8.1 - Collecting Gases

Choosing a way to collect a gas

• Two properties of a gas decide how it's collected:
  1. Whether it dissolves in water
  2. If it does dissolve, whether it's denser or less dense than air
• Ask about solubility first:
  • A gas that does not dissolve in water (or only slightly) is collected over water, regardless of density.
  • A gas that does dissolve in water is collected by displacement of air, where density decides the method.

A gas with a low solubility in water - Collection over water

• A gas jar or measuring cylinder is filled with water and inverted in a trough of water over a beehive shelf.
• The delivery tube runs into the trough and under the shelf, so the gas bubbles up through the shelf into the mouth of the jar.
• As gas bubbles into the jar, it displaces the water downwards.

A soluble gas that is denser than air

• The gas jar stands upright with the delivery tube reaching near the bottom.
• The gas sinks to the bottom and pushes the less dense air up and out.
• This is called downward delivery (or upward displacement of air).
• Used for sulfur dioxide and hydrogen chloride.

A soluble gas that is less dense than air

• The gas jar is held upside down with the delivery tube pointing up into it.
• The gas rises to the top and pushes the denser air down and out of the mouth.
• This is called upward delivery (or downward displacement of air).
• Used for ammonia.