Charged ends at work

Section 4 of 4

Polar molecules turn to line up with an electric field. A microwave oven uses this to heat the water in your food.

An electric field has a positive side and a negative side. It twists a polar molecule so that its δ+ end faces the negative side and its δ− end faces the positive side. A non-polar molecule has no charged ends for the field to twist.

Inside a microwave oven, the microwaves' electric field flips direction about 5 billion times a second. Every flip tugs each water molecule in the food to turn the other way. As the molecules turn, they knock into their neighbours, and that jostling is heat.

Water molecules turning in an electric field Two panels. Left, a negative plate on the left and a positive plate on the right, with three water molecules between them, each turned so its two hydrogen atoms point to the negative plate. An arrow leads to the right panel, where the plates have swapped sides and every water molecule has turned round, its hydrogen atoms now pointing right, towards the new negative plate. − + + − hydrogen ends face − the field flipped
A simple picture of water in an electric field: the hydrogen ends, δ+, face the negative side. When the field flips, they are tugged round to face the other way.

Ice and plastic tubs

Ice warms slowly in a microwave oven, because the water molecules in an ice crystal are held in place and can barely turn. A defrost setting heats in short bursts with pauses between them, so heat from the thawed patches has time to spread into the frozen parts.

The poly(propene) of many food tubs, a polymer you have met, is made of long chains of C–⁠C and C–⁠H bonds, so it is non-polar. The field has nothing to turn, so the tub warms up only from the hot food inside it.

Quick check

A microwave oven warms the water in food. What lets it do that?

That's the end of lesson 11

In this lesson you found out:

  • Why some atoms pull harder on shared electrons than others
  • What partial charges are, and how chemists mark them
  • Why water is a polar molecule and carbon dioxide is not
  • How a microwave oven heats food by turning water molecules
Start Lesson 12: Molecules that stick together All lessons