8.20 - How the oceans formed
Follow the sequence from water vapour in the hot early atmosphere to liquid oceans. Use changes of state to explain why cooling mattered.
Cooling changes water vapour into liquid
The early atmosphere contained water vapour, H₂O(g), supplied by volcanic activity. When Earth's surface and atmosphere were very hot, liquid water could not persist at the surface. As Earth cooled sufficiently, water vapour condensed into liquid droplets.
Condensation
Condensation is the change of state from gas to liquid.
H₂O(g) → H₂O(l)
The water molecules remain H₂O. Condensation is a physical change: no new substance forms. Energy is transferred from the water to its surroundings as it condenses, and its molecules become much closer together.
From droplets to oceans
Condensation produced droplets that fell as rain. Once conditions were cool enough, liquid water could remain on the surface and flow into low areas. Repeated rainfall and accumulation over a long time formed oceans.
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