CHEMISTRY              COMPOUNDS: Compounds are substances that are formed by two or more elements bonded chemically in a fixed proportion by mass. Their properties are different from that of their constituents. But compounds should not be mistaken with mixtures as the constituents of mixtures can be separated by mechanical means but the constituents of compounds cannot be separated easily. Also, the mixtures are made mechanically but all compounds are made chemically only. All compounds tend to decompose on the application of heat and the temperature at which they decompose is called the decomposition temperature. Also, all compounds can exist in a solid state. Simple compounds are also used for chemical synthesis for the production of some complex compounds.     The following are the properties of impure substances:   • They can usually be separated back to their original constituents   • They are formed by the mechanical bonding.   • They are usually formed by the combination of two or more elements or compounds.   • They retain the chemical properties of their constituents.   • Their melting points usually differ from that of their constituents   • They are separated by mechanical means.     HOMOGENEOUS MIXTURES: Homogeneous mixtures are those which have a uniform composition and similar properties at all points. They are uniform in appearance and have only one phase i.e. they are either all solid or liquid or gas. However, there are differences while studying them very minutely. But most of the pure substances are considered homogeneous for practical purposes.     HETEROGENEOUS MIXTURES: These are mixtures which are non uniform in composition and appearance i.e. different portions/samples from the same mixture show different composition. They are composed of two or more substances which retain their own identifying properties. Also, they may have mixed phases.