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Reversible Changes:

1. Melting of Ice: When ice melts into water upon heating, it undergoes a reversible change. Upon cooling, water can freeze back into ice.
2. Dissolution of Sugar in Water: When sugar dissolves in water, it forms a solution. This process is reversible, as upon evaporation of water, sugar can be obtained back in its crystalline form.

Irreversible Changes:

1. Burning of Paper: When paper is burned, it undergoes an irreversible change. Once the paper is converted into ash and gases, it cannot be restored to its original state.
2. Cooking an Egg: When an egg is cooked, the proteins denature and solidify, resulting in irreversible changes in texture and structure. Once cooked, the egg cannot revert back to its raw state.

Burning of a piece of paper is an irreversible change. Once the paper undergoes combustion, it transforms into ash and gases, and it cannot be restored to its original form.

Sure, let's classify each of these:

(i) Growth of a plant: Reversible. The growth of a plant involves processes like cell division, elongation, and differentiation, which are reversible to some extent. For example, if a plant experiences stunted growth due to unfavorable conditions, it may resume growth when conditions improve.

(ii) Ploughing a field: Reversible. Ploughing involves turning over and breaking up the soil to prepare it for planting. While the act of ploughing itself is reversible (you can plough a field multiple times), the specific arrangement of soil particles and the disruption caused by ploughing may not be exactly reversible in the short term, but it's reversible in the sense that the field can be ploughed again to return it to a similar state as before.

Curd setting is a process that involves the coagulation of milk proteins, primarily casein, by the action of lactic acid bacteria. Here's how it typically occurs:

1. Milk is heated to a specific temperature (usually around 40-45°C or 104-113°F).
2. A small amount of curd (containing live bacterial culture) or a coagulating agent like lemon juice or vinegar is added to the warm milk.
3. The bacteria present in the curd or the added culture begin to ferment lactose (milk sugar) into lactic acid.
4. The increase in acidity caused by the lactic acid lowers the pH of the milk, which causes the casein proteins to denature and coagulate, forming a gel-like substance, which is curd.

Is this change reversible? The process of curd setting is generally considered irreversible. Once the proteins in the milk have coagulated and formed curd due to the action of lactic acid, it is not possible to return the curd to its original liquid milk state. However, you can further process curd to obtain other dairy products like cheese or whey, but the original curd itself cannot be reversed back into milk.