Key Idea.
Section 1
The Disappearing Act
Have you ever seen a puddle dry up on a hot day? The water didn't just vanish into nothingness. It changed form.
When liquid water warms up, it turns into a gas. This process is called evaporation. The liquid water becomes water vapor, an invisible gas that rises and mixes with the air. The water is still there; you just can't see it anymore.
Section 2
Molecules on the Move
To understand evaporation, we need to zoom in to the molecules. In liquid water, water molecules are huddled close together.
When they get warm, they gain energy and start moving fast—really fast. They break free from their group and fly apart into the air. This shows that matter is conserved: the number of molecules stays the same, they just spread out to become a gas.
Section 3
Spreading Out
The main difference between liquid water and water vapor is space. In liquid water, molecules are packed tight. In water vapor, molecules are far apart.
Because the molecules in the gas are so spread out, light passes right through them. This is why water vapor is invisible to our eyes, unlike the puddle it came from.
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Section 1
The Disappearing Act
Have you ever seen a puddle dry up on a hot day? The water didn't just vanish into nothingness. It changed form.
When liquid water warms up, it turns into a gas. This process is called evaporation. The liquid water becomes water vapor, an invisible gas that rises and mixes with the air. The water is still there; you just can't see it anymore.
Section 2
Molecules on the Move
To understand evaporation, we need to zoom in to the molecules. In liquid water, water molecules are huddled close together.
When they get warm, they gain energy and start moving fast—really fast. They break free from their group and fly apart into the air. This shows that matter is conserved: the number of molecules stays the same, they just spread out to become a gas.
Section 3
Spreading Out
The main difference between liquid water and water vapor is space. In liquid water, molecules are packed tight. In water vapor, molecules are far apart.
Because the molecules in the gas are so spread out, light passes right through them. This is why water vapor is invisible to our eyes, unlike the puddle it came from.
Book overview
Jump across lessons in the current chapter without opening the full course modal.
Continue this chapter