Changes Around Us: Physical and Chemical - Are All Changes Desirable?

Physical Change: A change in which a substance undergoes a change in its physical properties like shape, size, color, or state. It is generally reversible, and no new substance is formed. Example: Melting of ice ($H_2O(s) \rightarrow H_2O(l)$).

Chemical Change: A change in which one or more new substances are formed. It is often irreversible and accompanied by heat/light release, gas production, or color change. Example: Burning of paper.

Rusting: When iron objects are exposed to oxygen and moisture (water vapor), they form a flaky brown substance called rust ($Fe_2O_3 \cdot xH_2O$).

Galvanization: The process of depositing a layer of zinc ($Zn$) on iron to prevent it from rusting.

Crystallization: A physical process used to obtain large crystals of pure substances from their solutions. Example: Obtaining pure Copper Sulphate ($CuSO_4$) crystals.

Reaction of Vinegar and Baking Soda: Mixing vinegar (acetic acid) and baking soda (sodium hydrogen carbonate) produces Carbon dioxide ($CO_2$), which turns lime water milky due to the formation of Calcium Carbonate ($CaCO_3$).

Reaction of Iron with Copper Sulphate: When an iron nail is placed in blue Copper Sulphate solution, it turns green due to the formation of Iron Sulphate ($FeSO_4$) and a brown deposit of Copper ($Cu$) is seen.

Desirability of Changes: Changes can be classified as desirable (e.g., ripening of fruits, cooking of food) or undesirable (e.g., rusting of iron, spoiling of food) based on their utility to humans.