Chemical Properties of Metals and the Reactivity Series for CBSE Class 10

While physical properties give us visual and tactile clues about elements, their chemical properties reveal how they bond, exchange electrons, and react with the surrounding environment. Metals tend to lose electrons easily to form positive ions (cations), which makes them electropositive elements. However, not all metals lose electrons with the same ease: some react vigorously with cold water, while others do not react even with boiling steam or concentrated acids.

In CBSE Class 10 Science, Chapter 3 (Metals and Non-Metals) synthesizes these varied behaviors into an orderly hierarchy called the Reactivity Series. Understanding this sequence is the key to predicting chemical reactions, displacement outcomes, and metallurgical extraction techniques.

What You Will Learn

• Reactions of metals with oxygen: basic oxides vs. amphoteric oxides ($\text{Al}_2\text{O}_3, \text{ZnO}$)
• The industrial protective technique of anodising
• Reactions of metals with water: cold water, hot water, and steam
• Reactions of metals with acids and why nitric acid ($\text{HNO}_3$) rarely produces hydrogen gas
• Aqua Regia: the solvent for noble metals
• The Reactivity Series and predicting displacement reactions
• Board exam observations, reaction conditions, and common traps

1. Reaction of Metals with Oxygen (Air)

Almost all metals combine with oxygen to form metal oxides: $\text{Metal} + \text{Oxygen} \longrightarrow \text{Metal Oxide}$

• High Reactivity: Potassium ($\text{K}$) and Sodium ($\text{Na}$) react so vigorously with oxygen and moisture that they catch fire if kept in the open. To prevent accidental fires, <u>sodium and potassium are stored immersed in kerosene oil</u>.
• Moderate Reactivity: Magnesium, aluminium, zinc, and lead react slowly at room temperature, forming an imperceptible, protective oxide layer that prevents the underlying metal from further oxidation. $2\text{Mg}(s) + \text{O}_2(g) \longrightarrow 2\text{MgO}(s)$ $4\text{Al}(s) + 3\text{O}_2(g) \longrightarrow 2\text{Al}_2\text{O}_3(s)$
• Low Reactivity: Copper does not burn, but hot copper is coated with a black layer of copper(II) oxide: $2\text{Cu}(s) + \text{O}_2(g) \xrightarrow{\quad \Delta \quad} 2\text{CuO}(s) \text{ [Black]}$
• Noble Metals: Silver and gold do not react with oxygen even at very high temperatures.

Amphoteric Oxides (CBSE Board Classic)

Most metal oxides are basic in nature and dissolve in acids. However, certain metal oxides show both acidic and basic behavior:

Amphoteric oxides are metal oxides that react with both acids and bases to produce salt and water.

Primary Examples: Aluminium oxide ($\text{Al}_2\text{O}_3$) and Zinc oxide ($\text{ZnO}$).

1. $\text{Al}_2\text{O}_3$ Acting as a Base (Reacting with Acid): $\text{Al}_2\text{O}_3(s) + 6\text{HCl}(aq) \longrightarrow 2\text{AlCl}_3(aq) + 3\text{H}_2\text{O}(l)$
2. $\text{Al}_2\text{O}_3$ Acting as an Acid (Reacting with Base): $\text{Al}_2\text{O}_3(s) + 2\text{NaOH}(aq) \longrightarrow 2\text{NaAlO}_2(aq) \text{ [Sodium Aluminate]} + \text{H}_2\text{O}(l)$

What is Anodising?

Anodising is an electrochemical process of forming a thick, protective oxide layer on aluminium. During electrolysis of dilute sulphuric acid with an aluminium anode, oxygen gas evolved at the anode reacts with the aluminium to produce a clean, uniform, corrosion-resistant oxide coat that can be dyed in attractive colours.

2. Reaction of Metals with Water

Metals react with water to produce a metal oxide (or hydroxide) and hydrogen gas: $\text{Metal} + \text{Water} \longrightarrow \text{Metal Hydroxide/Oxide} + \text{Hydrogen Gas} \uparrow$

The temperature and physical state of water required depends strictly on the metal's reactivity:

                             Metals Reacting with Water
                                         |
       +---------------------------------+---------------------------------+
       |                                 |                                 |
Cold Water                        Hot Water                         Steam Only
- K, Na (Violent; catches fire)   - Mg (Forms hydroxide; floats)    - Al, Fe, Zn (Forms oxides)
- Ca (Less violent; floats)                                         - Pb, Cu, Ag, Au (NO reaction)

