Which one of the following is the Physical and Chemical Change?
Baking of a chapatti Cutting of cloth Heating of a needle Burning of paper
In your previous classes you have already learnt about physical and chemical changes of matter. Pictures 1 and 4 show physical changes, because there is no change in the nature and the identity of their initial substance .However, pictures 2 and 3 show changes in their initial substances, hence are chemical changes. Whenever a chemical change occurs, we say that a chemical reaction has taken place. You may perhaps be wondering as to what is actually meant by chemical change. How do we come to know that a chemical reaction has taken place? Let us see some activities to find the answer to these questions.
What have you observed in this video?
You might have observed that magnesium ribbon burns with a dazzling white flame and changes into a white powder. This powder is magnesium oxide, a new substance. It is formed due to the reaction between magnesium and oxygen present in the air.
Did you observe any change in colour after mixing lead nitrate and potassium iodide solution?
On mixing lead nitrate and potassium iodide solution, new products lead iodide and potassium nitrate are formed.
Did you observe anything happening around the zinc granules?
You might have observed bubbles of gas around zinc granules. This shows evolution of gas taking place in this reaction. In this reaction the conical flask also gets warm, this means that there is change in temperature also.
From the above three activities, we can say that usually any of the following observations help us to determine whether a chemical reaction has taken place –
change in state
change in colour
evolution of a gas
change in temperature.