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Thermodynamics is the study of how energy moves through the world．Heat is energy moving from one place to another．When heat adds energy to a substance, the atoms and molecules begin moving more quickly．The amount of thermal energy in a substance determines whether it will be solid, liquid, or gas.
The molecules in solids are packed tightly and move slowly．Heat increases the thermal energy, so the molecules move more quickly and bounce against each other with more force．They need more room to move, so the solid expands and the substance changes to liquid．More heat brings more thermal energy and the substance expands again to become gas．Energy follows the three laws of thermodynamics.
The first law is that energy is neither created nor destroyed． This means there is only so much energy in the universe．It moves around a lot, but the amount never changes．For example, if you drop an ice cube into a cup of coffee, thermal energy moves from the coffee to the ice．Solid turns to liquid — the ice melts．However, at the same time the coffee cools because it lost thermal energy to the ice．This happens on a much bigger scale in the universe.
The second law is entropy．When two substances have different amounts of thermal energy, this law states that energy will move between them until they have equal amounts．Heat always flows toward the substance with less thermal energy.
Finally, the third law states that at absolute zero entropy becomes total．It is at absolute zero that atoms and molecules stop moving．They have no energy because they’re spread evenly through the substance．The colder something is, the less energy it has.
Word List
packed： very close together
absolute： complete, total
entropy： randomness
absolute zero： the lowest temperature possible
You Practice
I．Answer the following questions.
1）What determines whether something is solid, liquid, or gas?
2）What is thermodynamics?
3）Why does dropping an ice cube into a hot cup of coffee make the coffee cooler?
4）Why do molecules stop moving at absolute zero?
5）What happens when hot water is poured on an ice cube? Why?
II．Word power—Use these words from the reading in sentences of your own.
1）substance ____________________
2）packed ____________________
3）energy ____________________
4）absolute ____________________
5）spread ____________________
III．True / False /Not Given—Write T if the sentence is true, F if it is not true, or NG if there is no information about this sentence.
____ 1）The amount of energy in the universe is always changing．
____ 2）Things that are hot have more energy．
____ 3）Energy moves around a lot in the universe．
____ 4）When the temperature drops, energy is destroyed．
____ 5）Absolute zero is too cold for people to survive．