Reactions with Water

Metals show very different behaviours when they come into contact with water. Some metals react extremely vigorously with cold water, some react slowly, some react only with steam, and others show almost no reaction at all.

These differences provide useful evidence about the reactivity of metals.

For many reactive metals, the general pattern with cold water is:

metal + water → metal hydroxide + hydrogen

However, metals that react with steam commonly form a metal oxide and hydrogen instead.

Studying these reactions allows us to use the reactivity series to predict how an unfamiliar metal may behave.

Why Metals React Differently with Water

Metals differ in how readily their atoms lose electrons and form positive ions.

More reactive metals generally lose electrons more readily.

For example:

Na → Na⁺ + e⁻

Sodium loses electrons readily, so it reacts strongly with water.

Copper is much less reactive and does not normally react with cold water or steam under ordinary classroom conditions.

This gives us a general pattern:

more reactive metal → greater tendency to react with water

But the exact behaviour depends on the metal and the conditions.

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The Reactivity Series and Water

A useful simplified section of the reactivity series is:

Most reactive

Potassium
Sodium
Lithium
Calcium
Magnesium
Aluminium
Zinc
Iron
Hydrogen
Copper
Silver
Gold

Least reactive

The metals near the top react much more readily with water than metals near the bottom.

A simplified comparison is:

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Metals Reacting with Cold Water

Very reactive metals can react directly with cold water.

For several important examples, the products are:

• a metal hydroxide
• hydrogen gas

The general word equation is:

metal + water → metal hydroxide + hydrogen

For example:

sodium + water → sodium hydroxide + hydrogen

The reaction can release a considerable amount of energy.

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Potassium and Water

Potassium is one of the most reactive metals commonly encountered when learning about the reactivity series.

It reacts extremely vigorously with cold water.

Word equation:

potassium + water → potassium hydroxide + hydrogen

Balanced symbol equation:

2K + 2H₂O → 2KOH + H₂

The reaction is strongly exothermic.

Typical Observations for Potassium

Depending on the conditions, observations can include:

• rapid movement across the water
• vigorous fizzing
• rapid disappearance of the metal
• considerable heat
• ignition of the hydrogen or metal
• a lilac-coloured flame

These observations indicate that potassium is extremely reactive.

Because of the hazards involved, reactions of highly reactive alkali metals with water are normally performed only as controlled demonstrations.

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Sodium and Water

Sodium also reacts vigorously with cold water.

Word equation:

sodium + water → sodium hydroxide + hydrogen

Balanced symbol equation:

2Na + 2H₂O → 2NaOH + H₂

Sodium is less reactive than potassium but still reacts rapidly.

Typical Observations for Sodium

Sodium may:

• float on the water
• move rapidly across the surface
• fizz
• melt into a small ball
• gradually disappear
• release hydrogen gas

Why might it melt?

The reaction releases heat.

Sodium has a relatively low melting point, so the heat generated can melt the metal.

The resulting sodium hydroxide dissolves in the water, producing an alkaline solution.

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Lithium and Water

Lithium also reacts with cold water.

Word equation:

lithium + water → lithium hydroxide + hydrogen

Balanced symbol equation:

2Li + 2H₂O → 2LiOH + H₂

Lithium reacts less vigorously than sodium or potassium.

Typical observations include:

• floating
• fizzing
• movement
• gradual disappearance

This provides a useful comparison within Group 1.

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Comparing Lithium, Sodium and Potassium

The order of reactivity is:

potassium > sodium > lithium

As we move down this part of Group 1, reactions with water become more vigorous.

A simplified comparison is:

Lithium

Fizzes and moves relatively slowly.

Sodium

Fizzes more rapidly and often melts.

Potassium

Reacts extremely rapidly and may ignite.

These observations provide experimental evidence for their relative positions in the reactivity series.

Calcium and Water

Calcium also reacts with cold water.

Word equation:

calcium + water → calcium hydroxide + hydrogen

Balanced symbol equation:

Ca + 2H₂O → Ca(OH)₂ + H₂

Calcium is less reactive than the alkali metals discussed above, so its reaction is generally less violent.

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Observing Calcium and Water

Typical observations can include:

• bubbles forming on the calcium
• gradual disappearance of the metal
• hydrogen gas being produced
• the water becoming cloudy

The cloudiness can result from calcium hydroxide, which is only slightly soluble in water.

The solution becomes alkaline because calcium hydroxide is a base.

Testing the Hydrogen

When a suitable metal reacts with water, hydrogen gas can be produced.

The familiar laboratory test for hydrogen is the squeaky pop test.

A small sample of gas is exposed to a flame.

Hydrogen can burn rapidly, producing a characteristic pop.

The reaction is:

hydrogen + oxygen → water

Balanced:

2H₂ + O₂ → 2H₂O

Hydrogen testing should only be carried out using appropriate laboratory procedures and supervision.

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Magnesium and Cold Water

Magnesium is less reactive than calcium.

It reacts only very slowly with cold water under ordinary conditions.

A simplified reaction can be represented as:

magnesium + water → magnesium hydroxide + hydrogen

Balanced:

Mg + 2H₂O → Mg(OH)₂ + H₂

However, the reaction can become slow partly because magnesium hydroxide can form on the surface and reduce further contact between magnesium and water.

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Magnesium and Steam

Magnesium reacts much more readily with steam.

Importantly, the product changes.

With steam:

magnesium + steam → magnesium oxide + hydrogen

Balanced symbol equation:

Mg + H₂O(g) → MgO + H₂

Notice the difference:

Cold water → metal hydroxide + hydrogen

Steam → metal oxide + hydrogen

for the relevant metals and conditions.

Why Does Steam Make a Difference?

Steam involves much higher temperatures than cold water.

At higher temperatures:

• particles have greater kinetic energy
• collisions occur with greater energy
• a larger proportion of collisions can lead to reaction

Therefore, metals that react too slowly with cold water to be easily observed may react more readily with steam.

This is an important reminder that:

reactivity and reaction conditions both influence what we observe.

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Zinc and Water

Zinc does not react significantly with cold water under ordinary conditions.

However, hot zinc can react with steam.

Word equation:

zinc + steam → zinc oxide + hydrogen

Symbol equation:

Zn + H₂O(g) → ZnO + H₂

This reaction provides evidence that zinc is reactive enough to remove hydrogen from water under sufficiently energetic conditions, but not reactive enough to react readily with cold water.

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Iron and Water

Iron does not normally react significantly with cold water in the same way as calcium or sodium.

However, sufficiently hot iron can react with steam.

A simplified word equation is:

iron + steam → iron oxide + hydrogen

One commonly encountered equation is:

3Fe + 4H₂O(g) → Fe₃O₄ + 4H₂

The exact iron oxide produced depends on reaction conditions.

This is why word equations may sometimes simply refer to iron oxide unless a particular oxide is specified.

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Copper and Water

Copper is much less reactive than magnesium, zinc or iron.

Copper does not normally react with:

• cold water
• hot water
• steam

under the conditions normally considered in introductory chemistry.

Therefore:

copper + water → no reaction

This behaviour is consistent with copper's low position in the reactivity series.

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Silver and Gold

Silver and gold are even less reactive.

They do not react significantly with water under ordinary conditions.

This contributes to their usefulness in applications where chemical stability is important.

Gold, in particular, can remain relatively unchanged for extremely long periods.

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Cold Water vs Steam

This distinction is very important.

Very Reactive Metals

Can react with cold water.

Examples:

• potassium
• sodium
• lithium
• calcium

General pattern:

metal + water → metal hydroxide + hydrogen

Moderately Reactive Metals

May react little or not at all with cold water but can react with steam.

Examples include:

• magnesium
• zinc
• iron

General pattern with steam:

metal + steam → metal oxide + hydrogen

Low-Reactivity Metals

Show little or no reaction even with steam under ordinary classroom conditions.

Examples:

• copper
• silver
• gold

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Predicting Reactions Using the Reactivity Series

The reactivity series allows us to make predictions.

Suppose you are asked:

Will calcium react with cold water?

Calcium is relatively high in the reactivity series.

Prediction:

Yes.

Products:

calcium hydroxide + hydrogen

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Worked Prediction: Copper

Question:

Will copper react with cold water?

Copper is relatively low in the reactivity series.

Prediction:

No significant reaction.

Copper does not readily displace hydrogen from water under these conditions.

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Worked Prediction: Zinc

Question:

Will zinc react vigorously with cold water?

No.

However, zinc can react with steam when heated.

Products:

zinc oxide + hydrogen

Word equation:

zinc + steam → zinc oxide + hydrogen

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Comparing Metal Reactivity Experimentally

Suppose four metals are tested with cold water.

Based only on these observations, a reasonable preliminary order is:

A > B > C > D

However, if D reacts strongly with steam, we gain additional information.

