The decreasing order of reactivity of metals with water, based on the provided reactivity series, is Sodium (Na) > Magnesium (Mg) > Calcium (Ca) > Iron (Fe) > Silver (Ag).
This order indicates how readily these specific metals react with water or steam, with Sodium exhibiting the highest reactivity among this group and Silver being the least reactive.
Understanding Metal Reactivity with Water
Metal reactivity with water is fundamentally determined by a metal's tendency to lose electrons and form positive ions. Metals that readily donate electrons are considered highly reactive and react vigorously with water, whereas less reactive metals require more energy (such as high temperatures for steam) or do not react at all. This property is systematically categorized in the Reactivity Series of Metals, which helps predict the outcome of various chemical reactions.
Generally, metal reactions with water involve the metal displacing hydrogen from water to produce either a metal hydroxide (with cold or hot water) or a metal oxide (with steam), alongside hydrogen gas.
- General reaction with cold/hot water:
Metal + H₂O → Metal Hydroxide + H₂ - General reaction with steam:
Metal + H₂O(g) → Metal Oxide + H₂
Detailed Reactivity Breakdown of Specific Metals
Let's explore the reactivity of each metal within the specified sequence:
-
Sodium (Na)
- Reactivity: Extremely high. Sodium reacts very vigorously and exothermically with cold water.
- Reaction Characteristics: When dropped into water, sodium floats, melts into a spherical ball due to the heat generated, and darts rapidly across the surface. It produces hydrogen gas, which often ignites, and sodium hydroxide.
- Equation:
2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)
-
Magnesium (Mg)
- Reactivity: Moderate. Magnesium reacts slowly with cold water, but significantly more readily with hot water or steam.
- Reaction with Cold Water: A slow reaction occurs, forming magnesium hydroxide and hydrogen gas. The magnesium hydroxide is sparingly soluble and can coat the metal surface, hindering further reaction.
- Reaction with Steam: When heated strongly in steam, magnesium reacts vigorously to produce magnesium oxide and hydrogen gas.
- Equation (with steam):
Mg(s) + H₂O(g) → MgO(s) + H₂(g)
-
Calcium (Ca)
- Reactivity: Moderate. Calcium reacts with cold water, though less vigorously than sodium.
- Reaction Characteristics: It sinks in water, and bubbles of hydrogen gas are noticeably produced. The reaction forms calcium hydroxide, which is slightly soluble and can make the water cloudy.
- Equation:
Ca(s) + 2H₂O(l) → Ca(OH)₂(aq) + H₂(g)
-
Iron (Fe)
- Reactivity: Low. Iron shows no reaction with cold or hot water. It only reacts under specific conditions with steam.
- Reaction with Steam: Red-hot iron reacts with steam to form iron(II,III) oxide (Fe₃O₄, also known as magnetite) and hydrogen gas. This reaction is used in industrial processes.
- Equation:
3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g)
-
Silver (Ag)
- Reactivity: Very low. Silver is a noble metal and is largely unreactive with water.
- Reaction Characteristics: It does not react with cold water, hot water, or steam under normal conditions, making it highly durable and resistant to corrosion by water.
Summary Table of Metal Reactivity with Water
| Metal | Chemical Symbol | Position in Reactivity Series (Decreasing Order) | Typical Reaction Conditions with Water | Products |
|---|---|---|---|---|
| Sodium | Na | 1st (Most Reactive) | Cold water; very vigorous | NaOH (sodium hydroxide) + H₂ |
| Magnesium | Mg | 2nd | Slow with cold water; vigorous with steam | Mg(OH)₂ (magnesium hydroxide) or MgO + H₂ |
| Calcium | Ca | 3rd | Cold water; moderately vigorous | Ca(OH)₂ (calcium hydroxide) + H₂ |
| Iron | Fe | 4th | Steam (when red-hot) | Fe₃O₄ (iron oxide) + H₂ |
| Silver | Ag | 5th (Least Reactive) | No reaction with cold water, hot water, or steam | No reaction |
