Write down a balanced chemical equation for reaction of magnesium with water. Give one
indicator activity to test the evolution of hydrogen gas from a test tube during a reaction.
Answers
Explanation:
Teaching goal
Students will likely already have seen the reaction of magnesium with acids to produce hydrogen gas and a salt (Equation 1). They may also have seen the reaction of lithium with water to produce hydrogen gas and a hydroxide, as evidenced by the use of a universal indicator or phenolphthalein (Equation 2).
Equation 1: Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
Equation 2: 2Li(s) + 2H2O(l) → 2LiOH(aq) + H2(g)
This demonstration shows how the reaction of metals with water can be dramatically sped up by an increase in temperature. Students can see the production of the hydrogen gas and draw a mental connection between the reactions seen in Equation 1 and Equation 2.
Here, the reaction initially produces magnesium oxide (Equation 3), which can continue to produce the hydroxide on reaction with liquid water (Equation 4).
Equation 3: Mg(s) + H2O(g) → MgO(aq) + H2(g)
Equation 4: MgO(s) + H2O(l) → Mg(OH)2(aq)
Students could then be invited to make predictions about what the reactants and products are for the reaction of magnesium and water at room temperature and what evidence we might collect for the reaction taking place (Equation 5) if they had several days to wait for the results.
Equation 5: Mg(s) + 2H2O(g) → Mg(OH)2 (aq) + H2(g)
Having seen the use of an indicator in metal–water reactions and the production of hydrogen gas in metal–acid reactions, students might suggest these as possible signs that the reactions are connected.
You can test this by leaving an inverted funnel and collecting tube over some magnesium ribbon which has been submerged in water with a few drops of phenolphthalein. The indicator will begin to change colour within a few minutes (Figure 2) but a few days may be needed to collect a significant volume of gas which could be tested in the next lesson.
Diagram of a measuring flask containing magnesium ribbon, under an inverted funnel with a test tube on the funnel to collect hydrogen