Water molecules (H2O) are polar molecules, as the oxygen is slightly negatively charged and the hydrogen is slightly positively charged. Due to this polarity, H2O molecules are attracted to one another and don’t move as freely as those in other materials. For this reason water has a high specific heat capacity – the amount of heat required to raise the temperature of the material by one degree Celsius.
Vegetables contain water but soup contains much more. Water’s high specific heat capacity means it needs more energy than other materials to raise its temperature, which for microwave cooking means the higher the water content in food, the longer it will take to heat up.
Answered by Rik Sargent, How It Works contributor.