Problem 8 - A Complete Resource Book in Chemistry for JEE Main

Question

Hydrogen burns in air or oxygen with (a) invisible pale blue flame (b) red flame (c) golden flame (d) blue flame

Step-by-Step Solution

Answer

Hydrogen burns with an invisible pale blue flame (option a).

Step 1: Understanding the Problem

The problem asks us to identify the color of the flame produced when hydrogen burns in air or oxygen. This involves knowing the properties of hydrogen combustion.

Step 2: Reviewing Hydrogen Combustion

Hydrogen is known to burn with a flame. In a pure oxygen environment or air, hydrogen typically produces a specific color flame due to the combustion process.

Step 3: Recalling Flame Colors

Hydrogen, when burned, produces a flame that is characteristically not very bright and is generally hard to see. Its color is known to be pale blue and can sometimes appear almost invisible in daylight.

Step 4: Drawing Conclusions

Based on the properties of hydrogen combustion, we can conclude that the correct answer among the given options is related to the flame being pale blue and not very visible.

Key Concepts

Flame Color

When hydrogen gas combusts, it produces a distinctive flame color. Unlike other common flames that you might see, such as the bright orange and yellow flames of a campfire, hydrogen burns with an almost invisible pale blue flame. This is because hydrogen combustion releases relatively little light compared to other burning materials. However, it burns at a high temperature.

In dark conditions, the pale blue flame of hydrogen can be easier to see. In daylight, it might appear nearly invisible due to its faintness. This is an essential safety concern in laboratories and industries that use hydrogen because detecting a flame by sight can be challenging.

• The pale blue color is due to the heat emitted by the hydrogen molecules reacting with oxygen.
• It’s important to handle hydrogen with care as its flame can be hard to detect visually.

Chemical Properties of Hydrogen

Hydrogen is the lightest and most abundant element in the universe. It is a colorless, odorless, and tasteless gas under normal conditions. These natural properties make hydrogen intriguing but also complex to handle safely.

Hydrogen combines with oxygen during combustion to form water. This is one of its most unique chemical properties as it defines hydrogen combustion’s environmental impact. Unlike fuels that produce carbon dioxide when burned, hydrogen combustion only results in water, making it a cleaner option.

• Hydrogen is highly flammable, requiring careful storage and handling.
• Due to its lightness, it quickly disperses in the air, which can prevent large accumulations in an open environment.
• Although it is non-toxic, it can still be dangerous because it can form explosive mixtures with air.

Combustion Reaction

The combustion of hydrogen occurs when it reacts with oxygen to produce heat and water. The reaction can be summarized by the chemical equation:

\[ 2H_2 + O_2 \rightarrow 2H_2O + ext{heat} \]

This reaction involves hydrogen molecules combining with oxygen molecules to form water (H₂O), releasing energy in the form of heat.

Combustion requires specific conditions to occur:

• Sufficient oxygen: The reaction needs a source of oxygen to sustain combustion.
• Ignition energy: A little extra energy is needed to start the reaction; this could be a spark or flame.
• Correct ratio: Hydrogen must be within a certain concentration range with oxygen to combust efficiently.

Understanding these factors helps ensure that hydrogen combustion is used safely and efficiently in various applications.