Olympiad Test: Changes Around Us - Class 6 MCQ

When the water vapours strike the cooler walls of container, they condense and form water droplets on the inner surfaces of metal tray and the beaker. This process is called condensation.

Lemon juice (acid) reacts with washing soda (base) and liberates carbon dioxide gas i.e., a new substance is formed. Thus, it is a chemical change. Whereas, addition of salt in lemon juice is a physical change as only salt gets dissolved in lemon juice, no chemical reaction takes place.

Boiling of an egg is a chemical, irreversible and slow change.

Metal (X), iron reacts with blue coloured solution of copper sulphate and forms a "new substance (green coloured solution of iron sulphate) and cannot be converted back to the original solution. Thus, it is an irreversible and chemical change.

Passage Analysis:
The passage discusses the changes that occur in iron nails when exposed to certain conditions.
Key Points:
- Iron nails are initially grey and bright in color.
- When exposed to certain conditions, they become covered with a coating.
- This process is called (iii).
- The change observed in the iron nails is a (iv) change.
Answer Explanation:
Based on the information provided in the passage and the given images, we can determine the appropriate options to fill in the blanks.
(i) The process described in the passage is the formation of a coating on the iron nails. The most suitable option for this would be "rusting" as it is a common process that occurs when iron is exposed to moisture and air.
(ii) The images provided in the passage show a reddish-brown coating on the iron nails. This corresponds to the typical appearance of rust. Therefore, the appropriate option to fill in the blank would be "rust".
(iii) The process of iron nails getting covered with a rust coating is called "rusting".
(iv) The change observed in the iron nails when they become covered with rust is a "chemical" change, as a new substance with different properties is formed.
Therefore, the correct option to fill in the blanks is "b" (rust, chemical).

Changes which occur again and again after fixed intervals of time are known as periodic changes. For example, change of day and night, occurrence of full moon, beating of heart, oscillation of pendulum of clock, high and low tides in sea, etc. Changes which do not repeat themselves at regular intervals are called non-periodic changes. You cannot predict when will they reoccur. For example, occurrence of earthquake, accidents, sneezing, rusting of iron, growing of a tree, etc.

A change in which a substance undergoes a change in its physical properties is called a physical change. It is generally reversible and in such a change, no new substance is formed. For example, boiling of water, dissolving of salt in water, melting an ice cube. A change in which one or more new substances are formed by means of a chemical reaction is called a chemical change. It is generally irreversible. For example, setting of milk into curd, making idlis, rusting of iron, cooking of food, etc.

On heating, iron reacts with sulphur to form black solid of iron sulphide. This is an irreversible, chemical change.

All these changes occur again and again after fixed intervals of time, hence are periodic changes.

In figure I, evaporation of water from sugar solution is a physical change because there is only change in state of matter from liquid (sugar solution) to solid state (crystals of sugar). In figure II, when sugar is heated, it turns black due to the formation of a new substance and it cannot be reversed. Therefore, it is a chemical change.

Irreversible change:
- Burning the pieces
Explanation:
- Tearing the sheet into pieces is a physical change. It does not alter the chemical composition of the paper and can be reversed by taping or gluing the pieces back together.
- Burning the pieces, on the other hand, is a chemical change. It involves the combustion of the paper, resulting in the release of heat, light, and gases such as carbon dioxide and water vapor. This change is irreversible as it involves the breaking and rearrangement of chemical bonds in the paper, resulting in the formation of new substances.
- Therefore, the irreversible change in this process is the burning of the paper pieces.

The water formed after melting of ice can be frozen again to form ice. If the air inside the blown balloon is allowed to escape, then the balloon regains its original shape. So, both these changes are reversible changes. While changes like cutting of paper and flowering of bud cannot be reversed and hence, are irreversible.

Explanation:

• Process 1: Shaping pots out of clay




• This process involves physically shaping the clay into pots.


• Physical changes are reversible changes that do not alter the chemical composition of a substance.


• Since the potter can reshape the clay if desired, process 1 is a reversible, physical change.




• Process 2: Baking the pots in an oven




• This process involves heating the pots in an oven to a high temperature.


• Chemical changes involve a transformation of substances into new substances with different properties.


• During the baking process, the clay undergoes a chemical change as it is transformed into a solid ceramic material.


• Since the chemical change is irreversible (the clay cannot be converted back into its original form), process 2 is an irreversible, chemical change.

Therefore, the correct statement is that Process 1 is a reversible, physical change while Process 2 is an irreversible, chemical change (Option C).

Digestion of food in our body - Slow change 
Earthquake - Natural, undesirable change
Falling of stone - Fast change       
Change of seasons - Natural, desirable change

Periodic changes are the changes which keep repeating themselves after a regular period of time. So, low and high tides and opening of a morning glory flower are the examples of periodic changes. 

Processes (i) and (ii) involve absorption of heat while processes, (iii), (iv) and (v) involve evolution of heat.

Opening of a bud is a chemical and irreversible process. 

While fixing the metal rim, it is heated. It becomes slightly larger in size and can be fixed into the wooden wheel. On cooling, it contracts and fits tightly on to the wheel.

Process 1 is boiling which is a fast process while process 2 is evaporation which is a slow process.