When metals react with water, a salt and a gas are produced. The gas is hydrogen. This can be tested with a lighted splint; if a pop is heard, the test is positive.
Not all metals will react with cold water; some metals that will not react with cold water will react with steam. However, some unreactive metals like gold will not react with water in any state.
- Open Yenka file Model 1.
- Turn on the hot plate and the bunsen burner. Observe the magnesium and wait until a reaction occurs before adding the gas syringe. Why do you not collect gas at the start?
AnswerThe gases at the start would be air from the apparatus and unreactive steam. This would create misleading results.
- Write a word and symbol equation for the reaction.
AnswerMagnesium + steam → magnesium oxide + hydrogen
Mg + H2O → MgO + H2
- By clicking on the button in the main toolbar which says 'Scenes' when hovered over, move to scene 'Cold water'. Do you think the reaction of magnesium with cold water will be faster or slower than with steam?
AnswerSlower, because there is less energy in the reaction.
- Put the magnesium in the water. Write a word and symbol equation for the reaction.
AnswerMagnesium + water → magnesium hydroxide + hydrogen
2Mg + 2H2O → 2MgOH + H2
- How can you test for hydrogen gas? Test the gas produced.
AnswerPut a lighted splint to the gas. If it pops, it is hydrogen.
- The reaction between cold water and magnesium is very slow. How could the gas be collected and tested in real life?
AnswerPut the metal in a beaker of cold water. Trap the metal in an upturned filter with a test tube balanced over the spout. All the equipment should be full of water so that the gas can be collected under displacement.
Magnesium is a relatively reactive metal. The reaction with cold water is very slow; in real life it would take days to collect and test the gas released. But when steam is used, the magnesium burns to produce magnesium oxide and hydrogen.
- Students in the lab can complete the reaction between magnesium and cold/warm water. The reaction between steam and magnesium should only be completed as a demonstration and is liable to force the bung out at high speed, as the pressure builds in the gas tube.
- See also the related demonstration Making and Testing Hydrogen.