Pure and impure substances

• Pure describes natural substances that have not been processed or changed. A pure substance contains 1 element or compound
• Impure substances contain more than 1 element or compounds. This means they are a mixture.
• It is difficult to obtain pure substances. Just 1 or more atom of something different makes a substance impure
• Even air is impure as it is a mixture. It contains a mix of nitrogen, CO², H²O, oxygen and other noble gases
• Many useful mixtures are many different substances often chosen because of the desirable properties.
• An alloy is a mixture of metals with1 or more other elements. Most of the time metals that are used are alloys.
• Pure gold is very soft so gold used in jewellery is an alloy of gold and copper
• You can tell whether a substance is pure or impure by whether is has a fixed melting point or melts at a range of temperatures.
• If a substance is impure, its mealting point will be less than that of a pure substance and will melt over a range of temperatures (not at 1 specific point)
• The greater the range between the measured temperatures of the melting substance, the lower its purity is.
• You determine the melting point of substances by heating it. You either measure the temperature it melts at or its temperature at regular tiem intervals
• When trying to find out the melting point of a substance, it is important to heat the substance slowly and stir the substance as it melts
• Heating a substance slowly allows the temperature of a whole sample to increase.
• Mixing a sample ensures that the entire sample is at the same temperature