Elements, Compounds and Mixtures

• Impure substances contain other substances also in trace amounts.
• The substances may be pure or impure, homogenous or heterogenous
• Substances which contain particles of only one kind are called pure substances.
• Pure substances are of 2 types - Elements and Compounds
• Elements are made up of only one kind of atom.
• Compounds are made up of only kind of molecules.
• The substances that do not have uniform composition are called heterogenous substances.
• An element is a substance that cannot be broken into simpler substances by any physical or chemical means.
• There are 118 elements known today.
• 92 elements out of these are naturally occurring and the remaining 26 elements are made artificially.
• An atom is the smallest unit of an element that shows all the properties of that element.
• A molecule represents the smallest unit in which a pure substance can be divided without losing its basic nature or identity.
• Sulphur

  
• A physical combination of two or more elements or compounds in any proportion is called a mixture.
• In a mixture, original atoms and molecules remain as they are because no chemical change is taking place.
• Mixtures are impure substances because their constituents have different kinds of molecules.
• The constituents in a mixture may be present in any ratio.
• The constituents of a mixture exhibit their own properties. Thus, the mixture has no properties of its own.
• There is no fixed melting or boiling point of a mixture.
• A mixture can be separated into its constituents by simple physical or mechanical means.
• A mixture in which the components cannot be seen separately and are uniformly distributes throughout its volume is called homogenous mixture.
• A mixture in which its components can be easily seen and are not uniformly distributed is called a heterogenous mixture.
• The process of separating the components of a mixture is called separation of mixtures.
• The principles of separation depends upon the type of mixture and the properties of components of mixture.
• Sieving helps to separate components of a mixture that differ in size.
• When a mixture of insoluble substance and water is kept undisturbed for some time, then the suspended insoluble particles settle down at the bottom under the influence of gravity. This process is called sedimentation.
• The insoluble solid substance that settles down is called the sediment.
• The clear liquid over the sediment is called supernatant liquid.
• The clear liquid is then poured into another beaker, leaving the sediments undisturbed. This process is called sedimentation.
• The components of an insoluble liquid and liquid cannot be completely separated.
• The components of a solid which is lighter than liquid do not settle, hence, they cannot be separated.
• Filtration is a method used to separate an insoluble solid from a liquid.