Physical and Chemical Changes, solution

(a) Blackening of silverware: As discussed previously, this is the formation of silver sulfide ($Ag₂S$) due to the reaction of silver ($Ag$) with sulfur compounds. This is a chemical change because new substances are formed.

(b) Burning of Candle: When a candle burns, the wax melts (a physical change), but the wick burns, and the wax undergoes combustion with oxygen in the air, producing carbon dioxide, water vapor, and soot. This is a chemical change as new substances are formed.

(c) Making of curd from milk: This process involves the fermentation of lactose (milk sugar) into lactic acid by bacteria. This changes the proteins in milk, leading to the solidification. This is a chemical change because new substances are formed.

(d) Dissolving of sugar in water: When sugar dissolves in water, the sugar molecules disperse evenly throughout the water. The sugar molecules themselves do not change their chemical composition; they are still sugar. If you evaporate the water, you will get the sugar back. This is a physical change.

Change of water into the vapour is a physical change. The change of water into vapor is a process where the physical state of water changes from liquid to gas. In this process, the chemical composition of water (H​₂O) remains the same; no new substance is formed. This is a characteristic of a physical change.

A change which alters the specific properties of a material by bringing about a change in its molecular composition, followed by a change in state, is called a chemical change. The chemical change is a change in which something new is formed. Cooking of vegetables is a chemical change as it brings irreversible changes in the composition of vegetables.

Crystallization of sodium chloride: This is the process where sodium chloride (salt) forms solid crystals from a solution. The chemical composition of sodium chloride remains the same throughout this process. It's a separation technique based on solubility and phase change. This is a physical change.

Melting of ice: Ice is solid water, and when it melts, it becomes liquid water. The chemical composition of water (H₂​O) does not change. This is a change of state. This is a physical change.

Souring of milk: As discussed earlier, souring of milk is caused by bacteria converting lactose (milk sugar) into lactic acid. This process forms new chemical compounds (lactic acid) and alters the properties of the milk. This is a chemical change.

Ripening of fruits, setting of cement, and burning of coal—all of these are chemical changes and all are also exothermic. So there is no difference. Hence, option (c) is the correct answer.

A chemical reaction generally becomes faster at a higher temperature because at higher temperature, molecular motion becomes more rapid and the molecules have higher kinetic energy. The process of a chemical reaction depends on the concentration, temperature, and physical state of molecules and their nature. Normally, higher the temperature of molecules, faster would be the reaction.

The "decomposition of water" most commonly refers to electrolysis of water, where water is split into hydrogen gas ($H₂$) and oxygen gas (O₂​). This process requires an input of energy.

The form of energy that is used to cause the decomposition of water is electrical energy.

In order to pasteurize milk, first, the milk is heated up to a longer time and then cooled suddenly within specified time. Pasteurization is a process invented by French scientist Louis Pasteur during the nineteenth century. In 1864, Louis Pasteur discovered that heating beer and wine was enough to kill most of the bacteria that caused spoilage and hence, prevented these beverages from turning sour. This was achieved by eliminating pathogenic microbes and lowering microbial numbers to prolong the quality of the beverage.

Pasteurization is the partial sterilization of a product, such as milk, wine or fruit juices to make it safe for consumption and improve its keeping quality. Pasteurization was developed by Louis Pasteur in 1864. It is not used for tea. Other pairs are correctly matched.