Distinguish between physical and chemical changes. Differentiate between reversible changes and irreversible changes. A piece of iron is heated till it becomes red hot. It then becomes soft and is beaten to a desired shape. What kind of change is observed in this process?

here are the answers to the questions:

1. Distinguish between physical and chemical changes.

   • A physical change alters the form or appearance of a substance but does not change its chemical composition. No new substances are formed, and these changes are often reversible.
   • A chemical change results in the formation of one or more new substances with different chemical properties from the original substances. These changes are generally irreversible.

2. Differentiate between reversible changes and irreversible changes.

   • Reversible changes are those that can be undone, meaning the original substance can be recovered. Examples include melting ice or dissolving salt in water.
   • Irreversible changes are those that cannot be undone, and the original substance cannot be recovered. Examples include burning wood or cooking an egg.

3. A piece of iron is heated till it becomes red hot. It then becomes soft and is beaten to a desired shape. What kind of change is observed in this process?

   • This process involves a physical change. Heating iron to make it soft and then shaping it changes its physical state and form, but the iron itself remains chemically the same. No new substance is formed.

4. Give one example in each case:

   • i) Change which occurs on heating but can be reversed:
     • Melting of wax (wax can be solidified again by cooling).
   • ii) Change which occurs on heating but cannot be reversed:
     • Cooking an egg (the proteins in the egg undergo irreversible chemical changes when heated).

5. Explain the type of changes involved in this sequence. Ice cubes placed in sun ---> puddle of water ---> disappearance of puddle

   • Ice cubes placed in sun $\rightarrow$ puddle of water: This is a physical change called melting. The ice (solid water) changes its state to liquid water, but its chemical composition remains $\text{H}_2\text{O}$.
   • Puddle of water $\rightarrow$ disappearance of puddle: This is also a physical change called evaporation. The liquid water changes into water vapor (gaseous water) and mixes with the air, but it is still $\text{H}_2\text{O}$.

6. Differentiate between desirable and undesirable changes.

   • Desirable changes are those that are useful or beneficial to us. For example, cooking food, ripening of fruits, or the growth of plants.
   • Undesirable changes are those that are harmful or not beneficial to us. For example, rusting of iron, spoilage of food, or forest fires.