ISE 5-14 Curriculum Support Materials                                                           Overview advice

Group 23 exemplar Earth & Space  - Acids & metals (Word)

Reaction of metals and water

Very few metals discussed in school will actually react with cold water. If they do, then hydrogen gas and the metal hydroxide are formed. Lithium, sodium, and potassium react too violently for pupils to investigate themselves.

These alkali metals can become coated with an oxide layer even when stored in oil, and if left for some time a layer of super oxide can form on them. This is a particular problem with old samples of potassium and sodium.  The super oxide gives the metal the appearance of having a crust and is yellow colour in colour.

There is the potential of an explosion if a piece of very old alkali metal coated with super oxide is added to water.  Any samples coated with the super oxide should not be used.

Even with fresh potassium, there is the possibility of it splattering as the reaction nears completion.  Staff may wish to place a perspex sheet over the trough of water when reacting small pieces of any of the alkali metals.  Ensure students are at least 3 metres away from the experiment and wearing safety goggles.

If some universal indicator solution is added to the trough before the start, a purple trail will form in the water as the metal reacts and moves around the surface, indicating the formation of an alkali. Ensure students hear the hissing of a gas being produced. A good movie clip to show is The Alkali Metals from The Brainiac Series, which shows Rubidium and Caesium reacting spectacularly in a bath of water.

Calcium reacts slowly enough for students to try the experiment on their own in a boiling tube but care should be exercised.  Only add 1-2 small pieces of calcium to half a tube of water.  The gas can be collected by placing an inverted boiling tube over the mouth of the reaction tube for a few minutes.  A lit taper held at the mouth of the inverted tube will show it is hydrogen being made.  Again, a few drops of universal indicator added to the boiling tube will turn purple. If the experiment is done immediately after the alkali metals demonstration, the two can be linked not only in speed of reactions, but in the products formed (both produce an alkaline metal hydroxide solution and a gas).  Students quickly grasp that the hissing from the alkali metals was hydrogen gas being produced.

The reaction with magnesium and cold water is extremely slow. A demonstration can be set up with the class one week before it is required.  Take a piece of magnesium ribbon approximately 5 cm long and clean it with emery paper.  Wrap in round a pencil and then wedge the spiral of magnesium in a boiling tube.  Fill the boiling tube with water, place a stopper on it and invert it in a 400 cm³ beaker which has been half filled with water. With the mouth of the boiling tube under water, remove the stopper to leave it filled with water.  Add a few drops of universal indicator to the water and leave it for a week, topping up the water in the beaker as it evaporates.  Show the results after the alkali metals and calcium reactions.  The tube should have some gas it and the water will again show a purple colour.  Invite speculation on the name of the gas.  Test it by carefully raising it out of the beaker, allowing the remaining water to drain out. Quickly test the gas in the inverted tube with a burning taper to prove it is once again hydrogen.

The same experiment set up with pieces of zinc, iron and copper wedged in the boiling tube produces no gas and the indicator remains green, showing there is no reaction between these metals and cold water.  It will be necessary to check these last demonstration prior to showing them to the class as dissolved air in the water can sometimes come out of solution and collect in the boiling tube, particularly in hot weather, giving the impression of a reaction. If this happens simply re-do the demonstration so there is no gas in the tube and immediately present them to the class!  The use of copper to make water pipes can be discussed. The rusting of iron is not a reaction of the metal with water, but with water and oxygen.  The preserved state of the Titanic can be discussed and related to the fact there is less oxygen dissolved in the water at that depth.  Discuss painting, coating with grease, plastic coated wire etc as all ways to stop the reaction of iron with oxygen and water.

Gold found buried for thousands of years remains untarnished showing this metal does not react with oxygen, dilute acids and water.

Reaction of metals and dilute acid

Do not use the alkali metals or calcium, as there is a serious risk of explosion.

As an introduction, it is worth having two test tubes, one containing 5 cm³ water and the other 5 cm³ dilute 1 mol l^-1 hydrochloric acid. Add a spatula of magnesium turnings to both and observe that the acid test tube reacts faster. The gas can be tested with a burning splint if desired. A simple experiment to show the relative reactivities of some metals is to add 5 cm³ of 1 mol l^-1 hydrochloric acid to four test tubes followed by a dropper full of washing up liquid.  If equal amounts of cleaned pieces of magnesium, zinc, iron and copper are added to each, the amount of frothing produced by the formation of the hydrogen can indicate the speed of the reactions which is Mg>Zn>Fe>Cu (which does not react).

Filter the test tubes into filter paper in a funnel and wash thoroughly with water prior to disposing of the residue carefully.  Extreme care should be taken with the zinc residue.  There is a danger of spontaneous combustion of moist zinc waste if zinc powder is used instead of pieces.

 Thanks to Don Sutherland of the DUSC Project (Development to Update School Chemistry)