Answer:
• Distinguish between pure substances and mixtures
• Identify mixtures as homogeneous or heterogeneous
Explanation:
Pure Substances
When we speak of a pure substance, we are speaking of something that contains only one kind of matter. This can either be one single element or one single compound, but every sample of this substance that you examine must contain exactly the same thing with a fixed, definite set of properties.
Lead (Pb)
element
lead atoms
Oxygen gas (O2)
element
oxygen molecules*
Water (H2O)
compound
water molecules
Ammonia (NH3)
compound
ammonia molecules
*Note: pure oxygen gas consists of molecules but it is still considered an element, rather than a compound, as the molecules are made up of a single type of element. Compounds are made up of one or more element.
Mixtures
If we take two or more pure substances and mix them together, we refer to this as a mixture. Mixtures can always be separated again into component pure substances, because bonding among the atoms of the constituent substances does not occur in a mixture. Whereas acompound may have very different properties from the elements that compose it, in mixtures the substances keep their individual properties. For example sodium is a soft shiny metal and chlorine is a pungent green gas. These two elements can combine to form the compound, sodium chloride (table salt) which is a white, crystalline solid having none of the properties of either sodium or chlorine. If, however, you mixed table salt with ground pepper, you would still be able to see the individual grains of each of them and, if you were patient, you could take tweezers and carefully separate them back into pure salt and pure pepper.
