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Why do we say grams per liter?Everything on earth has a density. And we measure density in grams per liter or other units of mass per volume.
These examples show you a bit about density, also called specific gravity. Here are some more. Wood varies from about 200 to over 1000 grams per liter. If it's less than a thousand, it floats in water. Ice is usually 920 grams per litre. It floats. Metals are very heavy. They vary from 2700 for aluminum to over 19,000 for gold, with average metals such as copper and steel in the 8000 to 10000 range. Rocks are quite dense too, with most coming in around 2500 to 3000 g/litre. Same goes for glass, which is made from rocks. The density for most things depend a bit on its temperature, as most things expand when heated without changing its mass. For gases, density changes very much with the pressure. Basic thermodynamics says that compressing a gas into half of its original volume will double its density. It will also increase in temperature and pressure. By how much? Well, that's harder to calculate than the grams per liter. And the density gasoline has? This stuff is a mixture of over 100 hydrocarbons and minerals, each with its own density. Every tank could have its own mix of these things, making the density different every time. Most petrol, as it is sometimes called, ranges from 700 to 800 grams per liter. Now what about concentration?
Concentrations are really densities, or is density really a concentration. Both are ways of expressing the amount of something spread out within a certain volume. But in this case, the rest of that volume is filled with a solvent, some other substance such as air molecules. We use grams per liter to indicate both concentration and density at times. If you were to take your sugar cube, three grams, and dissolve it in one litre of water by mixing it really well, you end up with a sugar concentration of three grams per liter. You should be able to taste that. How does concentration differ from density. This concept of concentration implies that the sugar is distributed quite evenly thoughout the entire liter. That depends on some degree on the solvent used. The water in this case. It needs to hold the tiny sugar particles off the ground, the bottom or the container, and air might not do that so well. Sugar powder would slowly settle. In air quality modelling, we do express concentrations of dust, which is little bits of solids suspended in the air, in grams per liter. Even though they eventually settle. Or we use similar, smaller units such as micrograms per cubic meter. We call airborne dust particulate matter, total suspended particulates (TSP) or just particulates. Concentrations are also given in terms of a fraction of the total mass or volume of the solvent (air or water) in a space. So you sometimes hear percent (%), parts per million (ppm) or even parts per billion (ppb). The concentration tells you how strong a vapour is, what the humidity, the density of water vapor, is outside or how salty the ocean is. It is what helps us make recipies with the proper proportions of sugar, flour and whatnot, determine how strong an alcoholic drink is and measure whether fumes have become deadly poisonous or explosive. The SI, Système international unit for density is kilogram per cubic metre, which happens to be equal to grams per liter. This system was invented by the French, which is why the official spelling is litre. The same goes for metre.
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