Collecting Gases

Why this skill matters

Gas collection is tested directly on Paper 5 and 6 (planning experiments that produce gases) and indirectly on Papers 2-4 (describing laboratory preparations and choosing collection methods). The choice of method depends on two properties of the gas: its solubility in water and its density relative to air.

Method and technique

Collection over water

Suitable for gases that are insoluble or only slightly soluble in water (H2, O2).

1. Set up a trough or basin of water with an inverted measuring cylinder or gas jar filled with water.
2. Connect the reaction vessel to a delivery tube. Place the end of the delivery tube under the water surface and into the mouth of the inverted container.
3. As gas is produced, it bubbles up through the water and collects at the top of the inverted container, displacing the water downward.
4. When the container is full of gas, slide a glass plate over the mouth while still underwater, then remove.

Advantages: you can see when the container is full; you can measure the volume of gas collected. Disadvantage: does not work for soluble gases (NH3, HCl) because they dissolve.

Upward displacement of air (collection in an upright gas jar)

Suitable for gases denser than air (CO2, Cl2, SO2, HCl).

1. Place a gas jar upright with the mouth facing upward.
2. Direct the delivery tube to the bottom of the gas jar.
3. The dense gas sinks to the bottom and pushes the lighter air up and out of the jar.
4. Test that the jar is full using the appropriate gas test (e.g., a burning splint for CO2 is extinguished).
5. Cover with a glass plate when full.

Downward displacement of air (collection in an inverted gas jar)

Suitable for gases less dense than air (H2, NH3).

1. Invert a gas jar so the mouth faces downward.
2. Direct the delivery tube upward into the mouth of the inverted jar.
3. The light gas rises and fills the jar, pushing denser air downward and out.
4. Test that the jar is full (e.g., a lighted splint produces a squeaky pop for H2).

Gas syringe

For measuring gas volume precisely:

1. Connect the gas syringe to the reaction vessel with a delivery tube.
2. As gas is produced, the plunger moves outward.
3. Read the volume from the scale on the syringe.
4. Ensure the syringe moves freely (not stuck with grease or corrosion).

Common errors and how to avoid them

Choosing the wrong method for a soluble gas: Trying to collect ammonia or HCl over water fails because they dissolve. Always check solubility first.

Not filling the gas jar with water first (for collection over water): If the jar contains air at the start, the collected gas is a mixture of gas and air. Fill completely with water before inverting.

Delivery tube not reaching the bottom (for upward displacement): If the tube ends near the top of the jar, the dense gas mixes with air. Push the tube to the bottom so the gas fills from below.

Gas syringe sticking: If the plunger does not move freely, the measured volume is too low. Check before starting.

Marking points examiners look for

• Name the collection method and justify it using gas properties (solubility and density)
• Draw or describe the apparatus correctly (delivery tube, trough, gas jar orientation)
• State how you know the gas jar is full (gas test or water fully displaced)
• For volume measurement: use a gas syringe or a graduated collection cylinder over water