Grade 7, Science Olympiad (CBSE) - Chemical changes 

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Grade 7  |   Science  |   Chemical changes, Physical and Chemical changes, Olympiad, CBSE, ICSE, SOF, ITO

Chemical changes
A change where the original substance changes into a new substance are called the chemical change. We consider chemical changes as irreversible changes. In chemical change the properties of the substance changes. For example- Souring of milk, curding of milk, burning of curd, rusting of iron etc.
Characteristics of chemical changes 

1. In a chemical change exchange of energy takes place.

2. It can be represented by a chemical equation.

3. A chemical change also involves a chemical reaction. A chemical reaction may also be accompanied by a change in color, production of sound, formation of gas etc.

4. A new substance is formed as the end product of the reaction. 

Examples of chemical changes
  • When a pinch of baking soda is added to vinegar, production of a hissing sound is accompanied with the formation of the bubbles of gas. This has is carbon dioxide. 
  • If we want to confirm it whether the gas is carbon dioxide or not, pass the gas through the lime water and observe the solution. If this lime water turns milky it means the gas evolved is carbon dioxide. 
  • Burning of magnesium ribbon is accompanied with a brilliant white light, leaving powdery ash behind. Equation supporting this reaction has been shown below.

Mg+ O—› MgO ( powdery ash)

When this magnesium oxide is reacted with water it gives Magnesium hydroxide as the end product. 

MgO+ H2O-—> [Mg(OH)2]

  • Spoilage of food is a chemical change. We have noticed the change in color  if the apple is cut with the knife after some time. The slice appears brown on color which changes the taste of the apple slice. This happens because of the reaction occuring between an enzyme tyrosinase and oxygen in the air. 
  • The neutralization reaction between an acid and base is also a chemical change. 
  • Photosynthesis is also a chemical change as carbohydrate is formed as the product of the reaction between carbon di oxide an water. 

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