Question

Magnesium does not decompose in (a) hot water (b) steam (c) semi hot water (d) cold water

Short answer

Magnesium does not decompose in (d) cold water.

Step-by-step solution

Understand Magnesium Reactivity

Magnesium is a metal that reacts with some water forms but not with others. It is important to understand which conditions prompt a chemical reaction and which do not.

Analyze the Given Options

Look at the options: (a) hot water, (b) steam, (c) semi hot water, and (d) cold water. Consider how magnesium usually reacts with these forms of water. Magnesium reacts with steam to produce magnesium oxide and hydrogen gas, and slowly with hot water to form magnesium hydroxide and hydrogen gas.

Eliminate the Active Reaction Options

Since magnesium reacts with steam (b) and hot water (a) to some extent, these two options can be eliminated. This leaves us with semi hot water (c) and cold water (d) as possibilities where magnesium does not decompose.

Determine the Reaction with Cold Water

Magnesium does not react with cold water at any significant rate. Therefore, when the options are weighed, the only one that magnesium truly does not decompose in is cold water.

Key concepts

Magnesium and Water Reaction

Magnesium is a silvery-white metal known for its reactivity, especially in the presence of different forms of water. The interaction between magnesium and water changes based on the temperature of the water. When magnesium comes in contact with **steam**, a vigorous reaction occurs, resulting in the production of magnesium oxide (\( \text{MgO} \)) and hydrogen gas (\( \text{H}_2 \)). The chemical equation for this reaction is: \[ \text{Mg (s) + H}_2\text{O (g)} \rightarrow \text{MgO (s) + H}_2 \text{(g)} \]In **hot water**, magnesium reacts more slowly, giving rise to magnesium hydroxide (\( \text{Mg(OH)}_2 \)) and hydrogen gas:\[ \text{Mg (s) + 2H}_2\text{O (l)} \rightarrow \text{Mg(OH)}_2 \text{(s) + H}_2\text{(g)} \]However, in **cold water**, magnesium displays minimal reactivity. The layer of magnesium oxide that forms on the magnesium surface acts as a protective barrier, preventing rapid reactions.

Chemical Reaction Conditions

Understanding the specific conditions under which a chemical reaction occurs is essential for predicting the behavior of elements like magnesium. Reaction conditions include factors such as temperature, concentration, and the physical state of the reactants. - **Temperature**: The temperature of the water significantly impacts magnesium's reactivity. Steam, being at a much higher temperature than hot water, accelerates the reaction with magnesium. - **Physical State**: Magnesium reacts more readily with steam compared to liquid water because the gaseous state provides more energy and facilitates the breaking of bonds in the reactants. - **Protective Layer**: Over time, a layer of magnesium oxide forms naturally on magnesium, which shields it from further reaction unless disrupted by higher temperatures or mechanical means.

Metal and Steam Reaction

The reaction between metal, specifically magnesium, and steam is a classic exothermic reaction. This means the reaction releases heat. The interaction is characterized by the conversion of magnesium to magnesium oxide and the liberation of hydrogen gas. When magnesium is exposed to steam, the higher temperature and energy in steam break the bonds in the magnesium oxide layer and enable the reaction to proceed rapidly. **Key Points to Remember:** - **Rapid Reaction**: Steam facilitates a faster and more complete reaction compared to water. - **Products of the Reaction**: The chemical process produces magnesium oxide and hydrogen gas, which can be seen as a vigorous reaction when steam suddenly meets the metal surface. - **Practical Applications**: Understanding how metals react with steam can help in designing steam systems for industries that utilize such reactions for energy or material production.