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• Physical changes are generally reversible and the change is reversed by reversing the conditions. I.e. it is possible to regain the shape, size, color or state of the substance
• A change in which new substances are formed is called a chemical change. A chemical change is called chemical reaction.
• Difference between physical change and chemical change
  

  Physical Change	Chemical change
  Changes can be reversed	Changes cannot be reversed
  New product are not formed	New products are formed
  No energy is absorbed or released	Energy is absorbed or released
  E.g. melting of ice/td>	E.g. Rusting of iron

• Corrosion – The gradual destruction of metal due to the effect of its surrounding environment is called corrosion. Most observed corrosion of metals are iron, silver, aluminium, copper, etc
• Prevention of corrosion – Corrosion can be prevented if the contact between metal and the air is cut off. e.g. coating metals with paint, Oil, grease or varnish prevents the corrosion temporarily.
• Some methods of prevention from corrosion are – galvanising, tinning, electroplating, anodising, alloying
• Galvanising – The process of forming a thin coating of zinc on iron or steel is called galvanising. e.g. shiny iron nails, pins etc.
• Tinning – The process of giving a tin coating to other metal is called tinning. Cooking vessels which are made of copper and brass are given a coating of tin to prevent corrosion.
• Electroplating – Electroplating is a method in which coating is formed on other metal by using Electrolysis. E.g. Silver plated spoons, Gold plated jewellery etc.
• Anodising: In this method metals such as copper and aluminium are coated with thin strong layer of their Oxide.
• Alloying: An alloy is an homogeneous mixture of two or more metals or metal and non-metal in Specific Proportion. Alloy did not Corrode easily. e.g. Brass is made of copper and zinc.
• Crystallization – The solidification of matter, forming a definite geometric shape is called crystallization. The substances which are formed by crystallization are crystalline substances. Examples are table salt, sugar, copper sulphate, diamond etc.
  • Take water and add few drops of sulphuric acid
  • Start boiling water, and go on adding small amount of sulphuric acid
  • At a point you will get saturated solution of copper sulphate and water
  • Filter the solution to separate copper sulphate from water
  • Allow it to cool for a while
  • You will get crystals of copper sulphate solution