Why you can never obtain 100% ethanol from distillation of a ethanol-water mixture

Distillation is an imperfect method for separating mixtures of liquids that formazeotropes. An azeotrope, also called a constant boiling mixture, is a mixture of two or more liquids at a specific ratio, whose composition cannot be altered by simple distillation. Every azeotrope has a characteristic boiling point, which may be lower (a positive azeotrope or minimum-boiling mixture) or higher (a negative azeotrope ormaximum-boiling mixture) than the boiling points of the individual liquids that make up the azeotrope.

Ethanol forms a positive azeotrope with water. The boiling point of a mixture of 95.6% ethanol (by weight) with 4.4% water is 78.1 °C, which is lower than the boiling point of pure water (100 °C) or pure ethanol (78.4 °C). Because the azeotropic mixture boils at a lower temperature, it is impossible to use simple distillation to produce ethanol at concentrations higher than 95.6%.

VSEPR theory

VSEPR which stands for Valence shell electron pair repulsion, is a model in chemistry used to predict the shape of individual molecules based upon the extent of electron-pair electrostatic repulsion. VSEPR theory proposes that the geometric arrangement of terminal atoms, or groups of atoms about a central atom in a covalent compound, or charged ion, is determined solely by the repulsions between electron pairs present in the valence shell of the central atom.

History- The idea of a correlation between molecular geometry and number of valence electrons (both shared and unshared) was first presented in a Bakerian lecture in 1940 by Nevil Sidgwick and Herbert Powell at Oxford University.

To start, we need to know the Lewis structure of a molecule. Then we count how many pairs of electrons are around the central atom. 

If there are two pairs of electrons, they must be positioned 180° apart from each other and the shape is therefore linear. 

Three pairs are best positioned 120° apart and the shape is thus trigonal planar. Here the shape is referred to include the non-bonding electron pairs. For the shape of a molecule without counting non-bonding electron pairs, make a normal prediction then look at the molecule without non-bonding electrons showing. 

Four pairs of electrons are best positioned as tetrahedral shape. Depending upon the number of non-bonding electron pairs, the shape of the molecule not counting non-bonding electron pairs can be: 

a) tetrahedral (no non-bonding pairs); 

b) trigonal pyramidal (one non-bonding pair); or 

c) "bent" or "V" (two non-bonding pairs). For five pairs of electrons, the shape is predicted to be trigonal bipyramidal. 

Lastly, the octahedral is the shape predicted for six pairs of electrons.

Firework chemistry

Firework colours:

There are two main mechanisms of color production in fireworks, incandescence and luminescence. 

Incandescence is light produced from heat. Heat causes a substance to become hot and glow, initially emitting infrared, then red, orange, yellow, and white light as it becomes increasingly hotter. Metals, such as aluminium, magnesium and titanium, burn very brightly and are useful for increasing the temperature of the firework. 

Luminescence is light produced using energy sources other than heat. Luminescence can occur at room temperature and cooler temperatures. To produce luminescence, energy is absorbed by an electron of an atom or molecule, causing it to become excited, but unstable. When the electron returns to a lower energy state the energy is released in the form of a photon (light). The energy of the photon determines its wavelength or color.

The following table illustrates the compounds required to produce specific coloured fireworks.

Colour	Compound	Wavelength of Light
Red	Strontium Salts & Lithium Salts Li2CO3 SrCO3	600-646nm
Orange	Calcium Salts CaCl2 CaSO4.2H2O	591-603nm
Gold	Incandescence of Iron or Charcoal	590nm
Yellow	Sodium Compounds NaNO3 Na3AlF6	589nm
Electric White	White Hot Metal BaO	564-576nm
Green	Barium compounds with Chlorine BaCl+	511-533nm
Blue	Copper Compounds and Chlorine Cu3As2O3Cu(C2H3O2)2	460-530nm
Purple	Mixture of Strontium (red) and Copper (blue) compounds	432-456nm
Silver	Burning aluminium, titanium or magnesium powder.	412nm

An interesting fact

It has taken pyrotechnicians years to solve the problems which lie behind coloured firework production. They have mostly succeeded except for the production of the ocean and forest green coloured firework.

A combination of green and blue coloured emmiters (such as BaCl and CuCl) is the obvious choice. However BaCl and CuCl emissions are difficult to obtain. Hence, the quest for the forest and ocean green coloured fireworks still continues.

More about fireworks:

Fireworks are usually made out of the following items; an oxidizing agent, a reducing agent, a coloring agent, binders and regulators. 

Oxidizing agent - These produce oxygen to burn the mixture. Oxidizers are usually nitrates, chlorates or perchlorates. The common oxidizers are nitrates, which are made up of a metal ion and the nitrate ion. An example of such a chemical equation: . Hence, oxygen is produced.