Therefore, several experimental conditions may be needed to place metals accurately in a reactivity series.

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What Does Vigour Tell Us?

A more vigorous reaction can provide evidence of greater metal reactivity when the experimental conditions are comparable.

For a fair comparison, we should control variables such as:

• mass of metal
• surface area
• water volume
• water temperature
• purity of metal
• observation time

Otherwise, we may incorrectly attribute differences in reaction rate to differences in reactivity.

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Surface Area

Imagine comparing:

• a large magnesium block
• finely divided magnesium

The finely divided magnesium has a much larger surface area exposed to the surroundings.

It may therefore react more rapidly.

This does not mean the powdered magnesium is a different position in the reactivity series.

It is still magnesium.

The difference is reaction rate, not fundamental chemical reactivity.

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Protective Surface Layers

Surface coatings can also affect metal-water reactions.

Aluminium is an important example.

Aluminium is relatively high in the reactivity series but normally shows little obvious reaction with water because its surface is protected by a thin layer of aluminium oxide.

This oxide layer prevents water from readily reaching the aluminium underneath.

Aluminium and the Reactivity Series

If we judged aluminium only by placing aluminium foil in water, we might conclude that it is not very reactive.

That conclusion would be incorrect.

Other chemical evidence shows that aluminium is more reactive than metals such as:

• zinc
• iron
• copper

The protective oxide coating hides its underlying reactivity.

This demonstrates why chemists use multiple experiments when establishing a reactivity series.

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Why Is Hydrogen Produced?

During many metal-water reactions, the metal loses electrons.

For example, sodium forms sodium ions:

Na → Na⁺ + e⁻

Species derived from water gain electrons, ultimately producing hydrogen gas.

Therefore, metal-water reactions involve electron transfer.

The metal is oxidized because it loses electrons.

Hydrogen-containing species are reduced as hydrogen gas is formed.

These are therefore redox reactions.

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Oxidation and Reduction

Remember:

Oxidation = loss of electrons

Reduction = gain of electrons

A useful memory aid is:

OIL RIG

Oxidation Is Loss

Reduction Is Gain

During a metal-water reaction:

metal → positive metal ions

The metal loses electrons and is oxidized.

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Why More Reactive Metals React More Vigorously

More reactive metals have a stronger tendency to undergo oxidation under suitable conditions.

Their atoms can lose electrons more readily.

This means electron transfer can occur more readily when they encounter water.

As a result, metals higher in the reactivity series generally react with water more easily than metals lower in the series.

However, the observed reaction can also depend on:

• temperature
• surface area
• oxide coatings
• concentration or availability of reactants

So the reactivity series describes an underlying chemical tendency, while the visible rate depends on conditions too.

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Energy Changes

Many reactions between reactive metals and water are exothermic.

This means they transfer energy to the surroundings.

The temperature can increase.

For highly reactive metals, enough energy may be released to ignite the hydrogen produced.

This helps explain why reactions involving metals such as potassium can become particularly vigorous.

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Metal Hydroxides

When some reactive metals react with cold water, they form metal hydroxides.

Examples include:

sodium hydroxide — NaOH

potassium hydroxide — KOH

lithium hydroxide — LiOH

calcium hydroxide — Ca(OH)₂

Metal hydroxides are generally basic, and many produce alkaline solutions in water.

For example, sodium reacting with water produces sodium hydroxide, causing the solution to become strongly alkaline.

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Metal Oxides from Steam

When metals such as magnesium and zinc react with steam, they form metal oxides rather than metal hydroxides under the usual conditions studied.

Examples:

Mg + H₂O(g) → MgO + H₂

Zn + H₂O(g) → ZnO + H₂

This distinction is important when predicting reaction products.

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Writing Word Equations

Sodium and Water

sodium + water → sodium hydroxide + hydrogen

Calcium and Water

calcium + water → calcium hydroxide + hydrogen

Magnesium and Steam

magnesium + steam → magnesium oxide + hydrogen

Zinc and Steam

zinc + steam → zinc oxide + hydrogen

Iron and Steam

iron + steam → iron oxide + hydrogen

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Balancing Metal-Water Equations

Consider:

Na + H₂O → NaOH + H₂

This is not balanced.

Balanced:

2Na + 2H₂O → 2NaOH + H₂

Check:

Left:

Na = 2
H = 4
O = 2

Right:

Na = 2
H = 4
O = 2

The equation is balanced.

