Changes Around Us Online Test 7th Science Lesson 9 Questions in English

Explanation
Changes take place around us all the time. A change refers to an alteration in physical properties
or alteration in the composition of matter. For example, ice melts on heating, that is, it changes
from a solid to liquid. On further heating, water starts evaporating; it changes from a liquid to gas.
Here, there is a change in the physical state of the substance

Explanation

Let us look at another change, that is, when objects made of iron are exposed to moist conditions,
a reddish-brown new substance called rust forms on the surface of these objects. In this instance
of rusting, there is change in the composition of the substance. Thus, the physical changes involve
an alteration in the properties such as colour, texture and the state of the substance since there is
formation of a new substance.

Explanation

Explanation
In case of burning of paper, changes to carbon dioxide and other substances. Now we cannot get
back the paper after burning. As there is a change in the chemical nature, it is called as chemical
change.

Explanation
Upon heating, particle arrangement within the state of matter gets disturbed. The disturbance is
seen either as expansion or contraction.

Explanation
When heated or cooled, the object may expand or contract based on the nature of the object, but the
mass remains the same. That is, the number of particles that was inside the object does not undergo
any change, only the arrangement of the particle changes.

Explanation
When a glass of water is heated, its volume increases and in case a glass of water is cooled its volume decreases.

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Explanation
Physical changes are the changes in which only physical properties of a substance undergo
a change and there is no change in its chemical composition. There is no new substance formed in
a physical change.

Explanation
Physical properties include lustre, malleability (flexibility), and ductility (ability to be drawn into a
thin wire), density, viscosity, solubility, mass, volume and so on. Any change in these physical
properties is referred to as a physical change.

Explanation
Any change in these physical properties is referred to as a physical change. For example, when a
rubber band is stretched, it elongates. However, when then stretching is stopped, the rubber band
comes back to its original state and shape. In this example, there is no new substance formed but
the rubber band remains the same before and after elongation.

Explanation
A physical change has following characteristics:
 During a physical change, no new substances are formed. In a physical change, the chemical
properties of a substance do not change. For example, when ice cube melts, water is formed.
In this change, there is no new substance, but water is same both in ice and in water
 A physical change is usually temporary and reversible in nature. For example, when water
is heated, water vapours are formed, once water vapours are cooled, water can be obtained
again.
 In a physical change, the chemical properties of a substance do not change. For example,
when a piece of gold is melted, its chemical composition remains the same in the solid form
and also in the liquid form.

Explanation
In a physical change, the physical properties such as colour, shape and size of a substance may
undergo a change. For example, cutting of vegetables and inflating a balloon are some examples of
physical changes in which size and shape of a substance undergoes a change. we know it is not

Explanation
The following are some of the changes of state:
from Solid → to Liquid is Melting
from Liquid → to Gas is Vaporization
from Liquid → to Solid is Freezing
from Gas → to Liquid is Condensation
from Solid → to Gas is Sublimation

Explanation
In an endothermic process, heat is absorbed and the speed of the molecules is increased hence
they move faster.

Explanation
Melting, vaporization, and sublimation occur when heated and hence it is called as endothermic process.

Explanation
In contrast, such as in freezing and condensation, heat is removed, resulting in the decreasing the
speed of the molecules causing them move slower. Such processes are called as exothermic
process

Explanation
When you put a wet cloth to dry, the water evaporates into air, leaving the clothes dry. That is there
are two types of vaporization: boiling and evaporation, the first one is by heating and the second
type of vaporization is natural.

Explanation
Boiling is the process of conversion of a liquid into vapours on heating. In gaseous state, only the
arrangement of molecules changes, there is no change in their chemical composition. So, boiling
is a physical change.

Explanation
Upon heating a liquid, the particles gain energy and vibrate more vigorously. When the particles
possess enough energy, they overcome the strong forces of attraction between one another. The
particles break free from one another and move randomly. For example, when liquid water is
heated to 100ºC, it boils to become steam. Boiling occurs when the boiling point is reached. The
liquid changes to its gaseous state.

Explanation
Evaporation is the technique used to separate dissolved solids from a solid-liquid mixture. This is
the technique used to extract salt from sea water in salt pans.

Explanation
Salt is formed by the evaporation of sea water. Shallow level of sea water is impounded. Slowly the
water evaporates due to action of Sun. Ultimately salt deposits over the ground we can understand.
Evaporation makes use of the fact that the solvent in a solution can vapourise at any temperature,
leaving behind a residue of the solid that was dissolved in the liquid.

Explanation
From drying clothes to drying fish, evaporation is used. Evaporation is a slow process and occurs
only at the surface of the liquid.

Explanation
Upon cooling a liquid, the particles loose energy and vibrate less vigorously. When the particles
possess less energy, they can experience strong forces of attraction between one another. The
particles move closer to each other and movement of particles is also restricted.

Explanation
When liquid water is cooled to 0º C, it freezes to become ice. Freezing occurs when the freezing
point is reached. The liquid changes to its solid state.

Explanation
Condensation is the conversion of gas into its liquid state. The liquid obtained after condensation
can be converted back into gas on heating. So, condensation is also a physical process. During this
process, only the arrangement of molecules changes from the gaseous state to liquid state. So,
condensation is a physical change.

Explanation
Condensation is the changing of a gas into its liquid state and it happens by cooling, whereas
Evaporation is the changing of a liquid into its gas state and it happens by heating.

