How do you make transparent ice? Also, why does just-boiled water freeze faster than cold?

15 Dec

Snow is frozen water, and that’s not transparent either. But massive Antarctic glaciers are clear and beautiful. It’s all the same water, so why the difference?

The truth is, not all water is the same.

The water cycle teaches us that water in lakes and rivers evaporates, forms clouds, rains on the mountains and runs back into the lakes and reservoirs. Understandably, it’s not often that water really gets a chance to travel far. Most cloud formations stay within a certain geographical area, usually limited by the nearest hills for example. Even the clouds that are lucky enough to get harvested for their helium content have the water released locally.  This means that in any given area, the water system there is made up of around 85% water that has been local to the area for many years.

Ice formations are similar to crystal and mineral formations in that the formation will begin at a starting point known as the ‘seed’ and build to match the existing crystals. What this means is that water will form ice in accordance with the way that water has always formed ice. It also means that the behaviour of any given water system can be different from area to area.

Take East Anglia in the UK, for example. The water system is limited by the Chiltern Hills which run all the way right through Bedfordshire, Buckinghamshire and Hertfordshire. This means that most of the water over the three counties stays in it’s own cycle.

The ice it forms is white, crunchy, often unevenly lumpy/slushy and most definitely opaque. And if you use local tap water to make ice cubes, that’s the type of ice cubes you’ll get.

Simply boiling the water is not enough to reset the seeding properties and make it form a different type of ice. The trick is to make the water believe it’s from an iceberg.

Doing this at home can be awkward, but it is possible without any specialised equipment.

First, get your biggest saucepan and fill it 3/4 full of water. Put it on the stove to heat. This simulates the water being in a lake and evaporating naturally.

Next, take the lid of that pan (hopefully it’s domed or has a handle in the centre) and turn it upside-down on top of the pan. This simulates the sky and cloud formations. Fill it with ice to replicate the atmospheric conditions up high.

Then you’ll need a small bowl to float in the middle of the pan of water, to collect the ‘rain’ which will start to drip from the upturned pan lid. The water you collect in this bowl will now have been reset.

Once you have as much water as you would like to collect, you need to provide it with a new ‘seed’ to tell it what ice to form. You do this by using a very simple item that most people have in their kitchen – cling film. Lay a piece of cling film on the surface of the water. It doesn’t need to form a seal or cover the whole surface. As long as it’s touching the water it will simulate a clear sheet of ice like the one we are trying to create. Put this in the fridge overnight to simulate Arctic conditions.

Finally, the freezing process. You can’t just go ahead and freeze the whole lot at once. That never happens in nature. Glaciers have sunlight on the surface and freeze from the bottom upwards, so this is how your ice needs to develop. The logistics of this are up to you. Ideally you may choose to simulate the sun by keeping something warm above the surface of the water, although this method may make your spouse uncomfortable if your freezer is also full of food. If you have the time to go back and add a small amount of water every hour or so this could work. If not, it may be possible to set up a drip system (bearing in mind that icicles are also clear), but you would need to ensure the source does not freeze before it gets a chance to drip.

If you manage to convince the water it is forming an iceberg or icicle, you should end up with a nice, transparent lump of ice.

As for boiled water freezing faster, this is known as the Mpemba effect and has been a mystery as far back as Aristotle and Descartes. This summer the Royal Society of Chemistry offered a £1,000 reward for an answer to the question. Many plausible theories were submitted, but none hit on the truth:-

Water does not like to be hot.

When you boil a kettle, science teaches us that the hotter the water is, the more energy it takes to make it hotter. The amount of energy required to reach 100 degrees from 99 is much more than to reach 99 from 98 and so on. The water molecules are resistant to taking in more heat energy because this means they will turn into steam and float away and possibly get lost and never see their friends again. This principle applies exactly the same in reverse. They know that if they freeze they get to stay together and be safe so the closer they are to danger the more quickly they get to safety by losing their heat and freezing.

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Posted by on December 15, 2012 in How?, Why?


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