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Worked Example: Calcium

Start with:

Ca + H₂O → Ca(OH)₂ + H₂

Calcium is already balanced.

To provide the required oxygen atoms, place 2 before H₂O:

Ca + 2H₂O → Ca(OH)₂ + H₂

Check:

Ca = 1 on both sides

O = 2 on both sides

H = 4 on both sides

Balanced equation:

Ca + 2H₂O → Ca(OH)₂ + H₂

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Worked Example: Magnesium and Steam

Word equation:

magnesium + steam → magnesium oxide + hydrogen

Formula equation:

Mg + H₂O → MgO + H₂

Count the atoms.

Left:

Mg = 1
H = 2
O = 1

Right:

Mg = 1
H = 2
O = 1

The equation is already balanced:

Mg + H₂O(g) → MgO + H₂

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Connecting Water Reactions to the Reactivity Series

The pattern can be summarized approximately as:

very high reactivity → reaction with cold water

moderate reactivity → reaction may require steam

low reactivity → little or no reaction with water

This is one of several ways chemists establish and apply the reactivity series.

Real-World Importance

Metal-water reactions help explain why metals must be stored, handled and used differently.

Sodium and Potassium

Because they react strongly with water and moisture, they require special storage conditions.

Aluminium

Its protective oxide layer allows aluminium to be used widely despite its underlying reactivity.

Iron

Iron can be used in many structures even though it can undergo corrosion under suitable environmental conditions.

Copper

Its relatively low reactivity contributes to its usefulness in water pipes and other applications.

Understanding reactivity therefore helps engineers choose appropriate materials.

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Common Mistakes

Assuming All Metals React with Cold Water

They do not.

Some require steam, while others do not react significantly with water.

Assuming Metal + Water Always Produces an Oxide

For many highly reactive metals with cold water:

metal hydroxide + hydrogen

is produced.

With steam, certain metals produce:

metal oxide + hydrogen

Forgetting Hydrogen

Hydrogen gas is an important product in many metal-water reactions.

Confusing Hydrogen with Oxygen

The gas commonly released is:

H₂

not O₂.

Thinking No Visible Reaction Means a Metal Is Completely Unreactive

It may require higher temperature or steam, or it may have a protective surface layer.

Thinking Aluminium Is Low in the Reactivity Series

Aluminium is reactive but protected by an oxide coating.

Comparing Reactions Under Different Conditions

A fair comparison requires similar mass, surface area, temperature and other conditions.

Confusing Reactivity with Reaction Rate

Reactivity is an underlying chemical tendency.

Reaction rate also depends on experimental conditions.

Forgetting That Steam Is Water

Steam is gaseous water:

H₂O(g)

Forgetting the Product Difference

Remember:

cold water → often hydroxide + hydrogen

steam → often oxide + hydrogen

for the metals typically studied.

Changing Chemical Formulas When Balancing

Use coefficients.

Do not alter the formulas of the compounds.

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Key Terms

Reactivity — The tendency of a substance to undergo chemical reactions.

Reactivity series — A ranking of metals according to their relative reactivity.

Cold water reaction — A reaction between a metal and liquid water without requiring strong heating.

Steam — Water in the gas phase, H₂O(g).

Metal hydroxide — A compound containing a metal ion and hydroxide ions.

Metal oxide — A compound containing a metal chemically combined with oxygen.

Hydrogen gas — H₂, a gas produced in many reactions between reactive metals and water.

Alkali — A soluble base that produces an alkaline solution in water.

Exothermic reaction — A reaction that transfers energy to the surroundings.

Oxidation — Loss of electrons.

Reduction — Gain of electrons.

Redox reaction — A reaction in which oxidation and reduction occur together.

Electron transfer — Movement of electrons from one species to another.

Metal ion — A positively charged particle formed when a metal atom loses electrons.

Surface area — The amount of a material exposed to its surroundings.

Reaction rate — How quickly reactants are converted into products.

Passivation — Protection of a metal by a surface layer that slows further reaction.

Oxide layer — A layer of metal oxide formed on a metal surface.

Word equation — A chemical equation written using substance names.

Symbol equation — A chemical equation written using chemical formulas.

Balanced equation — An equation with equal numbers of each type of atom on both sides.

Coefficient — A number placed before a chemical formula when balancing an equation.

Squeaky pop test — A common laboratory test used to identify a small sample of hydrogen gas.

