Physical and Chemical Changes, solution

Physical changes affect the form of a chemical substance, but not its chemical composition. This fact contrasts with the concept of chemical change in which the chemical composition of the substance changes or one or more substances combines or break up to form new substances. In general, a physical change is using physical means. For example, sugar dissolved in water can be recovered by allowing the water to evaporate.

Change of water into the vapour is 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.

A chemical change is a change in which something new is formed (irreversible). The starting materials change into an entirely different substance or substances. This new substance has a different chemical composition than the starting materials. Examples of chemical change would be the reaction of iron with air (rusting) or the reaction of a metal and acid. The crystallization of sodium chloride and melting of ice are examples of physical change while souring of milk is an example of 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.

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.