Saturday, 5 February 2011


One of the most challenging tasks of a science teacher in a chemistry class is how to show that gases exist and it can be collected from simple reactions. Most common amongst these gases are OXYGEN, HYDROGEN, and CARBON DIOXIDE.

Yet there is a simple activity that could be done as a demo or class experiment to confirm those scientific explanation about acid-metal reaction.  Be sure the materials are readily available in your laboratory. There are other processes to produce these gases but these are the ones helpful and practical in a school setting.


There are few ways to collect hydrogen gas and test it. But the most exciting amongst these, is to collect it into a balloon and pop it afterwards.
You need a regular size conical flask, zinc chips/zinc granules, and 2M sulphuric acid. Be very careful of the corrosive nature of acids. 2M sulphuric acid may irritate or burn your skin.


Put reasonable amount of zinc granules (10-15 chips) into the conical flask. Add 20 ml of sulphuric acid. The reaction takes place right away, cover the flask with the balloon and wait for at least 5 minutes. As gas is produced from the reaction, the balloon would slowly inflate, the gas being trapped inside the balloon is a hydrogen gas. Consider the chemical equation below:
Zinc + sulphuric acid --> zinc sulphate + hydrogen

Hydrogen gas can also be produced with the reaction of magnesium and hydrochloric acid. If these materials are available in your science laboratory, you may try it as well.

Magnesium + hydrochloric acid --> magnesium chloride + hydrogen


To test the hydrogen balloon, you need an iron stand and clamp to hold it. Prepare a stick at least one meter long and attach/tape a wooden splint on one end. Light the splint and put it under the balloon. It would burst out with a considerable bang.

Warning: Please ask the kids to stay away while you pop the balloon and remind them to cover their ears slightly. 

Collecting and Testing Oxygen Gas

Combustion is another term for burning and it happens when substances react with oxygen in air. Oxygen is needed for burning. Fuels burn faster in pure oxygen than in air. Manganese (IV) oxide catalyses the decomposition of hydrogen peroxide to form water and oxygen. Refer to the chemical equation below:

Hydrogen peroxide + manganese oxide --> water + oxygen


Pour 10ml of hydrogen peroxide into a test tube. Take a spatula of manganese oxide and tip it into the test tube. It would start to fizz and if you cover the tube tightly with your thumb, you would feel the oxygen gas trying to escape from the tube.


If a smouldering wooden splint is put into the test tube of reacting hydrogen peroxide and manganese oxide, but not touching the solution, the splint will burst into flame.

Collecting and Testing Carbon Dioxide

Carbon Dioxide is an odourless, colourless, and tasteless gas, which is stable, inert and non-toxic in nature.  The simplest way to collect carbon dioxide is by reacting calcium carbonate with hydrochloric acid.  For classroom demos, you only need a small amount of Hydrochloric acid, which is diluted to 1M. ( 2M concentration works best but should be used with extra care).


Put 20ml of HCl into a conical flask. Add a spoonful of calcium carbonate (or limestone chips) to the HCL. When the reaction starts, cover the conical flask with a bung and delivery tube so that you can collect the gas through the delivery tube and into an upturned test tube. (Test tubes should be immersed in bowl of water). You will notice that gas is collected, since water in the test tube is displaced. Cover the test tube with a bung and put in the racking. As long as there is still a reaction, you can continue to collect the gas. Here is the chemical equation to show that reaction:

Calcium Carbonate  +  HCL    --> calcium chloride + water    +   carbon dioxide


To test that carbon dioxide is produced from the reaction, you can modify the procedure from above and instead of using test tube, use a boiling tube half filled with  LIMEWATER or calcium hydroxide (slaked lime).  Why Limewater? Because carbon dioxide turns a clear limewater cloudy. As the reaction takes place in the conical flask, cover it with the bung and insert the delivery tubing into the boiling tube with limewater. Wait and see, it should turn the limewater hazy or milky after few seconds.


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