Chapter Review:

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  There are many changes around us which is continuous process.
• Some changes occur naturally and some changes are made by us. 
• Some such changes are: Raw egg to boiled egg, milk to curd, flour to bread, bud to flower, cow dung to biogas, rusting of iron, freezing water into ice, burning of coal, woods into furnitures, fibres into fabric, Changing of season, changing shape of moon etc.
• Some changes are reversible and some are non-reversible. 
• A change may occur by heating a substance or by mixing it with some
  other.
• There are different types of changes, i.e physical changes, chemical changes, reversible, non-reversible periodic changes and aperiodic changes. 
• The changes which can be reversed to their original forms are called reversible changes. Like water changes into ice, 
• The changes which can not come into their original forms are called non-reversible or irreversible changes. Like changing milk into curd, rusting of iron, burning of coal or papers etc. 
• Changes which are repeated after regular intervals of time are called periodic changes. like (i) Rising of sun. (ii) Motion of pendulum. (iii) Occurring of seasons one after the other.
• There are changes that do not take place regularly. These are called aperiodic change.
• When an object is heated it becomes larger in size and on cooling it contracts this is known as contraction. 
• When an object expands or melts on heating this is known as expansion. 
• Changing of water into gas on heating is called evaporation.