Our next experiment comes courtesy of a friend of mine after an experience he had while serving overseas. On hot days in the desert the boys would get some refreshing entertainment by pulling out a bottle of water from their extra-cold fridge and slamming it down on a table. Every so often one of the bottles would instantly turn to ice! He was baffled by the paradoxical effect. If you take water from a cold fridge into the hot air and add energy by hitting it, it should only get warmer and stay liquid, not freeze, right? Well, no actually.
The false assumption was that the water was warmer than 0°C to begin with, when in fact it was still liquid below the freezing point!
Very pure water can become supercooled. This semi-stable state is not favoured and is easily disrupted by impact, resulting in almost instant freezing.
A liquid (like water) at its freezing point will crystallize when enough water molecules bump into each other in just the right way to form a seed crystal or nucleus, around which a larger crystal structure can form (ice!). Nucleation (the localized budding of a distinct thermodynamic phase – like freezing) occurs more easily at nucleation sites on surfaces contacting the liquid, such as the side of a bottle or small particles in water.
Supercooling is possible when using “pure” water (such as distilled) because the likelihood of a large enough crystal lattice forming is reduced due to the presence of fewer nucleation sites. However, adding a sudden burst of energy through shaking increases the probability of the supercooled water forming a spontaneous seed crystal resulting in homogeneous nucleation (i.e. lots of ice all at once). And, of course, manually adding ice will also initiate the change.
This is the same process (roughly) that results in freezing rain and is used for cloud seeding. On to the experiment!
We started out with 3 – 500ml bottles. One with natural spring water (as a control), a second with distilled water, and one with demineralised water. These were then placed in the freezer to get cold. After an hour we realized that it was going to take
forever a while for that much water to freeze – and we’re impatient. On to experiment 2.0 – a bowl of salted ice with a glass of water in it.
We ran into a small problem (about 10 mm) a few minutes in. The beaker used was a little too thin on the bottom and ended up breaking.
Experiment 2.2. Same set-up, less water, longer time.
Experiment 2.3. Less less water, more salt, longer longer time.
Experiment 2.4. Back to experiment 1.0 – the water bottles. This also did not work.
Break for dinner. After renewing our energy with quiche we tried the water in a bowl of ice experiment one more time. Only to have it not work. Again. In frustration I threw an ice cube into the cup with “regular” tap water – to only watch it freeze instantly. Off camera. Sigh. You’ll have to take our word for it – but it totally worked! And after the way this day has gone it was the most likely time for something to go right.
Yesterday we put five solid hours into watching water freeze with nothing to show for it…
So we broke down and bought 8 bottles of commercial water. These were placed in the freezer for 50 minutes until the spring water (i.e. regular or unpurified water) began to freeze. And guess what????????Illustration adapted from The Gashlycrumb Tinies: or, After The Outing by Edward Gorey, published in 1963.