GCSE OCR Gateway C4:2a Tests for gases: identifying the products of chemical reactions

GCSE OCR Gateway C4:2a Tests for gases: identifying the products of chemical reactions

Learning Objective:

C4.2a To be able to describe tests to identify selected gases

oxygen, hydrogen, carbon dioxide and chlorine

Test for Oxygen (O₂)

This is one of I think the easier GCSE chemistry topics.  I would hope that you have been aware of tests for simple common gases for a couple of years before you get to GCSE stage.

I’d be disappointed of you had not already carried out these tests before you come to GCSE work.

So I’m expecting that you know pretty well the test for oxygen gas.

Oxygen gas relights a glowing spill because it accelerates combustion reactions.

What you might not be so certain of is a reaction at school chemistry level that produces oxygen gas because there are not that many.

One is the catalytic decomposition of hydrogen peroxide (H₂O₂(aq))solution.

2H₂O₂     ⟶     2H₂O    +    O₂

The catalysts here are many and varied such as blood, potato, celery and MnO₂ (and other transition metal oxides) to name four.

And another reaction, one incidentally you ought not to try seeing as it can have explosive consequences, is the effect of heat on potassium manganate(VII) containing a trace of manganese (IV) oxide catalyst.

2KMnO4       ⟶       K₂O    +   2MnO₂    +  1½O₂

Finally there is the classic demonstration called “Elephnt’s Toothpaste”!!

Just add finely ground potassium iodide crystals to a mix of 10ml 100vol hydrogen peroxide and fairy liquid in a very large (say 500 -1000ml) measuring cylinder and stand back!!

Plunge a glowing spill into the resultant foam and see that it is an oxygen foam.

Here is the reaction:

2KI   +   2H₂O₂       ⟶      I₂    +   ½O₂   +  2KOH  +  H₂O

The iodine stains the foam brown.

Tests for Hydrogen (H₂)

Hydrogen is another of those common gases you ought to have met long before you start a GCSE course.

Its explosive properties are exploited in the test since it will ignite with a lighted spill and produce what we all call a squeaky pop!!

So in this picture, we have what looks like magnesium ribbon in hydrochloric acid releasing hydrogen to be tested.

Mg   +   2HCl    ⟶    MgCl₂    +    H_2(g)

Follwed by the test for the hydrogen

2H₂    +    O₂     ⟶     2H₂O   (+  pop!!  )

When you do this test look for the product, water vapour, on the sides of the test tube and look too for bands in the vapour.  I think the bands in the water vapour are due to the standing wave from the noise of the squeaky pop.

Test for Carbon dioxide (CO₂)

Here is yet another gas whose test you ought to be familiar with because you should have been testing for this gas in Biology classes and Chemistry classes from your earliest days in your school.

Carbon dioxide (CO₂) turns limewater (calcium hydroxide solution: Ca(OH)₂) cloudy or chalky.

Ca(OH)₂    +    CO₂   ⟶      CaCO₃     +   H₂O

Best not to say it turns limewater milky since calcium carbonate (CaCO₃) is not milk even if it might look like it in the test tube.

The chalkiness is a calcium carbonate precipitate.

Carbon dioxide is formed in many reactions at GCSE level such heating some carbonates or reacting them with an acid.

ZnCO₃   ⟶      ZnO    +   CO₂

And

ZnCO₃      +    2HCl     ⟶   ZnCl₂     +   H₂O +    CO₂

Test for Chlorine (Cl₂)

Now here is a gas that you may not have come across before.  Nor might you have come across its test.

Do you know how to test for chlorine?

Chlorine is poisonous and coloured green but those properties won’t do as tests.

But chlorine will bleach damp indicator paper.  Bleach means chlorine will turn the coloured paper white.

There is no equation for this test.

And chlorine is not usually made in chemical reactions in the Lab.

In my next post, I’m going to discuss the tests for cations and anions.