In which of the following cases can we be CERTAIN that a chemical change has taken place?
A)when a solid P is heated in a beaker for some time, it changes into a liquid.
B)when a liquid Q is taken in a beaker and powder R added to it and stirred, R dissolves in Q.
C)_when two liquids S and T are mixed, the beaker gets hot and a new liquid U is formed.
D)when a white powder V and black powder W are mixed, powder X with black and white particles is got.
A)when a solid P is heated in a beaker for some time, it changes into a liquid.
B)when a liquid Q is taken in a beaker and powder R added to it and stirred, R dissolves in Q.
C)_when two liquids S and T are mixed, the beaker gets hot and a new liquid U is formed.
D)when a white powder V and black powder W are mixed, powder X with black and white particles is got.
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option B on a broader range would include dissolution which is not a chemical change...
option A can include melting which also is not a chemical change
option D just includes mixing... (yeah u guessed it right!) which is also not a chemical change
On the other hand the release of heat is a major giveaway that a chemical reaction is taking place...
By the release of heat we can conclude that the substances that are mixed had a higher energy before the mixing and now they r achieving a more stable state thus releasing energy..... and the release of energy of a system to form a more stable component can be considered as a chemical reaction taking place.
Hope this helps