In which of the following are the substances arranged in descending order of their thermal conductivities?

Analyze the question: You're asked to pick the list where the metals go from the highest thermal conductivity to the lowest.

Concept: Thermal conductivity (k) measures how well a material conducts heat. Metals are good conductors because of their free electrons, but they differ in how good they are. Approximate values at room temperature (in W/m·K):

| Metal | Thermal conductivity | |---|---| | Copper | about 385-400 | | Aluminium | about 205-237 | | Iron | about 80 |

Solution:

1. Copper is the best conductor of the three (about 400).
2. Aluminium comes next (about 200-237).
3. Iron is the poorest of the three (about 80).

So the descending order is copper > aluminium > iron.

Checking the options:

• A. aluminium, copper, iron: wrong, because copper should come first.
• B. aluminium, iron, copper: wrong, because copper is last here.
• C. copper, aluminium, iron: correct.
• D. copper, iron, aluminium: wrong, because iron is placed above aluminium.
• E. iron, copper, aluminium: wrong, because iron is first here.

Tip: Remember the order with "Car And Iron" (Copper, Aluminium, Iron). Silver is the best conductor of all, followed by copper, then gold, then aluminium. Iron is much lower down the list.