Compounds and Mixtures

1. A compound is a substance made up of two or more elements chemically combined together. A compound may be formed in any of the following ways.
   (i) (element) + (element) --> compound
       (eg) Sodium + Chlorine --> sodium chloride
   
   (ii) (element) + (compound) --> compound
        (eg) oxygen + carbon monoxide --> carbon dioxide
   
   (iii) compound + compound --> compound
        (eg) carbon dioxide + water --> carbonic acid
   
2. Properties(i) The properties of the compound are different from the properties of its constituent    elements. (Eg) Water (compound) is made of hydrogen and oxygen. When hydrogen catches fire easily, oxygen helps in burning, the compound water made of the two is used to put out fire.
   
   (ii) Compounds can be broken down into simpler substances only by chemical methods.   Water is a compound and it can be spilt into H2 and O2 through chemical process called electrolysis.
   
   (iii) The proportion of the elements in a compound are fixed.
         In NaCl, Na and Cl are in the ratio 23 : 35.5 by mass.
   
3. MixturesA mixture consists of two or more substances not chemically combined. Constituents of a mixture may be all elements, all compounds or combination of elements and compounds. These substances could be in the solid, liquid or gaseous states.
   
4. Properties of a mixture(i) No chemical change occurs when a mixture is formed.     When we mix salt and water, we get saltwater a mixture. No heat or light is given off in this
        process.
   
   (ii) The properties of a mixture are the properties of the substances with which it is made.
         The saltwater can also be used to put out fire like water.
   
   (iii) A mixture can be separated easily by physical methods.      The salt and water can be separated easily from saltwater by evaporation.
   
   (iv) The proportion of the constituents in a mixture are not fixed.       There is no fixed ratio in which they should be mixed. The proportion may change. We can add
          1tsp of salt to 200ml water or 2tsp of salt to 100ml water, the result is saltwater.