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Thursday, February 25, 2016

Chemistry for Preppers: Chemical Solutions

A lot of chemistry occurs while the components are suspended in a neutral carrier that doesn't add anything to the reaction, but merely allows it to happen.These are called solutions.

If you add table salt (NaCl) to a cup of water, it will dissolve. This is a simple solution where the water is the solvent and the NaCl is the solute. Solvents come in two forms, polar and non-polar.
  • Water is a polar solvent due to the lop-sided shape of the water molecule. It has one end with a more negative charge and the other end has a more positive charge, like a magnet. Polar solvents work best to dissolve ionic molecules.
  • Non-polar solvents have a more balanced charge across the molecule and tend to be less conductive of electricity. Most petroleum products are non-polar and also make good electrical insulators, which is why large transformers are filled with oil.
There are limits to how much NaCl you can dissolve in water depending on the temperature of the solvent, technically known as the solubility. Most common tables of properties (found in reference books) will give solubility in water. Wikipedia has a solubility table for many common chemicals. Solutions are generally easy to separate, evaporation of the solvent (paint drying is one example) is a common method. Ionic compounds like NaCl dissociate into their ions in solution, but reform when the solvent is removed. This comes in handy when you need to replace an ionic component with something more useful.

Solubility is a common method used to separate out a compound formed in a solution. Using black powder as an example; most DIY recipes for black powder start with the potassium nitrate and how to make it. If you have access to caves full of guano (bat excrement that has been anaerobically digested by nitrifying bacteria) or manure piles that are over a year old (same bacterial action) you have a source of ammonium nitrate (NH4NO3) which has a solubility of about 240g/100g of H2O at room temperature. Rinsing the source material with water to dissolve the NH4NO3 and then adding potassium hydroxide, commonly known as lye, (KOH) will produce potassium nitrate (KNO3) and ammonium hydroxide (NH4OH). KNO3 has a solubility of only 43g/100g H2O at room temperature, about a fifth of NH4NO3. Evaporating off the water will cause the ratio of solute/solvent to shift higher and once the solubility for the ambient temperature has been reached, the solute will begin to form crystals that fall out of solution (precipitate). This means that KNO3 will precipitate out of solution as crystals long before any leftover NH4NO3 would, making it easy to separate out of the liquid with a paper filter.

Here's the chemical equation in proper form: NH4NO3 + KOH → KNO3 + NH4OH

Mixing two liquids, like alcohol and water, is another type of solution but it is harder to define which is the solvent and which is the solute. Generally, whichever is present in a larger percentage is the solvent. Separating liquid portions out of a solution is commonly done through:
  • Distillation uses the differences in boiling/condensation point to separate the liquids. Most petroleum products are “fractioned” out of crude oil through distillation.
  • Freezing uses the differences in freezing points to separate components. Sea water can be made drinkable by freezing it and melting the top layer of ice since salt water freezes at a lower temperature than fresh water.
  • Membrane technology, of which reverse osmosis is one form, allows only one part of the solution to pass through.
  • Centrifuges use the differences in  liquid densities to separate them. If you've ever seen a cream separator, with its stack of spinning plates, then you've seen a centrifuge. Spinning a liquid will cause the more dense portion to move towards the outside of the container, while the less dense portion will remain closer to the center.
  • Adsorption is a method that uses a material with a chemical affinity for some of the portions but not the others. Activated charcoal is an adsorption filter that most of us are used to seeing in water purification.
Ethyl alcohol (grain alcohol) is one of the inevitable exceptions, as it forms an azeotrope where the water molecules get locked into the alcohol molecules. This is why distillation of fuel alcohol gets tricky as you approach 90% pure alcohol: to get above 90% pure grain alcohol, you'll need to use something with a stronger affinity for water than the alcohol has. Concentrated sulfuric acid and adsorption media are the most common methods of making 100% pure (anhydrous) ethyl alcohol. Fortunately, most engines will run on alcohol that is 65% or more pure, so simple distillation will suffice for making fuel.

Next week, I'll get into ways of measuring the parts of a solution, so get ready for some math and constants that don't make a whole lot of sense to common folks. Just remember that chemistry was started by alchemists, and really only took off  because of competition between chemists who used systems of measurement and naming that were designed to keep their work hard to understand. Being the first to isolate an element or refine a useful compound was their way to get into the history books, so they put some effort into secrecy.

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