Reducing Agents - burn the oxygen produced by the oxidizers to produce hot gasses. Two examples of reducing agents are sulfur and charcoal(carbon). These react with the oxygen to form sulfur dioxide and carbon dioxide respectively. 

Keywords explained:

1) Incandescence - light produced from heat.

2) Luminescence - light produced using energy sources other than heat. 

3) Pyrotechnicians - an individual responsible for the safe storage, handling, and functioning of pyrotechnics (the science of materials capable of undergoing self-contained and self-sustained exothermic chemical reactions for the production of heat, light, gas, smoke and/or sound) and pyrotechnic devices.

4) Perchlorates - salts derived from perchloric acid (HClO₄). They occur both naturally and through manufacturing. They have been used as a medicine for more than 50 years to treat thyroid gland disorders.

The chemistry behind developing film photos

An introduction:

When we are taking a photograph with our camera, we are recording the visible light reflected from the objects in the camera's field of view. In order to do that, the reflected light causes a chemical change to the photographic film inside the camera. Light's energy is distributed as photons. It is the energy in each photon of light that causes a chemical change to the photographic detectors that are coated on the film. The process whereby electromagnetic energy causes chemical changes to matter is known as photochemistry. 

The film:

The film is coated with 20 or more layers of gelatin which are less than one thousandth of an inch thick. Some of the layers coated on the transparent film do not form images. They are there to filter light, or to control the chemical reactions in the processing steps. The imaging layers contain sub-micron sized grains of silver-halide crystals that act as the photon detectors. They undergo a photochemical reaction when they are exposed to various forms of electromagnetic radiation -- light. These silver-halide grains have been chemically modified on their surface to increase their light sensitivity.  Organic molecules known as spectral sensitizers were added to the surface of the grains to make them more sensitive to blue, green and red light, since the unmodified grains are only sensitive to the blue portion of the spectrum, and they are not very useful in camera film.  

Developing colour films: An explanation

Light gets reflected off surfaces and through the lens of a camera, past an open shutter, and falls onto a piece of plastic coated with layers of light-sensitive emulsion. Silver halide crystals in the emulsion react by forming clusters of silver ions, creating a latent image. When the film is submerged in developer, it transforms the silver ions into pure silver, leaving behind the silver halide crystals that were not struck by light. Those excess silver halide crystals are washed away with a second chemical, leaving metallic silver grains that are denser where the light was more intense during the exposure, producing a visible negative image. The negative images are then turned into positive images.

Key words explained:

1) photographic detectors - sensors of light or other electromagnetic energy.

2) gelatin - a translucent, colorless, brittle, nearly tasteless solid substance, derived from the collagen inside animals' skin and bones.

3) negative images - A positive image is a normal image. A negative image is an inversion of a positive image, in which light areas appear dark and vice versa. A negative color image is also color reversed, with red areas appearing cyan, green appearing magenta and blue appearing yellow.

The chemistry behind heat and cold packs

Hot/cold packs are used by athletes to minimize swelling of injuries such as muscle and joint sprains. Chemicals can store energy and release it in the form of heat. Exothermic reactions are chemical reactions that release heat. Endothermic reactions are chemical reactions that absorb heat.  

The chemistry behind cold packs:

The liquid inside the cold pack is water. In the water is another plastic bag or tube containing ammonium-nitrate fertiliser. When you hit the cold pack, it breaks the tube so that the water mixes with the fertilizer. This mixture creates an endothermic reaction -- it absorbs heat. The temperature of the solution falls to about 35 F for 10 to 15 minutes.

The chemistry behind reusable heat packs: 

The liquid inside the pouch is actually a supersaturated solution of sodium acetate. A supersaturated solution is very unstable and crystallizes very easily. However, the crystals require a 'site of crystallization' in order to kick-start the crystallization process. By flipping the disc, the gratings on it snap together and form a "crystallization site" which causes the solution to crystallize into solid sodium acetate again. Crystallization of sodium acetate is an exothermic reaction, hence the pouch heats up. After all of the sodium acetate has been crystallized, the heat will be lost and the pouch will cool down leaving a clump of solid sodium acetate inside the pouch. This reaction is reversible. To get the crystals to redissolve back into the solution, simply throw the pouch into hot water.

Key words explained:

1) Exothermic reactions - chemical reactions that release heat. 

2) Endothermic reactions - chemical reactions that absorb heat.  

3) Ammonium-nitrate fertiliser - a chemical compound made up of about 27-percent nitrogen and 8-percent calcium carbonate 

4) Supersaturation - a solution that contains more of the dissolved material than could be dissolved by the solvent under normal circumstances