• Question: Why does water evaporate from puddles when it is not boiling?

    Asked by mrenglishblonde to Donna, Jo, Mark, Stuart, timcraggs on 21 Jun 2010 in Categories: .
    • Photo: Donna MacCallum

      Donna MacCallum answered on 18 Jun 2010:

      Hello again Blondie – I think that I might have answered this in the previous answer. Basically, on the surface of the puddle the liquid heats faster and turns to water vapour (evaporating!) – the rest of the puddle is colder. Have you ever seen the massive amount of vapour that leave a playing field after it’s been heavily raining then gets really warm?

    • Photo: Joanna Buckley

      Joanna Buckley answered on 18 Jun 2010:

      Hi there! 🙂

      The puddle is spread over quite a large area and so the surface of the puddle is warmer than the inside. Things don’t just evaporate at 100 degrees. If you leave a glass of water out for ages it would go pretty mouldy eventually but the amount of water would also decrease as some of the surface has evaporated and that wasn’t boiling… it’ll just take ages to happen. Try it… but don’t drink the water afterwards 🙂

    • Photo: Mark Lancaster

      Mark Lancaster answered on 21 Jun 2010:

      Evaporation is a surface effect, boiling occurs throughout the entire liquid. Evaporation is just the molecules near the surface escaping the bulk liquid. The molecules all have slightly different energies – the small fraction of energetic ones have enough energy to escape the surface forces – they can get this rogue extra energy from colliding with another molecule that happens to kick it out of the liquid. So this can happen at temperatures below boiling point – just slowly.

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