Which of the following has the least thermal conductivity?

Analysis

The question asks us to identify the substance with the least thermal conductivity among the given options. Thermal conductivity is a measure of a material's ability to conduct heat. Materials with low thermal conductivity are known as thermal insulators.

Concept Explanation

Thermal conductivity depends on how easily energy (heat) can be transferred through a substance.

• Solids (like glass) generally conduct heat better than liquids or gases because their particles are packed closely together, allowing for efficient vibration transfer.
• Liquids (like water) conduct heat better than gases because their molecules are closer together than those in a gas.
• Gases (like air) are the poorest conductors of heat because their molecules are very far apart, making it difficult for kinetic energy to be transferred through collisions.

Among the options provided:

• Glass is a solid and a relatively poor conductor compared to metals, but it is still a better conductor than air.
• Water is a liquid and has a higher thermal conductivity than air.
• Ash is a porous solid; while it is a good insulator, it is still denser than air.
• Air is a gas. In its still state, it is one of the best natural insulators because its molecules are widely spaced, significantly hindering the transfer of heat by conduction.

Step-by-Step Solution

1. Compare states of matter: Generally, the order of thermal conductivity is Solids > Liquids > Gases.
2. Evaluate the options: • Glass (Solid) • Ash (Solid/Powder) • Water (Liquid) • Air (Gas)
3. Conclusion: Since air is a gas, it has the largest intermolecular spacing, which makes it the most difficult medium for heat to travel through via conduction. Therefore, it possesses the lowest thermal conductivity of the four.

Exam Tip

Whenever you see a question about "least thermal conductivity," always look for the gas first. Gases are almost always the best insulators (poorest conductors) in these types of physics problems. Think of how double-glazed windows use a trapped layer of air to keep heat inside a room—that is because air is an excellent insulator!