Bubbles or no bubbles?

The idea

At the boiling point vapour forms very fast, both at the surface and within the liquid, seen as bubbles; evaporation is the slow formation of vapour only at the surface, at all temperatures below the boiling point.

Spilled water

Spilled water on the floor disappears after some time, at any temperature. It never boils.

How does the spilled water disappear?

• It slowly turns into vapour from its surface
• It boils without anyone noticing
• It turns into ice

Where does vapour form?

Choose a temperature. Watch where the vapour forms, and how fast.

Boiling and evaporation

At the boiling point, vapour forms very fast, both at the surface and within the liquid. We see this as bubbles. Evaporation is the slow formation of vapour only at the surface, at all temperatures below the boiling point.

• Boiling | Evaporation
• Only at the boiling point | At all temperatures below the boiling point
• Very fast | Slow
• At the surface and inside the liquid: bubbles | Only at the surface: no bubbles

Notes

Boiling: at the boiling point, vapour forms very fast, at the surface and within the liquid (bubbles). Evaporation: slow, only at the surface, at all temperatures below the boiling point.

• Boiling | Evaporation
• At the boiling point | Below the boiling point
• Very fast, with bubbles | Slow, only at the surface

Check yourself

Where does vapour form during evaporation?

Wet clothes dry on a line on a cool day. What is happening to the water?

Big bubbles keep rising through a pot of water on a stove. What is happening?

At which temperatures does water evaporate?

• At the bottom of the container. Evaporation happens only at the surface.
• Inside the liquid, as bubbles. Vapour forms inside the liquid, as bubbles, only at the boiling point.
• Only at the surface of the liquid — correct. Yes. That is why there are no bubbles.
• It evaporates — correct. Yes. Vapour forms slowly at the surface, far below the boiling point.
• It boils. The clothes are far below 100 °C. Boiling happens only at the boiling point.
• It melts. Melting is a solid turning into a liquid.
• The water is evaporating slowly. Evaporation forms vapour only at the surface, without bubbles.
• The water is boiling: vapour forms inside the liquid — correct. Yes. Bubbles show vapour forming within the liquid.
• The water is turning into ice. Ice forms on cooling, not on heating.
• Only on hot, sunny days. Spilled water disappears at any temperature, even on cool days.
• Only at 100 °C. At 100 °C water boils. Evaporation happens below the boiling point.
• At all temperatures below its boiling point — correct. Yes. That is why spilled water dries at any temperature.