H2WOW! The Explosive Side of Water
As any Melbournian will tell you, springtime weather can sometimes feel like winter is back for an encore. There are few greater pleasures at this time of year than collapsing on the couch at the end of a busy day with a good TV show and a warm cuppa’. Could anything possibly go wrong in this fantasy?
Everyone loves their cuppa’ tea! (Rebecca Sims via Flickr)
Yes. A whole lot apparently. In fact, I was horrified to learn that for a few poor souls every year, dipping a bag of English Breakfast into their cups ended in scalding hot water being thrown back into their faces as it all instantly boiled.
The culprit? Superheated water. Now, before mass panic begins ensues, science tells us the chances of accidently falling victim to this phenomenon is almost zero. That said, there are some very easy steps we can all take to really put a lid on it.
The Hidden Dangers of Boiling Water
From as early as primary school, most kids have the following drilled into their brains: “water boils at 100 C°”. Right?
Well, not always.
In fact, under delicate conditions, water can actually be heated above its boiling point.
The old adage should really be “water boils at 100C at the boundary between water and the surrounding air at sea level”. It isn’t as catchy, but it is much more telling.
At its boiling point, water has enough energy to turn into a gas; this is what boiling is! If you’ve ever cooked before, the tell-tale bubbling is how we know that our pot of water is nice and hot.
It actually takes quite a bit of energy to push the surrounding water out of the way and fill that space with vapour. That’s why water boils easily if it’s already in contact with a bubble; it is hard to form the initial bubble, but very easy for it to grow in size once formed. It’s a bit like inflating a balloon; you have to blow quite hard to squeeze air into it at the start, but it gets easier as the balloon gets bigger!
We call these small air bubbles ‘nucleation sites’.
Usually there’s lots of these in cups, pots, kettles, or any other strange utensil you are using to brew your tea. They hide in microscratches: tiny imperfections in an object (sometimes invisible to the human eye) that come from natural wear and tear. When you heat up water, these trapped bubbles allow the water to boil easily.
Bubbling water as it starts to boil (Scott Akerman via Flickr)
It’s when your mug has no microscratches, allows little of the water to be in contact with air, and is kept very still while being heated that conditions are suitable for superheating…
…say, if you heated up a brand new mug in the microwave.
Disturb this stuff even slightly by dipping in a teabag (or even just sloshing it around), and KABOOM! An instantaneous, violent boiling process that sends hot water and steam flying in every direction. Voila, exploding water!
Why have I never seen water explode?
Remember when I said that water can only boil at a boundary with air? Luckily for us, this isn’t the whole truth. Impurities in the water like dissolved salts and bits of dust can also act as nucleation sites and start the boiling process. This makes exploding water an extremely hard feat to accidently perform.
Improbable, but not impossible.
It’s for this reason why you’ve probably never seen this process in reverse, also known as ‘supercooling water’. But that’s a story for another time…
How can we make tea safe again?
There are some easy ways we can protect ourselves against the wrath of “explosive cuppa’s”.
Your old, scratched up, favourite mug will have sentimental value. It will also have many, many microscratches. Heat up your water in this and it should boil as per usual.
If you’re using a brand new cup, simply place a wooden stirrer into it before you turn on the microwave (but do NOT put a metal teaspoon in the microwave!!!). This will disturb the water and force it to boil when it’s hot enough, stopping it from superheating.
But the easiest fix? Boil your water in a kettle, and not in the microwave!
Now if you’ll all excuse me, my tea is getting cold.