Why did my hot water bottle go cold?


The weather has turned colder here and we have started giving the children hot water bottles to keep them warm in bed at night. For my 3-year-old, this is a new thing – she was pleased to have a snuggly warm toy to cuddle up to all through the night. But the next morning, when she woke up, she cried because her hot water bottle had gone cold!

So, why does a hot water bottle go cold overnight? Where does all the heat go?

Heat is a kind of energy, known as thermal energy. It comes from the movement (or jigglyness) of atoms and molecules that make up everything around us.

Heating something up gives its atoms and molecules more energy, so they jiggle about more; cooling something down takes away energy from its atoms and molecules, so they jiggle about less. And you might remember from my answer to how does temperature work? that temperature describes how much energy atoms and molecules have.

When I make a hot water bottle for my daughter, I put cold water into the kettle and heat it up using electricity. This adds energy to the water molecules, making them jigglier and hotter, and I use this heat-filled water to fill up the hot water bottle. The water molecules jiggle about inside the hot water bottle but, as they move about, they gradually lose some of their extra energy – they pass it to my daughter (warming her up), to her bed (warming it up), and to the air around her (warming that up too)! Eventually, they lose all their extra energy and, by morning-time, the water bottle feels cool.

If you want to know more about the science of heat, which is a kind of physics known as ‘thermodynamics‘, there are some rules that scientists use to describe how heat energy works.


3 thoughts on “Why did my hot water bottle go cold?

  1. Skyscapes for the Soul December 7, 2017 / 3:08 am

    I was thinking about your blog when I was ironing last night and wondered if, somewhere along the line you addressed one of my childhood questions that remains unanswered: Why does fabric crease, and why does ironing straighten it out?

    Liked by 1 person

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