Monthly Archives: December 2012

The Science of Snowflakes…


Snow crystals form when tiny supercooled cloud droplets (about 10 μm in diameter) freeze. These droplets are able to remain liquid at temperatures lower than −18 °C (−0 °F), because to freeze, a few molecules in the droplet need to get together by chance to form an arrangement similar to that in an ice lattice, then the droplet freezes around this “nucleus.” Experiments show that this “homogeneous” nucleation of cloud droplets only occurs at temperatures lower than −35 °C (−31 °F). In warmer clouds an aerosol particle or “ice nucleus” must be present in (or in contact with) the droplet to act as a nucleus. The particles that make ice nuclei are very rare compared to nuclei upon which liquid cloud droplets form, however it is not understood what makes them efficient. Clays, desert dust and biological particles may be effective, although to what extent is unclear. Artificial nuclei include particles of silver iodide and dry ice, and these are used to stimulate precipitation in cloud seeding.


A snowflake often exhibits six-fold radial symmetry. The initial symmetry can occur because the crystalline structure of ice is six-fold. The six “arms” of the snowflake, or dendrites, then grow independently, and each side of each arm grows independently. Most snowflakes are not completely symmetric. The micro-environment in which the snowflake grows changes dynamically as the snowflake falls through the cloud, and tiny changes in temperature and humidity affect the way in which water molecules attach to the snowflake. Since the micro-environment (and its changes) are very nearly identical around the snowflake, each arm can grow in nearly the same way. However, being in the same micro-environment does not guarantee that each arm grows the same; indeed, for some crystal forms it does not because the underlying crystal growth mechanism also affects how fast each surface region of a crystal grow.


Snowflakes form in a wide variety of intricate shapes, leading to the popular expression that “no two are alike”. Although statistically possible, it is very unlikely for any two snowflakes to appear exactly alike. Initial attempts to find identical snowflakes by photographing thousands of them with a microscope from 1885 onward by Wilson Alwyn Bentley found the wide variety of snowflakes we know about today.

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Why does salt melt ice?

­If you live in a place that has lots of snow and ice in the winter, then you have probably seen the highway department spreading salt on the road to melt the ice.­ You may have also used salt on ice when making home-made ice cream. Salt lowers the freezing/melting point of water, so in both cases the idea is to take advantage of the lower melting point.

 Ice forms when the­ temperature of water reaches 32 degrees Fahrenheit (0 degrees Celsius). When you add salt, that temperature drops: A 10-percent salt solution freezes at 20 F (-6 C), and a 20-percent solution freezes at 2 F (-16 C).

On a roadway, this means that if you sprinkle salt on the ice, you can melt it. The salt dissolves into the liquid water in the ice and lowers its freezing point.

If you ever watch salt melting ice, you can see the dissolving process happen — the ice immediately around the grain of salt melts, and the melting spreads out from that point. If the temperature of the roadway is lower than 15 F or so, then the salt really won’t have any effect — the solid salt cannot get into the structure of the solid water to start the dissolving process. In that case, spreading sand over the top of the ice to provide traction is a better option.

When you are making ice cream, the temperature around the ice cream mixture needs to be lower than 32 F if you want the mixture to freeze. Salt mixed with ice creates a brine that has a temperature lower than 32 F. When you add salt to the ice water, you lower the melting temperature of the ice down to 0 F or so. The brine is so cold that it easily freezes the ice cream mixture.

Two things happen when ice and water are placed in contact:

  • Molecules on the surface of the ice escape into the water (melting), and
  • molecules of water are captured on the surface of the ice (freezing).

When the rate of freezing is the same as the rate of melting, the amount of ice and the amount of water won’t change on average (although there are short-term fluctuations at the surface of the ice). The ice and water are said to be in dynamic equilibrium with each other. The balance between freezing and melting can be maintained at 0°C, the melting point of water, unless conditions change in a way that favours one of the processes over the other.

The balance between freezing and melting processes can easily be upset. If the ice/water mixture is cooled, the molecules move slower. The slower-moving molecules are more easily captured by the ice, and freezing occurs at a greater rate than melting.

Conversely, heating the mixture makes the molecules move faster on average, and melting is favoured.  Adding salt to the system will also disrupt the equilibrium. Consider replacing some of the water molecules with molecules of some other substance. The foreign molecules dissolve in the water, but do not pack easily into the array of molecules in the solid. This leads to fewer water molecules on the liquid side because the some of the water has been replaced by salt. The total number of waters captured by the ice per second goes down, so the rate of freezing goes down. The rate of melting is unchanged by the presence of the foreign material, so melting occurs faster than freezing. That’s why salt melts ice.

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It’s a Cracker!!!

Silver fulminate (AgCNO) is the highly explosive silver salt of fulminic acid.
Silver fulminate is a primary explosive that has very little practical value due to its extreme sensitivity to impact, heat, pressure and electricity. The compound becomes progressively sensitive as it is aggregated, even in small amounts; the touch of a falling feather, the impact of a single water droplet or a small static discharge are all capable of explosively detonating an unconfined pile of silver fulminate no larger than a penny and no heavier than a few milligrams. Aggregation of larger quantities is impossible due to the compound’s tendency to self-detonate under its own weight.

Silver fulminate was first prepared in 1800 by Edward Charles Howard in his research project to prepare a large variety of fulminates. Since its discovery, its only practical usage has been in producing non-damaging novelty noisemakers as children’s toys and tricks – and Cracker snaps!

Silver fulminate, often in combination with potassium chlorate, is used in trick noise-makers known as “crackers”, “snappers”, “whippersnappers”, “pop-its”, or “bang-snaps”, a popular type of novelty firework. They contain approximately 200 milligrams of fine gravel impregnated with a minute quantity (approximately 80 micrograms) of silver fulminate. When thrown against a hard surface, the impact is sufficient to detonate the tiny quantity of explosive, creating a small report from the supersonic detonation. Snaps are designed to be incapable of producing damage (even when detonated against skin) due to the buffering effect provided by the much greater mass of the gravel medium. It is also the chemical found in Christmas crackers. The chemical is painted on one of two narrow strips of card, with abrasive on the second. When the cracker is pulled the abrasive detonates the silver fulminate.

Remember this next time you pull a cracker!!!!!

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