懂你英语 Level 3 - Unit 2 1/4 Listening 

Matter is made of atoms and molecules.

Water, for example, is the H2O molecule.

This means that a molecule of water has 3 atoms.

A water molecule has 2 hydrogen atoms and one oxygen atom.

Substances like sugar have many atoms in their molecule.

A molecule of sugar has many atoms, including carbon, hydrogen, and oxygen.

Matter is made of molecules such as H2O, which is the water molecule.

Matter can be in one of three stages, solid, liquid, and gas.

Water and ice are the same substance, but they are in different states.

These stages depend on the temperature of molecules.

When we heat a substance, the molecules move faster and try to take up more space.

When we cool a substance, the molecules move more slowly. When we cool a substance to it’s freezing point, it becomes a solid.

In the solid, the molecules move very little. Their positions are almost fixed. To be fixed means that their positions don’t change.

If  we heat the molecules, they move faster and away from each other.

The solid begins to melt, like ice cream on an hot day.

At a certain temperature, a solid begins to change into a liquid.

The  temperature at(of) which a solid changes into a liquid depends on the substance.

For water, the solid begins to change into a liquid when it's temperature rises to above 0 degree Celsius.

For some substances, such as steel, the temperature at which it becomes a liquid is much higher.

Steel often melts at around 1370 degrees Celsius.

If we continue to heat a liquid, the molecules move even faster.

At a certain temperature, the liquid begins to change into a gas.

For water, the liquid begins to change into a gas at 100 degree Celsius. This is the boiling point of water.

Inside the star, such as our sun, the temperature is very high. Every thing inside the sun is a gas.

According to scientists, there are over 65 elements inside the sun. This include oxygen and iron.

Over 90 percent of the sun is hydrogen gas.