring to density. They do not indicate mass for a given volume.
2.06 Carrying out experiments When carrying out experiments, we need to decide: • the apparatus to be used • the conditions needed, e.g. heat, catalyst • the measurements to be made
2.06 What two different apparatuses you could use for measuring the volume of gases. [2]
• what variables are involved
2.07 Why would you not use a measuring cylinder to measure 25.0 cm3 of a solution into a beaker? [2]
• the accuracy and reliability of the measurements
• how to make the experiment a fair test • safety, for example if harmful gases are given off, use a fume cupboard
11