Theory:

Expansion and Contraction:
Solids, liquids and gases are affected by heat. When we heat, they start to expand, and they start to contract when we cool them. This principle is used to measure the temperature. The thermometer is used to measure the temperature.
 
In this exercise, an activity is explained to understand how expansion and contraction happens.
 
Materials required:
  • Big bottle
  • Balloon
  • Threads
  • Candle
  • Water
  • Fork
Procedure:
  • Take one big bottle and fill some water in it.
  • Attach one balloon to the mouth of the bottle and fix it with thread.
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  • Hold the bottle with a fork. Heat the bottle with a candle and take an observation.
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Important!
What change occurs in the state of the balloon after heating the bottle?
  
Answer:  When the bottle is heated, the air molecules inside start moving faster. These molecules now collide into the balloon with more energy resulting in increased pressure. This results in the air inside the balloon getting heated and expanded. This increase in temperature causes the balloon to stretch and expand.
  • Now, let the bottle get cooled down.
Important!
What change occurs in the state of the balloon after the bottle gets cool down?
 
Answer: The temperature within the bottle reduces, causing the air to cool down. This reduction in temperature causes the balloon to contract.
Result to understand:
When the substance (solid, liquid, or gas) gets heated, it expands; when the substance cools down, it contracts.