Explanation
There are certain solid substances like camphor, naphthalene that get converted into gas directly
upon heating without becoming liquid. This process in which a solid is converted directly into gas
is called sublimation. In each of the above said processes, there is a change of state due to change
in temperature. But there is no change in chemical composition.

Explanation

Crystallization is also a special form of physical change. Large crystals of pure substances can be
obtained from their solutions by the process of crystallization. Crystallization is a method of
separation as well as a method of purification.

Explanation
Changes that occur with the formation of new substance with different chemical composition or
transformation of a substance into another substance with the evolution or absorption of heat or
light energy are termed as chemical changes.

Explanation
Chemical changes are very important in our lives. All the new substances which we use in various
fields of our life are produced as a result of chemical reactions. Some of the examples of the
importance of chemical changes are given below: Rusting of iron, burning, curdling of milk,
reaction of baking soda with lemon juice, fermentation are some examples of chemical changes.

Explanation
In addition to new products, the following may also accompany a chemical change:
 Heat, light or any other radiation may be given off or absorbed.
 Sound may be produced
 A change in smell may take place (or) a new smell may be given off.
 A colour change may take place.
 A gas may be formed.

Explanation
Explosion of a firework is a chemical change. We know that such an explosion produces heat, light,
sound and unpleasant gases that pollute the atmosphere. That is why we are advised not to play
with fireworks.

Explanation
You must have noticed that a slice of an apple acquires a brown colour if it is not consumed
immediately. Colour of the potato remains the same when stored in water but there is change in
colour with the piece kept in air. Look at the cut brinjal kept in air.

Explanation
There is an iron pillar at the Qutub complex in Delhi which is more than 1600 years age. Even after
such a long period, the iron pillar kept in open spaces has not rusted at all. This shows that Indian
scientists made great advances in metal making technology even at 16th century which enabled
them to make this iron pillar having the quality of great rust resistance.

Explanation

iron + oxygen + water →rust
2Fe + 2O2 from air + 2H2O 2Fe2O3. H2O

Explanation
Another way of preventing rusting is to deposit a layer of a metal like chromium or zinc on iron.
This is called galvanization. Iron articles can be prevented from making contact with oxygen,
water/water vapour. A simple way is to apply a coat of paint or grease. These coats should be
applied regularly to prevent rusting.

Explanation
Burning a piece of paper gives entirely new substances such as carbon-dioxide, water, water
vapour, smoke and ash. Heat and light are also given out during the burning of paper. We cannot
combine the products of burning of paper to form the original paper again. So, it is a permanent
change.

Explanation
You can see that the magnesium ribbon starts burning with a dazzling white light. Hold the burning
magnesium ribbon over a watch glass so that the powdery ash being formed by the burning of
magnesium collects in the watch glass.

Explanation
When magnesium ribbon burns in air, then the magnesium metal combines with the oxygen of air
to form a new substance called magnesium oxide.
Magnesium + Oxygen -> Magnesium oxide
2Mg + O2 -> 2MgO

Explanation
We know that curdling of milk is an example of irreversible change since we cannot get back the
milk after curdling occurs. It is also called as a chemical change. Curdling is a process in which
liquid gradually turns into solid, forming clumps along the way.

Explanation
Fermentation is the process in which microorganisms such as yeast and certain bacteria break
down sugar solution into alcohol and carbon-di-oxide. It is an irreversible process as the alcohol
formed cannot be turned back into sugar

Explanation
Louis Pasteur (1822- 1895), a French chemist and microbiologist was the first person to describe the
process of fermentation.

Explanation
Louis Pasteur described that fermentation occurs in the absence of air and in the presence of
micro-organisms such as yeast. He discovered the cure for rabies.

Explanation
Baking soda is sodium hydrogen carbonate and lemon juice contains citric acid. So, when these
two substances are mixed together, then a chemical change takes place between sodium hydrogen
carbonate and citric acid to form three new substances: sodium citrate, carbon di-oxide and water.

Explanation
We know that firing of crackers is a chemical change. Some crackers explode only when thrown
against a wall or struck with a hard substance. Thus, we could see that change in pressure may
also bring about a chemical change. When lemon juice is mixed with soda water, they produce
brisk effervescence which is otherwise not possible when they are separate.

Explanation
Catalysts are substances that speed up the process of a chemical change and it will not undergo
any change during the course of the reaction. For example, yeast acts as the catalyst in the
fermentation of sugar. You will learn more about catalyst in your higher classes.

Explanation
We saw that the burning of magnesium ribbon gives out heat and light. Similarly, burning of wood
also releases heat and light. Such changes in which heat is released are known as exothermic
changes.

Explanation
There are some changes in which heat is absorbed. For example, water absorbs heat when it
evaporates to form water vapours. Similarly, ice absorbs heat when it melts to form water. Such
changes in which heat is absorbed are known as endothermic changes. Dissolution of glucose in
water is also an endothermic change.

Explanation
Changes that repeat themselves after a definite interval of time are called periodic changes.
Rotation and Revolution of earth, beating of the heart, clock striking every hour, motion of the
seconds-hand / minute-hand / hour-hand of a clock are some examples of periodic changes.

Explanation
Changes that do not repeat themselves after a definite interval of time and occur randomly are
called non-periodic changes. Eruption of a volcano, occurrence of an earthquake, a streak of
lighting flash across the sky during a thunderstorm, running of a batsman between the wickets,
movement of legs while dancing are a few examples of non-periodic changes.