CONCEPT:

• Good Conductor of heat: The material that allows the heat to transfer through them easily is called a good conductor of heat.
  • Examples: Copper, silver, iron, etc
• The bad conductor of heat or insulator: The material that doesn't allow the heat to transfer through them easily is called an insulator.
  • Example: Wood, ice, glass, plastic, etc.
• Heat transfer mainly takes place due to temperature differences.

EXPLANATION:

• Air changes temperature faster than water so, in winters, the air cools down before the water.
• The water on the surface is in direct contact with the air and therefore freezes first, before the water at the bottom.
• Now since ice is less dense than water, it continues to float and doesn't sink. This ice sheet keeps getting thicker and acts as an insulator between the cold air and warm water at the bottom. This is why the water at the bottom stays liquid.
• Hence we can say that ice is a bad conductor of heat that's why the water at the bottom of the lake remains in the liquid state. So option 3 is correct.
• The density of water is maximum at 4°C but it is not the correct explanation for the given question.

​EXTRA POINTS:

There are three modes of Heat transfer.

• Conduction: The mode of heat transfer in solids where heat transfer takes place without the movement of medium particles is called conduction.
  • For example: By heating one end of a metal rod we can feel hotness at the other end.
• Convection: The mode of heat transfer in fluids where heat transfer takes place due to the movement of particles of the medium is called convection.
  • For example- Heating of water in a pot
• Radiation: The mode of heat transfer where heat is transferred from one place to another without affecting the medium particles is called radiation.
  • For example: When we place our hand nearby the burning gas stove then we can feel the hotness. This is due to radiation.