Class 7-General Science-Ch-3-Properties of Natural Resources-Maharashtra Board

Properties of Air

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What is air? Air is a mixture of gases, water vapor, and other substances, and it has specific properties, or characteristics.

Air has a lot of atoms and molecules. Air is a gas (as opposed to a liquid or a solid) and contains about 78% nitrogen, 21% oxygen, and 1% argon. There are other trace gases in air, like helium, carbon dioxide, and neon, just to name a few.

Air also contains water vapor; the amount varies depending upon the location (think Sea beach versus the dry desert). Finally, air contains other things, like dust, pollen, and bacteria. [/responsivevoice]

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If you blow up a balloon with air, the balloon expands. It's expanding due to the gases, water vapor and other materials that give air mass.

The molecules of the gases in the air are in constant motion. When these molecules strike a body, they create pressure on that body. This is the pressure of air that we call ‘atmospheric pressure’. atmospheric pressure is the same in all the directions.

Under ordinary conditions, atmospheric pressure at sea level is about 1,01,400 Newtons per square metre. It can be measured with the help of a barometer.

As we go higher above sea level atmospheric pressure decreases. [/responsivevoice]

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In 1726, the Swedish scientist Daniel Bernoulli put forth the important principle :

The pressure of air decreases when its velocity increases while the pressure of air increases when its velocity decreases.
If a body is moving through air, the air pressure decreases perpendicular to the direction of the motion and then the air in its surroundings flows rapidly from higher pressure to the lower pressure. [/responsivevoice]

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Air is a mixture of very fine particles of some gases, dust, smoke and moisture. When rays of light fall on these minute particles, the particles spread the light in all the directions. This natural phenomenon 1s called scattering of light.

Temperature regulation : The earth receives energy from the sun. This energy is reflected by the earth in the form of
heat. The constituents of air surrounding the earth, such as water vapour, carbon dioxide, absorb a part of this heat and give it to the other constituents of air. As a result, the earth’s surface remains warm and thereby becomes suitable for the living world on the earth. If there were no air on the earth, the average temperature of the earth’s surface would have
been very low.
Transmission of sound : All the sounds that we hear reach us through the surrounding air. Air is useful as a medium for the transmission of sound.

The density of air also changes due to change in its temperature. [/responsivevoice]

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Lesson Summary

Air takes up space. It's made up of atoms and molecules.
Air is made of gases, including nitrogen, oxygen, water vapor, and other materials. These substances are what give air mass.
Air exerts pressure.
Air has weight.
Air can be compressed.
Air is affected by temperature.

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Properties of Water

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Water occurs in liquid state under ordinary conditions.

Water is a fluid substance.

Water does not have its own shape, but has a volume.

It can pass through small holes or seep through very small gaps or cracks.

Ice is lighter than water. When water freezes to form ice, it becomes lighter than the original liquid.

When water freezes, that is, when it transforms into the solid state, its volume increases. Therefore, the density of ice is less than that of water. That is why the ice floats on water.

The density of water is 1 g/cc. [/responsivevoice]

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Water shows anomalous expansion. It means that water expands when the temperature falls below 4° C. This is called the anomalous behaviour of water.

As the temperature increases from 0°C to 4°C, the density of water increases.
The density of water is maximum at 4°C.
The surface layer of water in a reservoir freezes at 0°C.
The temperature of water below the ice is about 4°C. Hence, the water stays below the ice and the aquatic animals in the reservoir can survive in it. [/responsivevoice]

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When the salt dissolves in water, its particles spread in water. Slowly they become smaller and smaller. Ultimately they become so small that they cannot be seen, that is, they mix completely with water. This is what is called dissolving.
Solute : the substance that dissolves - Salt
Solvent : the substance in which the solute dissolves - Water
Solution : what we get when the solute dissolves in the solvent.

Seawater consists of about 35 grams of salt per litre. Thus, the density of seawater is more than that of the ordinary water. Hence, this makes a person to swim with more ease in the seawater than in the ordinary water. [/responsivevoice]

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Uses of water according to its properties

1. Water is useful for water transport due to its fluidity. Water falling down from a height is used to generate electricity with the help of a generator.

2. Water is a good coolant and is used in motor vehicles to control the temperature of the engine.

3. Water is used as a solvent in factories, laboratories, foodstuffs and in various types of biological processes occurring in the body such as digestion, excretion, etc.

4. Water is useful for cleaning purposes such as for bathing, washing clothes. [/responsivevoice]

Properties of Soil

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All soils contain mineral particles, organic matter, water and air. The combinations of these determine the soil’s properties – its texture, structure, porosity, chemistry and colour.

