CHEMISTRY · Stage 8 · Chapter 9: Chemical Reactions 2
Chemistry 9.7 Metals and water — Alkali Metals, Metal Hydroxides and Hydrogen
5 min read 6 parts Lesson 7 of 12
Lesson summary
9.7 Metals and water — Alkali Metals, Metal Hydroxides and Hydrogen — Alhussan International Academy — Chemistry (Stages 7–9) notes: Describe the reactivity of metals with water and write word equations for the reactions.Everyday metals — the metals used most often do not react w…
- General equation
- Universal indicator
- Hydroxide ions
- Copper, silver and gold
- Metals in steam
- Order of reactivity
Section 1 of 6Which metals react with water
- Everyday metals
- the metals used most often do not react with water, which is why metal taps do not react with water or metal cars with rain.
- Products
- all the metals that react with water form two products: an oxide or hydroxide, and hydrogen.
- Pattern
- metals that react vigorously with oxygen also react vigorously with water.
Section 2 of 6Reactions with cold water
- General equation
- metal + water → metal hydroxide + hydrogen.
- Calcium
- it sinks and bubbles quickly, making hydrogen: calcium + water → calcium hydroxide + hydrogen.
- The water turns cloudy because calcium hydroxide is only slightly soluble.
- Sodium
- it zooms around the water surface, bubbling vigorously, and forms a silvery ball: sodium + water → sodium hydroxide + hydrogen.
- Potassium
- it reacts even more vigorously than sodium, getting hot enough to set fire to the hydrogen with a lilac flame: potassium + water → potassium hydroxide + hydrogen.
- Lithium
- it fizzes steadily without melting.
- Safety
- use small pieces of alkali metals, a safety screen and eye protection, and never touch them with bare hands.
- Store the metals under oil so they do not react with moisture or oxygen.
Section 3 of 6Alkaline solutions
- Universal indicator
- it turns purple in the solution made by sodium and water, showing the solution is alkaline.
- Hydroxide ions
- Group 1 metal hydroxides are soluble and produce OH- ions, which make the solution alkaline.
- This is why Group 1 elements are called the alkali metals.
Section 4 of 6Less reactive metals
- Magnesium
- it reacts very slowly with cold water,
- forming a few bubbles of hydrogen on the surface,
- but burns vigorously in steam to form magnesium oxide and hydrogen.
- Zinc
- it does not react with cold water but reacts with steam : zinc + water (steam) → zinc oxide + hydrogen.
- (water in the gas state)
- it does not react with cold water but reacts with steam : zinc + water (steam) → zinc oxide + hydrogen.
- Iron
- it does not react with water alone but corrodes with water and oxygen to make rust.
- (hydrated iron oxide)
- it does not react with water alone but corrodes with water and oxygen to make rust.
- Copper, silver and gold
- they do not react with cold water or steam.
- A shipwreck raised from the South China Sea after 800 years still held gold and silver pots and 6000 copper coins,
- because these metals do not react with cold water.
- Metals in steam
- they form oxides, not hydroxides.
| Metal | Reaction with water |
|---|---|
| Potassium | Reacts violently with cold water |
| Sodium | Reacts vigorously with cold water |
| Calcium | Reacts vigorously with cold water, steady bubbles |
| Magnesium | Reacts very slowly with cold water; burns in steam |
| Zinc | Reacts with steam |
| Iron | Reacts with water and oxygen to make rust |
| Copper, silver, gold | Do not react |
- Order of reactivity
- potassium > sodium > lithium > calcium > magnesium > zinc > iron > copper.
Section 5 of 6Test for hydrogen
- Test
- collect the gas in an upturned test tube and hold a lit splint near the mouth.
- Result
- the lit splint goes out with a squeaky pop, which shows the gas is hydrogen.
Section 6 of 6Explaining the trend
- Outer electrons
- potassium, sodium and lithium each have one outer electron.
- Potassium atoms are the largest, so the outer electron is furthest from the nucleus, held weakly and lost most easily.
- Calcium and magnesium must lose two electrons, which needs more energy.
- Rule
- the more energy needed to lose electrons, the lower the reactivity.
- Investigating
- use small equal-sized pieces of each metal and the same volume of water in a trough.
- Observe the speed of movement, the amount of fizzing and whether a flame appears, and confirm products with the gas test and universal indicator.
- Only the teacher may demonstrate sodium and potassium.
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