Physical and Chemical Changes

Reactivity series

Ncert Class 7:

Physical changes:

• properties such as shape, size, colour, state of substance are called physical properties
• change in physical properties is called physical change
• generally reversible
• no new substance is formed

Examples:

• dissolve sugar into water
• stretched rubber band
• cutting a piece of paper
• crush a small piece of chalk into dust
• ice in glass tumblr
• boil water
• heat a hack saw blade - colour changes
• crystallisation

Chemical changes

• change in which one or more new substances are formed is called chemical change
• the reaction is known as chemical reaction
• with addition to new products,
  • heat
  • light
  • radiation
  • sound
  • change in smell
  • colour change
  • gas
• Burning of any substance is chemical change and it is accompanied by production of heat
  • bursting of crackers

Examples:

• curd from milk
• rusting of iron - brownish substance is formed
• clean a strip of magnesium ribbon with sand paper and burn it - it burns into a brilliant white light - powdery ash is formed
  • Mg + O2 –> MgO (Magnesium oxide)
  • MgO + water –> test with blue and red litmus paper - classify as acidic (red) or basic (blue)
  • MgO + H2O –> Mg(OH)2 (Magnesium hydroxide) - is a base
• Copper sulphate (blue vitriol or neela thotha) + water + dil sulphuric acid –> blue coloured solution
  • drop a nail in the solution and observe the colour
  • blue changes into green
  • CuSO4 (blue) + Fe –> FeSO4 (green) + Cu (brown)
• vinegar + baking soda – hear a hissing sound and bubbles of gas
  • vinegar(acetic acid CH3COOH) + Baking soda (sodium hydrogen carbonate NaHCOOH) –> CO2 (gas) + other substances
  • pass this gas through limewater (freshly prepared)
  • CO2 + Lime water(Ca(OH)2) –> Calcium carbonate (CaCO3) + H2O
  • makes limewater milky
  • it is the standard test for CO2
• metal extracted from its ore
  • iron from iron ore
• medicine, plastics, detergents

Rusting of iron:

Fe + O2 + H2O –> Fe2O3 (ferrous oxide)

• more humid, faster the rusting

To prevent rusting:

• coating of paint or greese is applied
• deposit a layer of metal like chromium or zinc (galvanisation) on iron
• Stainless steel - mixing of metals like chromium, nickel, manganese with iron and carbon –rust free

Crystallisation:

• salt from sea by evaporation
  • not pure and very small in size
• process:
  • mix water and drops of dil sulphuric acid
  • heat it
  • when boiling starts add copper sulphate powder till no more powder can be added (supersaturation) and stir it
  • filter the solution (blue solution)
  • cool for sometime
  • crystals of copper sulphate is formed

"CuO (s) + H2​SO4​ (aq) → CuSO4​ (aq) + H2​O (l)"

Arjun - Class 7: