Physical and Chemical Changes

Q35. Why is melting of ice a physical change?

Ans. Melting of ice is a physical change because it causes change in the physical state of water i.e. from ice in the solid state to water in the liquid state. Moreover, no new substance is formed in the process and this change is reversible. So, melting of ice is a physical change.

Q36. What happens when carbon dioxide is passed through lime water?

Ans. When carbon dioxide is passed through lime water, calcium carbonate is formed, which makes lime water milky.

Carbon dioxide (CO₂) + Lime water [Ca(OH)₂] → Calcium Carbonate (CaCO₃) + Water (H₂O)

Q37. Why rusting of iron objects are faster in coastal areas than in deserts?

Ans. For rusting, the presence of both oxygen and water (or water vapour) is essential. In coastal areas moisture present in the air is higher compared to desert areas. Thus, rusting of iron objects is faster in coastal areas than in deserts.

Q38. Why stretching of rubber band is a physical change?

Ans. When rubber band is stretched only its size changes and it comes back in its original shape and size, once it is released. Moreover, it does not cause any change in its chemical composition. Hence, stretching of rubber band is a physical change.

Q39. Ozone acts as a natural shield against ultraviolet radiation. Explain

Ans. It protects us from the harmful ultraviolet radiations which come from the sun. Ozone absorbs this radiation and breaks down to oxygen. If ultraviolet radiation were not absorbed by ozone, it would reach the earth’s surface and cause harm to us and other life forms.

Q40. What happens when magnesium ribbon is burnt in air?

Ans. When the magnesium ribbon burnt in air, it reacts with oxygen found in the air to form Magnesium Oxide. After it burns, it forms a white powder of the magnesium oxide. The change can be represented by the following equation:

Magnesium (Mg) + Oxygen (O₂) → Magnesium oxide (MgO)

Q41. What is rusting?

Ans. If a piece of iron is left in the open for some time, it acquires a film of brownish substance. This substance is called rust the process is called rusting.

The process of rusting can be represented by the following equation:

Iron (Fe) + Oxygen (O₂, from the air) + water (H₂O) → rust (iron oxide Fe₂O₃)

Q42. When baking soda is mixed with lemon juice, bubbles are formed with the evolution of a gas. What type of change is it? Explain.

Ans. When baking soda is mixed with lemon juice, bubbles are formed with the evolution of a gas.

We can write the reaction as:

Lemon juice (Citric acid) + Baking soda (Sodium hydrogencarbonate) → Carbon dioxide + other substances

It is a chemical change.

Q43. Why are chemical changes important in our daily lives?

Ans. Chemical changes are very important in our lives because all new substances are formed as a result of chemical changes. For example, if a metal is to be extracted from an ore, such as iron from iron ore, we need to carry out a series of chemical changes. A medicine is the end product of a chain of chemical reactions. Useful new materials, such as plastics and detergents, are produced by chemical reactions. Indeed, every new material is discovered by studying chemical changes.

Q44. In addition to new products, what else may accompany a chemical change?

Ans. In addition to new products, the following may accompany a chemical change:

                            i.        Heat, light or any other radiation (ultraviolet, for example) may be given off or absorbed.

                           ii.        Sound may be produced.

                          iii.        A change in smell may take place or a new smell may be given off.

                          iv.        A colour change may take place.

                           v.        A gas may be formed.