18.7 - Preparing & Testing Gases
The technique
Aim
To prepare small amounts of hydrogen, oxygen and carbon dioxide in the laboratory, and to test each gas to confirm what it is.
Background
- Hydrogen is made by reacting a metal, such as zinc, with dilute hydrochloric acid.
- Oxygen is made by decomposing hydrogen peroxide solution, using manganese(IV) oxide to speed it up.
- Carbon dioxide is made by reacting calcium carbonate, as marble chips, with dilute hydrochloric acid.
- All three gases are colourless, so you can't tell them apart by looking, but each one gives its own result in a simple test.
- So in this practical you make each gas, test it, and use the result to confirm which gas it is.
Method
- Make hydrogen - Add a small piece of zinc to 10 cm3 of dilute hydrochloric acid in a test tube.
- Test for hydrogen - As soon as bubbles form, hold a lit splint at the mouth of the tube, just above the liquid. A squeaky pop shows hydrogen.
- Make oxygen - Add half a spatula of manganese(IV) oxide, a catalyst, to 10 cm3 of hydrogen peroxide solution in a test tube.
- Test for oxygen - Hold a glowing splint at the mouth of the tube, just above the liquid. The splint relights if the gas is oxygen.
- Set up the carbon dioxide flask - Put about 3 g of marble chips in a conical flask, and fit a bung carrying a thistle funnel and a delivery tube. The end of the thistle funnel must reach below the level of the liquid.
- Fill the gas jars - Fill several gas jars with water and stand them upside down in a trough of water, on a beehive shelf.
- Collect the carbon dioxide - Pour 50 cm3 of dilute hydrochloric acid down the thistle funnel. Once gas comes through steadily, stand a water-filled jar on the shelf, and watch the water level fall. When a jar is full of gas, slide it off upside down and replace it with another.
- Test the carbon dioxide - Put a lit splint into the first jar, and it goes out. Shake the second with about 1 cm3 of limewater, and it turns milky. Shake the third with a few drops of universal indicator, and it turns yellow or orange.
Safety
- Eye protection - Wear it throughout, since dilute hydrochloric acid is an irritant.
- Hydrogen is flammable - Keep the hydrogen tube away from other flames while it reacts.
- Hydrogen peroxide - It can bleach skin and clothing, so rinse spills at once.
- Gas jars - Handle them carefully over the trough, since a jar that slides off the shelf can crack.
Common exam questions
Question 1
Why is the splint held just above the liquid rather than dipped into it, for hydrogen and oxygen?
The gas collects in the space above the liquid, so that's where the splint meets it. Dipping the splint into the liquid would put the flame or glow out.
Question 2
Why must the end of the thistle funnel reach below the level of the acid?
If the end were above the liquid, the gas would escape back up the funnel instead of going along the delivery tube to the gas jar.
Question 3
Why is carbon dioxide collected over water, even though it's denser than air?
Carbon dioxide isn't soluble enough in water to stop it being collected this way. Collecting over water also shows the jar filling as the water level drops.
Question 4
Why must the gas jars be completely full of water before you start collecting?
Any air left in a jar would mix with the gas being collected, so the gas wouldn't be pure.
Question 5
Why does universal indicator show that carbon dioxide is acidic?
Carbon dioxide dissolves in the water in the gas jar to form a weak acid. The indicator's colour change detects that acid, not the gas itself.