If all portions of a material are in the same state, have no visible boundaries, and are uniform throughout, then the material is homogeneous. Examples of homogeneous mixtures are the air we breathe and the tap water we drink. Homogeneous mixtures are also called solutions. Thus air is a solution of nitrogen, oxygen, water vapor, carbon dioxide, and several other gases; tap water is a solution of small amounts of several substances in water.
The specific compositions of both of these solutions are not fixed, however, but depend on both source and location; for example, the composition of tap water in Boise, Idaho, is not the same as the composition of tap water in Buffalo, New York.
Although most solutions we encounter are liquid, solutions can also be solid. Solid solutions of two or more metals are commonly called alloys. If the composition of a material is not completely uniform, then it is heterogeneous e. Mixtures that appear to be homogeneous are often found to be heterogeneous after microscopic examination. Milk, for example, appears to be homogeneous, but when examined under a microscope, it clearly consists of tiny globules of fat and protein dispersed in water.
The components of heterogeneous mixtures can usually be separated by simple means. Solid-liquid mixtures such as sand in water or tea leaves in tea are readily separated by filtration, which consists of passing the mixture through a barrier, such as a strainer, with holes or pores that are smaller than the solid particles.
In principle, mixtures of two or more solids, such as sugar and salt, can be separated by microscopic inspection and sorting. More complex operations are usually necessary, though, such as when separating gold nuggets from river gravel by panning. First solid material is filtered from river water; then the solids are separated by inspection. If gold is embedded in rock, it may have to be isolated using chemical methods. Homogeneous mixtures solutions can be separated into their component substances by physical processes that rely on differences in some physical property, such as differences in their boiling points.
Two of these separation methods are distillation and crystallization. Distillation makes use of differences in volatility, a measure of how easily a substance is converted to a gas at a given temperature. A simple distillation apparatus for separating a mixture of substances, at least one of which is a liquid. The most volatile component boils first and is condensed back to a liquid in the water-cooled condenser, from which it flows into the receiving flask.
If a solution of salt and water is distilled, for example, the more volatile component, pure water, collects in the receiving flask, while the salt remains in the distillation flask. Mixtures of two or more liquids with different boiling points can be separated with a more complex distillation apparatus. One example is the refining of crude petroleum into a range of useful products: aviation fuel, gasoline, kerosene, diesel fuel, and lubricating oil in the approximate order of decreasing volatility.
Another example is the distillation of alcoholic spirits such as brandy or whiskey. This relatively simple procedure caused more than a few headaches for federal authorities in the s during the era of Prohibition, when illegal stills proliferated in remote regions of the United States! Crystallization separates mixtures based on differences in solubility, a measure of how much solid substance remains dissolved in a given amount of a specified liquid. Most substances are more soluble at higher temperatures, so a mixture of two or more substances can be dissolved at an elevated temperature and then allowed to cool slowly.
Alternatively, the liquid, called the solvent, may be allowed to evaporate. In either case, the least soluble of the dissolved substances, the one that is least likely to remain in solution, usually forms crystals first, and these crystals can be removed from the remaining solution by filtration. Most mixtures can be separated into pure substances, which may be either elements or compounds.
An element , such as gray, metallic sodium, is a substance that cannot be broken down into simpler ones by chemical changes; a compound , such as white, crystalline sodium chloride, contains two or more elements and has chemical and physical properties that are usually different from those of the elements of which it is composed.
With only a few exceptions, a particular compound has the same elemental composition the same elements in the same proportions regardless of its source or history. The chemical composition of a substance is altered in a process called a chemical change.
Technically, charcoal briquettes aren't actual charcoal, but a combination of charcoal and other ingredients molded into easy-to-light lumps. Kingsford Charcoal, for example, by far the most popular brand in the US, is made up of bits of charcoal, coal, starch as a binder , sawdust, and sodium nitrate to make it burn better. For the same reason that SPAM is cheaper than a whole ham, briquettes are cheaper to make than all-wood charcoal. And charcoal briquettes are nothing to sneer at--competition grillers have won national contests with them.
There are, however, three things that separate them from hardwood lump charcoal. First, briquettes are more consistent in their burn, since they're all of uniform size. Second, briquettes have no distinct flavor of their own.
Third, and maybe most importantly, all the binders and additives in the briquettes make for a much ashier burn. What this means for cooking is that the briquettes tend to top out at a lower heat than hardwood lump, as the ash acts to slightly suffocate and insulate the coals. The volume of ash that briquettes produces also means that you can't use them effectively in ceramic grills like the Big Green Egg. If you're using a Weber kettle, or a similar brand with a ton of extra room for ash, briquettes work just fine.
That said, avoid the briquettes that come pre-coated with lighter fluid. Iit's not rocket science to get a load of coals lit, and you don't need those kinds of fumes messing with your food. Hardwood lump charcoal is just charcoal made from chunks of hardwood, and there's a ton of variation within the category here are our favorite brands. With no fillers to burn down, hardwood lump produces a lot less ash than charcoal briquettes, and by weight, it burns for about 20 minutes longer.
Measured by the chimney-full, though, which is a more convenient measure unless you're busting out your scale each time you grill , hardwood lump burns faster than briquettes, since the irregular shape of the lump charcoal makes for less efficient packing in the chimney. Over time, the temperature and pressures the organic matter is exposed to will transform it into coal. Wood tends to form coal based on its high levels of carbon and relatively low levels of oxygen that get trapped underground with the wood.
However, any organic matter can form any hydrocarbon based on a myriad of factors, including the environment and surrounding material. Coal formed underground varies in its quality, which is based on the carbon content. The higher the carbon content, the more valuable the coal.
In our grill we will not be using natural coal, as it is better suited to industrial purposes. Charcoal used for grills is a specific type of man-made coal that is produce by heating wood in a low oxygen environment.
The result is charred wood, which is what we call charcoal. This combustion reaction for wood is shown below. Coal is formed by natural earth processes just like petroleum and natural gas.
The char is almost completely made up of carbon.
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