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Key Takeaways

• Different metals react with water in very different ways.
• The differences provide evidence about their relative reactivity.
• Metals near the top of the reactivity series generally react more readily with water.
• Potassium reacts extremely vigorously with cold water.
• Sodium reacts very vigorously with cold water.
• Lithium reacts with cold water but less vigorously than sodium and potassium.
• Calcium reacts readily with cold water.
• Magnesium reacts only slowly with cold water under ordinary conditions.
• Magnesium reacts more readily with steam.
• Zinc and iron can react with steam under suitable conditions.
• Copper, silver and gold do not normally react significantly with water.
• For many reactive metals:

metal + cold water → metal hydroxide + hydrogen

• For several moderately reactive metals:

metal + steam → metal oxide + hydrogen

• Hydrogen gas can be identified using the appropriate laboratory test.
• Metal hydroxides are basic, and soluble hydroxides form alkaline solutions.
• More reactive metals generally lose electrons more readily.
• Metal-water reactions involve electron transfer and are redox reactions.
• The metal is oxidized during the reaction.
• Many metal-water reactions are exothermic.
• More vigorous reactions generally indicate greater reactivity when experimental conditions are comparable.
• Temperature can strongly affect the observed reaction rate.
• Surface area can strongly affect reaction rate.
• Protective oxide layers can prevent an apparently reactive metal from reacting visibly.
• Aluminium is an important example of passivation.
• A lack of reaction with cold water does not necessarily mean that a metal cannot react with steam.
• Experimental evidence from water reactions can help establish the reactivity series.
• The reactivity series can then be used to predict how metals will behave with water.

The main patterns to remember are:

More reactive metal → generally reacts more readily with water

Reactive metal + cold water → metal hydroxide + hydrogen

Some moderately reactive metals + steam → metal oxide + hydrogen

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Check Your Understanding

1. Why do different metals react differently with water?

2. Write the general word equation for a reactive metal reacting with cold water.

3. What gas is commonly produced during metal-water reactions?

4. Describe two observations when sodium reacts with water.

5. Write the word equation for sodium reacting with water.

6. Write the balanced symbol equation for sodium reacting with water.

7. Why can sodium melt during its reaction with water?

8. Arrange lithium, sodium and potassium from least reactive to most reactive.

9. How does the reaction with water change as you move from lithium to potassium?

10. Write the word equation for calcium reacting with water.

11. Write the balanced symbol equation for calcium reacting with water.

12. Why might water become cloudy when calcium reacts with it?

13. How can hydrogen gas be identified in a laboratory?

14. Describe magnesium's reaction with cold water.

15. How does magnesium's reaction with steam differ from its reaction with cold water?

16. Write the word equation for magnesium reacting with steam.

17. Write the balanced symbol equation for magnesium reacting with steam.

18. What are the products when zinc reacts with steam?

19. Why can iron react with steam even though it does not react significantly with cold water?

20. Predict whether copper will react with cold water.

21. Predict whether gold will react significantly with steam under ordinary classroom conditions.

22. Explain why aluminium's behaviour with water can be misleading when judging its reactivity.

23. What is passivation?

24. Explain why a powdered metal might react faster than a large piece of the same metal.

25. Explain the difference between reactivity and reaction rate.

26. Why must experimental conditions be controlled when comparing metals?

27. What happens to metal atoms in terms of electrons during a metal-water reaction?

28. Why can metal-water reactions be described as redox reactions?

29. Explain how reactions with water can help establish a reactivity series.

30. Challenge: Four unknown metals, A, B, C and D, are tested.

• A reacts violently with cold water.
• B reacts steadily with cold water.
• C does not react with cold water but reacts with steam.
• D does not react with cold water or steam.

a. Arrange the metals from most reactive to least reactive based on the evidence.
b. Which metal is likely to be highest in the reactivity series?
c. Which is likely to be lowest?
d. Which metal might behave similarly to zinc or iron? Explain.
e. Predict the general products when A reacts with cold water.
f. Predict the general products when C reacts with steam.
g. Explain why hydrogen may be produced in both reactions.
h. Describe how you could confirm that the gas produced is hydrogen.
i. Identify three variables that should be controlled if the reactions are compared experimentally.
j. Explain why surface area could affect the observations.
k. Explain why temperature could affect the observations.
l. Explain why a protective oxide coating might make one of the metals appear less reactive than it really is.
m. Suggest another type of chemical reaction that could provide additional evidence for the relative reactivity of A, B, C and D.