How does the reactivity of Sodium with cold water compare to that of Magnesium?

Sodium reacts vigorously and exothermically, melting into a ball and fizzing. Magnesium reacts extremely slowly with cold water, producing only a few bubbles over a long period.

What is the difference in products when Magnesium reacts with cold water versus steam?

With cold water, magnesium produces magnesium hydroxide ($Mg(OH)_2$) and hydrogen. With steam, it produces magnesium oxide ($MgO$) and hydrogen.

Why is the pH of the solution formed by Sodium and water higher than that formed by Magnesium and water?

Sodium hydroxide is highly soluble and dissociates fully, creating a high concentration of $OH^-$ ions (pH 13-14). Magnesium hydroxide is only partially soluble, resulting in a lower concentration of $OH^-$ ions (pH 10).

What is a common error when writing the equation for Magnesium reacting with steam?

A common mistake is writing $Mg(OH)_2$ as the product instead of $MgO$. At high temperatures with steam, the oxide is the stable product formed.

What mistake is often made regarding the state symbols in the Sodium-water reaction?

Students often forget that sodium hydroxide is formed as an aqueous solution $(aq)$, while hydrogen is a gas $(g)$ and water is a liquid $(l)$.

Why is it incorrect to say Magnesium does not react with water at all?

While the reaction with cold water is extremely slow, it does occur. Furthermore, magnesium reacts vigorously when the water is in the form of steam.

What is the balanced chemical equation for Sodium reacting with cold water?

$2Na(s) + 2H_2O(l) \rightarrow 2NaOH(aq) + H_2(g)$. This shows the formation of soluble sodium hydroxide and hydrogen gas.

What is the balanced chemical equation for Magnesium reacting with steam?

$Mg(s) + H_2O(g) \rightarrow MgO(s) + H_2(g)$. Note that water is in the gaseous state and the product is solid magnesium oxide.

Define the oxidation state changes for Magnesium in its reaction with water.

Magnesium starts at an oxidation state of $0$ in its elemental form and is oxidized to $+2$ in both $Mg(OH)_2$ and $MgO$.

What is 'Milk of Magnesia' in a chemical context?

It is a suspension of magnesium hydroxide ($Mg(OH)_2$) in water. It is used medicinally to neutralize excess stomach acid due to its mild alkalinity.

Library Podcasts

Courses

Referral & Rewards

Advanced Inorganic Chemistry

Na & Mg with water

Summary

The reactions of sodium and magnesium with water demonstrate the trend in reactivity of Period 3 metals. Sodium reacts vigorously with cold water to form a strong alkali, while magnesium reacts extremely slowly with cold water but vigorously with steam to form different products depending on the state of the water.

1. Core Concepts of Period 3 Metal Reactivity

Sodium (Na) and Magnesium (Mg) are the first two elements of Period 3, belonging to Group 1 and Group 2 respectively. Their reactivity with water is a primary indicator of their metallic character and ease of losing electrons.

Sodium is more reactive than magnesium because it only needs to lose one electron to achieve a stable electron configuration, whereas magnesium must lose two. This difference in ionization energy and charge density ( $Mg^{2+}$ vs $Na^+$ ) accounts for the varying rates of reaction observed.

Both metals undergo redox reactions with water, where the metal is oxidized (loses electrons) and hydrogen in water is reduced (gains electrons) to form hydrogen gas.

2. Sodium Reaction with Cold Water

Diagram showing sodium reacting on the surface of water, melting into a ball and producing gas bubbles.

3. Magnesium Reaction with Cold Water

4. Magnesium Reaction with Steam

5. Key Distinctions

6. Exam Strategy & Tips

Na & Mg with water

Summary

1. Core Concepts of Period 3 Metal Reactivity

Both metals undergo redox reactions with water, where the metal is oxidized (loses electrons) and hydrogen in water is reduced (gains electrons) to form hydrogen gas.

2. Sodium Reaction with Cold Water

Sodium reacts very vigorously and exothermically with cold water. The reaction is so energetic that the sodium melts into a silvery ball and skims across the surface of the water due to the pressure of the hydrogen gas produced.

The chemical equation for this reaction is: $2Na(s) + 2H_2O(l) \rightarrow 2NaOH(aq) + H_2(g)$

The resulting solution contains sodium hydroxide, which is highly soluble and fully dissociates, leading to a strongly alkaline solution with a pH of approximately 13 to 14.

The oxidation state of sodium increases from $0$ in its elemental form to $+1$ in the hydroxide compound.

Diagram showing sodium reacting on the surface of water, melting into a ball and producing gas bubbles.

3. Magnesium Reaction with Cold Water

Magnesium reacts extremely slowly with cold water, often taking days to produce a visible amount of hydrogen bubbles on the surface of the metal ribbon.

The reaction produces magnesium hydroxide: $Mg(s) + 2H_2O(l) \rightarrow Mg(OH)_2(aq) + H_2(g)$

Because magnesium hydroxide is only sparingly soluble in water, the concentration of hydroxide ions is much lower than in the sodium reaction. This results in a weakly alkaline solution with a pH of approximately 10.

This sparingly soluble magnesium hydroxide is the primary active ingredient in 'milk of magnesia', used to neutralize stomach acid.

4. Magnesium Reaction with Steam

When magnesium is heated and reacted with steam, the reaction is significantly faster and more vigorous than with cold water. The increased thermal energy allows the reaction to overcome the activation energy barrier more effectively.

The products of this reaction differ from the cold water reaction; instead of a hydroxide, a solid metal oxide is formed: $Mg(s) + H_2O(g) \rightarrow MgO(s) + H_2(g)$

During this reaction, the magnesium burns with a characteristic bright white flame, leaving behind a white powdery residue of magnesium oxide.

The oxidation state of magnesium changes from $0$ to $+2$ in both the hydroxide and the oxide forms.

5. Key Distinctions

It is vital to distinguish between the products formed by magnesium depending on the state of water used.

Feature	Sodium + Cold Water	Magnesium + Cold Water	Magnesium + Steam
Main Product	$NaOH$ (Hydroxide)	$Mg(OH)_2$ (Hydroxide)	$MgO$ (Oxide)
Reactivity	Very Vigorous	Extremely Slow	Vigorous
Solution pH	13 - 14	~10	N/A (Solid product)
Observations	Floats, melts, fizzes	Few bubbles	Bright white flame

6. Exam Strategy & Tips

State Symbols Matter: Always check if the water is liquid $(l)$ or gas $(g)$ in equations. For magnesium, $H_2O(l)$ yields $Mg(OH)_2$ , while $H_2O(g)$ yields $MgO$ .
pH Justification: If asked why $NaOH$ creates a higher pH than $Mg(OH)_2$ , focus on solubility. $NaOH$ is highly soluble, while $Mg(OH)_2$ is only partially soluble, releasing fewer $OH^-$ ions.
Oxidation States: Remember that in all these reactions, the metal is oxidized. Sodium always goes to $+1$ and Magnesium always goes to $+2$ .
Visual Observations: Be prepared to describe the 'bright white flame' for magnesium with steam and the 'silvery ball floating/fizzing' for sodium with water.