Changes Around Us: Physical and Chemical: questions with answers

Adding beetroot extract to water is one of the changes listed in Table 5.1. What is observed is the water taking on the colour of the extract, and it is sight that records it. A change could be in the size, shape, smell, or any other property of a substance. So a change of colour is as much a change as a change of shape.

Folding a handkerchief is a physical change and it can be reversed. The material remains the same, though its appearance -- here its shape -- has changed, and no new substance is formed. The folding really does alter something, which is why it is a change and not nothing.

Physical changes do not form new substances. Some, such as melting ice, can be reversed; others, such as chopping vegetables, cannot restore the original object.

Such changes, in which one or more new substances are formed, are called chemical changes. Carbon dioxide from the air you breathe out reacts with lime water and forms a new white-coloured substance, calcium carbonate, which is insoluble in water. Nothing in the definition asks for heat, and rusting takes place without heating too.

Lowering the tube allows gas to pass through the lime water. Carbon dioxide reacts with it to form white calcium carbonate. Bubbling alone does not establish a chemical change.

Baking soda dissolves in water. The fizzing reaction with vinegar needs an acid, which plain water does not supply.

Rusting forms iron oxide from iron. Dust settling adds material to the surface without turning the iron into a new substance.

The candle that was not covered continues to burn, whereas the candle covered with a glass tumbler stops burning after some time. The covered candle does not have a continuous supply of air. The component of air that supports combustion is oxygen, so its flame gets extinguished soon. Both candles were identical and lit together, so the wax cannot be the difference.

Magnesium reacts with oxygen from air to form magnesium oxide. Magnesium oxide is the white product, not a starting substance.

For the combustion process to occur, there are three requirements: a combustible substance, also called fuel; oxygen; and heat that allows the fuel to reach its ignition temperature. Two sides of the fire triangle are named in the drawing, so the third is the one that remains: heat.

We observe that the paper starts to emit smoke, and then catches fire. Thus, we find that a substance can burn even without fire. Focusing sunrays on the paper heats it; the temperature of the paper increases with time, and after some time the paper becomes so hot that it starts burning.

The melting of wax, its solidification, and evaporation are physical changes, while the burning of vapour is a chemical change. This shows that the burning of a candle involves both physical and chemical changes, so it belongs where the two circles overlap and not in either circle alone.

This shows that the burning of a candle involves both physical and chemical changes. The melting, solidification and evaporation of the wax are physical, and the burning of the vapour is chemical; they happen at the same time in the same candle. Not every process is both, but this one is.

Erosion during a landslide is an example of a physical change, and erosion caused by flowing water and wind is a physical change. Nothing about the rock becomes a new substance: it is carried from one place to another. The material at the foot of the hill is the same rock and soil, only moved. That the change cannot be reversed does not make it chemical.

Some changes occurring over the years due to human activity can have a long-term environmental impact. For example, the increased consumption of fuels in cars, trains, aeroplanes, etc., is increasing the amount of carbon dioxide in the atmosphere. That the travel itself is wanted does not make the build-up harmless.