Science

Crystallization of sodium chloride and melting of ice are physical changes as they involve a change of state. Souring of milk is a chemical change because it results in the formation of new substances.

Source: 7th NCERT: Science Chapter

Options 1 and 2 are correct: Microbes like bacteria and many fungi can be grown on nutritive media to form colonies that can be seen with the naked eyes. Such cultures are useful in studies on micro-organisms.

Option 3 is not correct: Unlike bacteria, many of which can be grown on an artificial nutrient medium, viruses require a living host cell for replication. Infected host cells (eukaryotic or prokaryotic) can be cultured and grown, and then the growth medium can be harvested as a source of virus.

Source: NCERT: Chapter Microbes in Human Welfare

Crystallization of sodium chloride and melting of ice are physical changes as they involve a change of state. Souring of milk is a chemical change because it results in the formation of new substances.

Source: 7th NCERT: Science Chapter

Option 1 is right because viruses do not have the chemical machinery needed to survive on their own.

Option 2 is wrong because they seek out host cells in which they can multiply. Viruses can't be grown on synthetic media (e.g. agar [a type of gelatin]) but they can be grown on cultures of bacteria. Bacteria reproduce and live like any other living organism would; they take in nutrients and reproduce using binary fission (like mitosis).

Viruses, on the other hand, are like little aliens. They really aren't living since they don't absorb nutrients or produce waste. Rather, they hook onto another cell (e.g. a bacterium) and inject the cell with its own viral genetic code. This code then causes the cell to produce multiple copies of the virus, essentially doing the reproduction for the virus at the cost of its own resources. The virus can't reproduce itself without another cell to build its copies. Therefore, we can't grow a virus on synthetic media.

Option 3 is also wrong because viruses enter the body from the environment or other individuals from soil to water to air via nose, mouth, or any breaks in the skin and seek a cell to infect.

Source Citation: NCERT: Class X Science

Inside a pressure cooker, the boiling point is increased by increasing the pressure. Hence, cooking is faster. Therefore, the temperature at which food is cooked is affected by the pressure and not the temperature of the flame.

This can also be understood by understanding why cooking is difficult on hills. At high altitudes, atmospheric pressure is lower, reducing the boiling point of water as compared to that at sea level.

Source: NCERT