1. Reaction with Cold Water (K, Na, Ca):
   • Potassium and Sodium: Reaction is so violent and exothermic that the evolved hydrogen gas instantly catches fire: $2\text{K}(s) + 2\text{H}_2\text{O}(l) \longrightarrow 2\text{KOH}(aq) + \text{H}_2(g) \uparrow + \text{Heat energy}$ $2\text{Na}(s) + 2\text{H}_2\text{O}(l) \longrightarrow 2\text{NaOH}(aq) + \text{H}_2(g) \uparrow + \text{Heat energy}$
   • Calcium: Reaction is less violent; heat is insufficient for $\text{H}_2$ to catch fire. Calcium starts floating because bubbles of hydrogen gas stick to its surface: $\text{Ca}(s) + 2\text{H}_2\text{O}(l) \longrightarrow \text{Ca(OH)}_2(aq) + \text{H}_2(g) \uparrow$
2. Reaction with Hot Water (Mg):
   • Magnesium does not react with cold water. It reacts with hot water to form magnesium hydroxide and hydrogen gas, and it also starts floating due to adhering $\text{H}_2$ bubbles: $\text{Mg}(s) + 2\text{H}_2\text{O}(l) \text{ [Hot]} \longrightarrow \text{Mg(OH)}_2(aq) + \text{H}_2(g) \uparrow$
3. Reaction with Steam Only (Al, Fe, Zn):
   • Aluminium, iron, and zinc do not react with cold or hot water. They react only when steam is passed over red-hot metal, forming metal oxides (not hydroxides!): $2\text{Al}(s) + 3\text{H}_2\text{O}(g) \text{ [Steam]} \longrightarrow \text{Al}_2\text{O}_3(s) + 3\text{H}_2(g) \uparrow$ $3\text{Fe}(s) + 4\text{H}_2\text{O}(g) \text{ [Steam]} \longrightarrow \text{Fe}_3\text{O}_4(s) \text{ [Magnetic Iron Oxide]} + 4\text{H}_2(g) \uparrow$
4. Metals with No Reaction: Lead, copper, silver, and gold do not react with water or steam at any temperature.

3. Reaction of Metals with Dilute Acids

Metals react with dilute acids to produce salt and hydrogen gas: $\text{Metal} + \text{Dilute Acid} \longrightarrow \text{Salt} + \text{Hydrogen Gas} \uparrow$

• Reactivity order with dilute $\text{HCl}$: $\text{Mg} > \text{Al} > \text{Zn} > \text{Fe}$.
• Copper does not react with dilute $\text{HCl}$ (no gas bubbles, no temperature rise).

Why Does Nitric Acid ($ext{HNO}_3$) Not Liberate Hydrogen Gas?

When metals react with nitric acid, hydrogen gas is NOT evolved.

• Scientific Reason: Nitric acid ($\text{HNO}_3$) is a strong oxidizing agent. As soon as hydrogen gas is formed, $\text{HNO}_3$ oxidizes it into water ($\text{H}_2\text{O}$) and is itself reduced to nitrogen oxides ($\text{N}_2\text{O}, \text{NO},$ or $\text{NO}_2$).
• The Sole Exceptions: Only Magnesium ($\text{Mg}$) and Manganese ($\text{Mn}$) react with very dilute (1%) $\text{HNO}_3$ to liberate hydrogen gas: $\text{Mg}(s) + 2\text{HNO}_3(aq) \text{ [Very dilute]} \longrightarrow \text{Mg(NO}_3)_2(aq) + \text{H}_2(g) \uparrow$

Aqua Regia (Royal Water)

Aqua Regia is a freshly prepared mixture of concentrated hydrochloric acid and concentrated nitric acid in the ratio of $3 : 1$ by volume: $\text{Conc. } \text{HCl} : \text{Conc. } \text{HNO}_3 = 3 : 1$ It is a highly corrosive, fuming liquid capable of dissolving noble metals like gold and platinum, which do not dissolve in any single acid alone.

4. The Reactivity Series of Metals

The Reactivity Series is an activity list of metals arranged in the order of their decreasing chemical reactivity:

Displacement Rule:

<u>A metal located higher in the reactivity series can displace a metal located lower in the series from its aqueous salt solution. A lower metal can never displace a higher metal.</u>

• Example: Zinc placed in copper sulphate solution: $\text{Zn}(s) + \text{CuSO}_4(aq) \text{ [Blue]} \longrightarrow \text{ZnSO}_4(aq) \text{ [Colourless]} + \text{Cu}(s)$ (Reaction occurs because $\text{Zn}$ is higher than $\text{Cu}$.)
• Conversely: $\text{Cu}(s) + \text{ZnSO}_4(aq) \longrightarrow \text{No Reaction!}$

5. Summary and Examination Tips

Exam Tip: In board questions on displacement reactions, always state: "Metal X is more reactive than metal Y as it lies above Y in the reactivity series, hence it displaces Y from its salt solution."

Common Mistake: Writing that iron reacts with steam to form $\text{Fe}_2\text{O}_3$ (ferric oxide). It forms magnetic iron oxide, $\text{Fe}_3\text{O}_4$ (which is a mixed oxide: $\text{FeO} \cdot \text{Fe}_2\text{O}_3$)!