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 cold water.

Step-by-step solution

Understanding Decomposition

Decomposition of magnesium refers to breaking down or reacting under certain conditions. In this case, we are examining in which type of water magnesium does not react. Typically, reactions may vary based on temperature changes of the water such as cold, hot, or steam.

Identifying Magnesium's Reaction with Water

Magnesium reacts very slowly with cold water, and it may form a thin layer of magnesium hydroxide, but this reaction is not enough to be considered decomposition. It reacts more rapidly with hot water and steam, resulting in magnesium oxide and hydrogen gas.

Evaluating Options Based on Reaction Intensity

Given that reactions are stronger in hot water and steam, we should evaluate magnesium's stability in these different water conditions. Since magnesium reacts minimally or very slowly with cold water, it could be considered as a condition where it does not undergo decomposition.

Conclusion Based on Chemical Behaviors

Since magnesium reacts least or almost non-decomposingly in cold water, compared to hot water, steam, or semi-hot water where reactions are observable, we conclude that magnesium does not decompose mainly in cold water.

Key concepts

Decomposition

Decomposition is a type of chemical reaction where one substance breaks down into two or more simpler substances. It's an important concept in understanding how materials interact and transform under certain conditions. For magnesium, decomposition refers to how it reacts with water at various temperatures. When looking at different states of water like steam, hot water, or cold water, each state can cause different levels of reactivity. In the exercise, we're specifically interested in identifying whether magnesium undergoes significant reaction—essentially decomposition—in these conditions. With chemical reactions, temperature plays a crucial role. A higher temperature generally increases the reaction rate, leading to a more significant breakdown or decomposition. However, magnesium's interaction with cold water is characterized by a very slow reaction, not enough to produce a substantial amount of new substances, indicating minimal decomposition in this condition.

Magnesium Oxide

Magnesium oxide is a compound that forms when magnesium reacts with oxygen. In our scenario, when magnesium reacts with hot water or steam, it doesn't directly bond with oxygen from the water. Instead, a side reaction with the oxygen present in water forms magnesium oxide. In the presence of hot water or steam:

• Magnesium reacts energetically, warming the water which can contribute to additional breakdown processes.
• Steam specifically elevates this reaction due to higher mobility and energy at the molecular level.
• The generation of heat and the gaseous environment facilitates the formation of magnesium oxide as a solid, white powder.

Understanding how magnesium oxide forms is crucial since it signifies that a substantial reaction has taken place. In contrast, when magnesium interacts with cold water, these intense reactions that lead to magnesium oxide formation are notably absent.

Hydrogen Gas

Hydrogen gas is a byproduct of the reaction between magnesium and water, especially in higher temperature conditions. During the reaction with hot water or steam, when magnesium breaks down, it liberates hydrogen gas from water molecules. Here's how hydrogen gas plays into this:

• When magnesium reacts with water, it takes oxygen to form magnesium oxide.
• The hydrogen part of the water molecule is released as a gas, contributing to the emergence of hydrogen gas.
• This gas reaction is observable as fizzing or bubbling when magnesium is immersed in hot water or steam.

The presence of hydrogen gas indicates that a significant chemical reaction is taking place, marking a difference between the almost negligible reaction with cold water. With cold water, the generation of hydrogen is so slow that it shows limited activity, further implying that the decomposition process is minimal compared to reactions under hotter conditions.