We have previously studied some examples of changes. Changes like the ripening of a fruit, spoiling of milk occur naturally. These are called natural changes. Can you think of other examples of such natural changes ? What changes have you seen taking place in the materials in your surrounding ? Let us learn about changes in greater detail in this lesson.
Many changes such as sharpening a pencil, baking bread, cooking food are useful to us and these are, therefore, called useful changes. The changes that are not useful or changes that do us harm are called harmful changes.
Think about it.
Can you now classify the natural and man-made changes you have listed as useful and harmful changes ? So far, we have studied certain types of changes. What can you tell about two of these, namely, bursting of a balloon and ripening of a fruit, from the point of view of their duration i.e. the time they take to happen? The duration of the bursting of a balloon is far shorter than that of the ripening of a fruit. Changes that take place in a short period of time are called fast/quick changes. While changes that take place over a long period of time are called slow changes
Melting some wax and obtaining the original solid wax again is something that we can do repeatedly. The changes that can occur in a forward and reverse direction again and again are called reversible changes. A ripe mango, however, cannot be transformed back into a raw mango. Wood cannot be obtained from the ash formed on burning the wood.
On considering the above examples, we find that some changes occur again and again after a definite interval of time. Such changes are called periodic changes. On the other hand, it cannot be said for sure when certain changes will recur after one occurrence. Even if they recur, the time interval is not fixed. Such changes are called non-periodic changes.
- Which of the changes shown in the picture alongside are temporary?
- Which of the changes are permanent ?
- In which of the changes did the original matter undergo a change ?
- In which, did the original matter remain unchanged ?
- In which of the changes was a new substance with a new property formed?
In some of the above examples of changes, the properties of the original substances remain the same, that is, their composition remains unchanged. No new substance is formed. Such a change is called a physical change. The change due to which one substance is transformed into another substance having new and different properties is called a chemical change.
The process of formation of vapour from a liquid is called evaporation. Drying of clothes, formation of salt from seawater are possible due to evaporation. We have studied some processes that are a part of the water-cycle. Which are these processes ? Do the original properties of water change during those processes ? Previously, we have learnt about the processes of dissolving, boiling, melting. They are all examples of physical change.
Apparatus : Evaporating dish, sugar, burner, tripod, etc. 1. Which of the changes shown in the picture alongside are temporary?
Procedure : Take some sugar in an evaporating dish. Place the dish on a tripod and heat it. Observe the changes taking place in the sugar. Stop heating when a blackish substance is seen at the bottom of the evaporating dish. What kind of change does the above process bring about ?
When an iron article rusts, a reddish brown layer forms on it. A greenish layer is seen to form on a copper article. This process is called corrosion of metals. Things become weak due to corrosion. Corrosion is caused by oxygen, moisture, vapours of chemicals in the air.