Mixtures and pure substances – Pure substances are matter made up of only one type of atom or molecule – Mixtures are a collection of 2 or more pure substances physically mixed together that cannot be represented by a chemical formula. – A mixtures properties may vary, because the proportions of the substances in them may vary. Gold for example is too soft to maintain it’s shape in jewelry so copper silver, or nickel is usually added to create more hardness in the god this solid mixture is known as an alloy. – Pure substances have specific chemical and physical properties. Pure substances are often hard to find so if a substance is mixed with small amounts of impurities the substance is still considered to be pure.
– Mixtures may be classified as either homogeneous or heterogeneous. Homogeneous mixtures are ones where substances are uniformly distributed throughout. Gas, syrup, and saltwater are homogeneous mixtures. Heterogeneous mixtures contain substances which are not evenly distributed throughout. A salad, cereal, and Snickers bars are heterogeneous mixtures. – Mixtures contain phases in which part of a system has uniform composition and properties.
Elements – Elements are the simplest pure substances, since they contain only one kind of atom. – Every element has a set of chemical and physical properties, purified gold discovered today cannot be distinguished from gold found years ago because pure gold from any source is the same substance, an un reactive soft metal which melts at 1064 C. – There are over 109 elements but only about 12 make up things we see everyday. Hydrogen is the most common of all it makes up more than 90% of the atoms of the known universe. Oxygen and silicon make up over 70% of the Earth’s crust – Living things a mainly made up of four elements, those being carbon, oxygen, hydrogen, and nitrogen, these combine to create the molecules needed for life. – Allotropes are different molecular forms of an element in the same physical state.
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Oxygen is an allotrope, ozone and diatomic oxygen are allotropes of oxygen. Allotropes may have properties that vary widely, diatomic oxygen is colorless, odorless, and essential to life, whereas ozone is blue, toxic, and have a very sharp odor. Compounds – Compounds are pure substances composed of two or more different elements. – Molecular compounds are composed of molecules.
Ionic compounds such as table salt, are made of at least 2 different elements that are chemically joined, but the atoms are not made of discrete molecules but instead are distributed throughout a continuous network. – Compounds have unique sets of properties, these properties may vary greatly from those of the elements used to create them. – There are 3 major differences between compounds and mixtures. 1. Mixtures are never made of only a single compound. 2.
A mixtures properties reflect the substances used to create it, whereas a compound often bears no resemblance to the properties of the elements used to make it. 3. Compounds have a definite composition by mass of their combining elements. Pyrite for example always exists with a mass of 46. 55% iron 53. 45% sulfur.
While substances exist in any mass ratio. Methods of separating mixtures – Filtering, evaporating, centrifuging, or decanting a mixture brings about only physical changes. The substances gained from these processes are not chemically changed. – Chromatography can be used to separate the components of a solution so they may be identified.
It works because different substances are attracted to different solvents and other media. – Distillation is a method of separating substances that have different boiling points – In some spots around the world, people use distillation to obtain drinking water from sea water. But this process requires much energy and is very expensive, so other ways of getting drinking water are preferred. Distillation is used in the petroleum industry to separate crude oil. The separated components of oil have many uses, gas, heating oil, lubricants, and materials from which plastics are made all come from crude oil being distilled. – A relationship between 2 quantities can be shown on a graph.
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– Density is the measure of how tightly matter is packed. Osmium is the densest substance on the earth a piece the size of a football is too heavy to lift. – Density may be used to identify substances. Ice floats in water because it has a lower density than water.