७ वी सामान्य विज्ञान 3.Properties of Natural Resources नैसर्गिक संसाधनांचे गुणधर्म

Question 1:

Fill in the blanks with the appropriate term.

(Temperature, volume, mass, density, humidity, acidic, weight, neutral, shape.)

(a) The capacity of air to hold moisture depends upon the ........... of the air.

(b) Water does not have a ........... but has definite ........... and ........... .

(c) While freezing, the ........... of water is lowered.

(d) ........... soil has pH 7.

Answer 1:

(a) The capacity of air to hold moisture depends upon the temperature of the air.

(b) Water does not have a shape but has definite mass and volume.

(c) While freezing, the density of water is lowered.

(d) Neutral soil has pH 7.

Question 2:

Why is it said that –

(a) Air is a homogeneous mixture of various gases.

(b) Water is a universal solvent.

(c) There is no alternative to water for cleaning purposes.

Answer 2:

a. Air is a homogeneous mixture of gases as it is primarily made up of nitrogen and oxygen. Its elements are not readily separated or distinguished from one another.

b. Water is a very good solvent as it can dissolve almost every substance in it, therefore water is known as the “universal solvent”.

c. There is no alternative to water for cleaning purposes beacuse it dissolves everything that is added to it. The reason why it is called the universal solvent. The water dissolves all the dirt present in our clothes, the reason why we soak our clothes in detergent and water prior to washing. Also, water has a neutral pH which means it does not harm the object which has to be cleaned and also does not affect the skin.

Question 3:

What will happen if....

(a) The amount of water vapour in the air increases.

(b) Only one crop is grown repeatedly

Answer 3:

a. The amount of water vapor in atmosphere determines the level of humidity. As the amount of water vapor in the atmosphere increases so does the level of humidity. Higher humidity levels result in excessive sweating and this sweat does not dry quickly leaving us feeling hot and sticky. 

b. If only one crop is grown repeatedly in the soil, it loses its fertility and gets devoid of nutrients. For this reason crop rotation is an important procedure, so that the fertility of the soil is retained and there is no loss of nutrients. For example, after cultivation of wheat, the fertility of soil decreases and thus leguminous crops like peanut, moong etc. are cultivated after its harvest. These leguminous crops restore the fertility of soil.

Question 4:

With whom should I pair up?

 

Group 'A'		Group 'B'
(1)	Air	(a)	Excretion
(2)	Water	(b)	Scattering of light
(3)	Soil	(c)	Plasticity

Answer 4:

Group 'A'		Group 'B'
(1)	Air	(b)	Scattering of light
(2)	Water	(a)	Excretion
(3)	Soil	(c)	Plasticity

Question 5:

State whether the following statements are true or false.

(a) Sandy soil has low capacity for holding water.

(b) Sea water is a bad conductor of electricity.

(c) The substance in which a solute dissolves is called a solvent.

(d) The pressure exerted by air is called atmospheric pressure.

Answer 5:

(a) Sandy soil has low capacity for holding water.- False

(b) Sea water is a bad conductor of electricity.- False

(c) The substance in which a solute dissolves is called a solvent.- True

(d) The pressure exerted by air is called atmospheric pressure.- True

Question 6:

Explain the picture in your own words.

Answer 6:

Image 'a' represents a crevice or crack which is filled with water wheras image 'b' represents a crevice which has ice. It is evident from both the images that in image 'b', the width of the crevice increases as the water in the crevice freezes. This is one of the properties of water due to which it expands upon freezing. This expansion in water after freezing results in the widening of the crack in image 'b'.

Question 7:

Write answers to the following questions in your own words.

(a) How is light scattered by the air?

(b) Explain the various properties of water.

(c) Why is the density of seawater more than that of rain water?

(d) What is the importance of good soil structure?

(e) What are the various uses of soil?

(f) What is the need and importance of soil testing from the point of view of farmers?

(g) What is the importance of air in transmission of sound?

(h) Why should a glass bottle completely filled with water never be kept in a freezer?

Answer 7:

a. Scattering is the phenomenon of change in direction of light rays when they travel through Earth's atmosphere. Air is a mixture of gases, somke, dust and very fine particles. As the rays of light fall on these particles, these rays are spread/scattered in all directions. It is a natural phenomenon which is observed on our daily basis.

b. Properties of water: 

• Water is a colourless, tasteless, and transparent liquid. It also has no smell.
• Boiling point of pure water is 100°C at 760 mm of Hg pressure. However, boiling point of water increases due to increase in pressure and presence of dissolved impurities.
• Pure water freezes at 0°C at 760 mm of Hg pressure. However, freezing point of water decreases due to increase in pressure and presence of dissolved impurities.
• The state of water changes on heating or cooling. On heating, the state of water changes from liquid state to gaseous state (vapour). Similarly, on cooling, the state of water changes from liquid state to solid state (ice).
• Water is neither acidic nor basic. It is neutral.
• Water is a bad conductor of heat and electricity.
• Water is an universal solvent as it can dissolve many substances in it.

c. Sea water has more density than rain water because it contains salt ions. These salt ions make the seawater heavier, which means the mass of the water increases due to the presence of salt ions. From the relation, density= mass/volume, we can say that as the mass of an object increases, its density also increases simultaneously.

d. Soil structure basically refers to the type of constituents and the ratio in which they are present in soil. The soil structure is important with respect to the growth of plants and the type of plantation which can occur in a particular area. Good soil structure has advantages like:

• roots get sufficient supple of oxygen
• it has good water drainage and thus roots grow well

e. The following table represents the various types of soil: 

Soil type	Particle type	Properties
Sandy	Large	Loosely packed, with large air spaces. Water soaks into it easily. Thus it is dry, light and airy
Clayey	Fine, small	Tightly packed with no air spaces. This type of soil is heavy and holds more water.
Loamy/silt	A mixture of large and small particles	Holds water, contains humus.

f. Soil testing is a method of determining the constituents of soil.  A sample of soil is taken and is tested for its various characteristics for example, ph and electrical conductivity. It is important with respect to agricultural practices as it helps us in determining which crops can be grown in which region. It also gives us a knowledge about the deficient nutrients and this deficiency can then accordingly be dealt with. It is also useful in determining, if additional amounts of fertilizers are required.

g. Air acts as a medium for transmission of sound. When a sound wave passes through air, the particles of air vibrate back and forth, parallel to the direction of the sound wave. Thus, when a sound wave travels in the horizontal direction, then the particles of the medium also vibrate back and forth in the horizontal direction. Sound cannot travel without a medium and thus we cannot listen to sounds in the space because of absence of air. 

h. A glass bottle completely filled with water should not be kept in a freezer because the bottle can break and result in damage. We know that water expands after freezing and if a completely water filled glass bottle is placed in freezer, the water would freeze and would expand resulting in the bottle to break.