1. Colour

Colour is an important property of soil. Soils are of different colours. They are red white and black. The red colour of the soil is due to the presence of iron oxide. The black colour soil is rich in minerals and humus. It is good for the growth of wheat and jowar

The colours of soil are useful for its classification. Besides, they are indirectly useful in indicating several properties of the land. In this way, the properties of the soil such as its fertility, drainage of water, capacity to hold water become clear from its colour.

The colour of the soil depends upon its texture and its organic ingredients as well as on chemical ingredients like iron, lime. [/responsivevoice]

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2. Texture

The texture of the soil depends upon the relative amount of these particles.

Clay soil :Clay has the smallest sized particles. Because of very small size, the clay is felt smooth.

Clay soil has a high water holding capacity.

Silt soil : Silt particles are larger than the clay particles. There size range from 0.002 to 0.02mm in diameter. So silt does not feel smooth.

Silt soil is not as ploughable as sandy soil. However, it is much more ploughable than clay soil.

This soil contains a large proportion of organic materials. Its capacity to supply nutrients is much greater. Silt soil is also called sedimentary soil.

Sandy soil : Sand particles are largest sized particles of soil. Their diameter is more than 0.02mm

Water drains rapidly through sandy soil.

It is easy to plough this soil. But it is less fertile. This is because the particles of sandy soil are made of the mineral called silicon dioxide (quartz). These particles do not dissolve in water and, therefore, the soil has very little capacity to supply nutrients. [/responsivevoice]

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Soil structure : The structure of soil depends upon the different shapes of the particles in it. Soil occurs in the form of columnar, laminar, granular as well as block-shaped structures.

Importance of soil structure :
The fertility of soil depends upon soil structure.

Causes of diminished soil fertility:

1. Soil pH less than 6 or higher than 8.

2. Low proportion of organic matter.

3. No proper drainage of water.

4. Repeated cultivation of the same crop.

5. Continuous use of saline water.

6. Excessive use of chemical fertilizers and pesticides.

The advantages of good soil structure are as follows :
1. Roots get a sufficient supply of oxygen.
2. Water drainage is good. Therefore, the roots of plants grow well. |

Uses of soil :

1. Plant conservation : To help plants grow.

2. Water conservation : Soil holds water. As result, by means of bunds and lakes, we canwater for use throughout the year. Texture decides the water holding capacity of the soil. The soil has some amount of water inside it as a moisture. Water is usually present as a thin film around the soil particles. It is absorbed by the roots of the plants. The capacity of a soil to hold water is important for the growth of various crops. Even a dry soil has some water in it.

3. Plasticity : Soil can be given any required shape. This property of soil is called plasticity. Because of its plasticity, we can use it to make article of a variety of shapes. These articles can baked to make them hard.

Water storage earth pots, earthen lamps, idols, bricks, etc. are articles made from soil. [/responsivevoice]

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Useful types of soil

1. China clay : (Kaolin) It is white in colour. It used to make crockery, bathroom tiles, tank, laboratory apparatus, masks, jars, etc. China clay is an industrial mineral of the type ‘Kaolinite’. It is found in China and hence called China clay. Upon heating, this soil acquires a glaze and hardness and, therefore, is used for making crockery.

2. Shadu soil : It is whitish in colour. It is us for making statues and idols.

3. Terracotta soil : This soil is red in colour Decorative articles and pots used for growl plants are made from this soil.

4. Multani soil : This soil is used in cosmetics.

Soil testing : The proportions of the various ingredients of soil can be determined by ‘soil testing’.

During soil testing, the soil is examined for colour, texture and the proportion of organic matter in it. Soil is tested to find out if there is a deficiency of any ingredients and to decide what measures should be taken to remove the deficiency.

Two tests, namely, the pH and the electrical conductivity tests, are particularly useful in finding out the characteristics of soil. [/responsivevoice]

Q & A 1-7

All questions and answers from the General Science Solutions Book of Class 7 Science Chapter 3 are provided here for you -Free.

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

(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.

Answer

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 universal solvent.

Answer

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.

Answer

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.

Answer

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) Only one crop is grown repeatedly

Answer

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 leguminuos 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

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

(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

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?

Answer

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) Explain the various properties of water.

Answer

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

Answer

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) What is the importance of good soil structure?

Answer

d. Soil stucture 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 nad thus roots grow well

(e) What are the various uses of soil?

Answer

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

Answer

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

Answer

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

Answer

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.

KitabCd Academy

Class 7-General Science-Ch-3-Properties of Natural Resources-Maharashtra Board

Properties of Natural Resources

Based on Maharashtra Board General Science Chapter 3