Q: Which of the following is/are the example/examples of chemical change?
a) Crystallization of sodium chloride
b) Melting of ice
c) Souring of milk
The correct answer is 3 only.
Chemical change
When one chemical substance is transformed into one or more different substances, it is called a chemical change.
The most common example is when iron transforms into rust.
Crystallization of sodium chloride
When the solubility limit of NaCl exceeds, the crystallization of NaCl takes place.
It is a physical change since sodium chloride does not transform into some other chemical element after crystallization. Hence, Statement 1 is not correct.
Melting of ice
Ice is the solid form that liquid water takes when it is cooled below 0 degrees Celsius.
When the temperature rises, solid ice changes into liquid water.
Melting of Ice is not a chemical change since the composition of water molecules stays the same before and after melting. Hence, Statement 2 is not correct.
Souring of milk
Lactic acid bacteria ferment lactose at room temperature and turn it into lactic acid.
As a result, the acidity of the milk increases, which in its turn causes the protein to fold and liquid to separate into whey and a viscous mass.
The soured milk has new elements and it cannot be reversed. Therefore, souring of milk is a chemical change. Hence, Statement 3 is correct.
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