Science textbook solutions for Class 7 Science Chapter 3 Properties of Natural Resource


Important points to remember :

  1. 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’. 
  2. Under ordinary conditions, atmospheric pressure  at sea  level is about  1,01,400 Newtons per  square metre. 
  3. In 1726, the Swedish scientist  Daniel Bernoulli  put forth the important principle  that the pressure of air decreases when its velocity  increases while the pressure of air increases when its velocity  decreases. 
  4. Air is a mixture  of very fine  particles  of some gases, dust, smoke  and moisture.  
  5. When  rays of light fall on these minute particles,  the particles spread  the light in all the directions. This natural phenomenon  is called  scattering of light.  
  6. Air is useful as a medium  for the transmission of sound. 
  7. water expands when the temperature falls below 40  C. This is called  the  anomalous behaviour  of water. 
  8. The Danish Scientist  Sorensen put forth  the  concept of  pH, based on the concentration of hydrogen ions. World Soil Day : 5th  December 


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 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 : 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 : 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 : It is said that there is no alternative to water for cleaning purposes. Water is provided by nature. Water is available in abundance. So as there is more water on Earth we use water for cleaning purposes. We do not use other substances because some substances are very expensive and some substances are very scare.

Question 3: What will happen if....
a) The amount of water vapour in the air increases
Answer : If the amount of water vapor in the air increases, the relative humidity increases, and if the amount of water vapor in the air decreases, the relative humidity decreases.

b) Only one crop is grown repeatedly.
Answer : Is called monoculture. The practice of growing only one type of crop on the same field is correctly referred to as monoculture. Monoculture refers to the growing of the same kind of crops on the same field repeatedly.

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 :
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.- 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 crack which is filled with water wheras image 'b' represents a crack which has ice. It is evident from both the images that in image 'b', the width of the crack 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 : The phenomena occurring when a ‘parallel beam of light’ passing through a particle present in the air, the particle scatters the light and illuminates it in all direction apart from its incident direction.
The ‘atmospheric air molecules’ are smaller than the visible light wavelength range.
The lower wavelength light like blue is ‘scattered more than’ the higher ones like red and this is the reason for the sky appearing blue. The sunlight falling on a particle in the air is scattered according to their wavelength and not as white light.
This scattering of light by air is also termed as Rayleigh’s Effect.


b) Explain the various properties of water.
Answer : The main properties of water are its polarity, cohesion, adhesion, surface tension, high specific heat, and evaporative cooling.

  1. Polarity : A water molecule is slightly charged on both ends. This is because oxygen is more electronegative than hydrogen. 
  2. Adhesion : Similar to cohesion, but adhesion is when the hydrogen bonds in water allow for the water molecules to be held to another subtance.
  3. Heat : Specific heat is amount of heat absorbed or lost for 1g to change 1ºC, which in the case of water, is pretty high. This allows for evaporative cooling to occur, which is when heat energy is transferred to water molecules , and evaporating water removes a lot of heat energy from an organism 
  4. Cohesion : Hydrogen bonds hold water molecules together, as seen in the picture above. Cohesion creates surface tension which is why if you fill a spoon with water, drop by drop, the water volume will actually be bigger than the spoon's surface before the water falls off.
  5. Other important characteristics of water involve it being a universal solvent, along with its unusual density. Water, unlike any other solid-liquid, is more dense in its liquid form than as a solid, which is why ice floats, and this allows for entire habitats to exist underneath layers of ice floating on oceans. Its neutral pH (7) is also a relevant characteristic.


c) Why is the density of seawater more than that of rain water?
Answer : Sea water has more density than the fresh water or rain water.

It is because there are salt ions present in the seawater which makes the seawater heavier , it means the mass of the water increases due to the presence of salt ions.

Density of seawater ranges between 1020 and 1029 kg/m³. As we know that the mass is directly proportional to the density ,
Density = mass/ volume. that's why density of seawater is more than the fresh or rain water.

d) What is the importance of good soil structure?
Answer : Soil structure is the arrangement of soil particles (sand, silt, clay and organic matter) into granules, crumbs or blocks. It is the shape that the soil takes based on its physical, chemical and biological properties. Soil structure is often confused with soil texture, both of which affect the soil's drainage and aeration capabilities.

 A good soil structure is important to allow air and water into the soil which are vital for healthy plant growth. It will improve drainage and reduce soil erosion caused by excess surface run-off. Without structure, soils will suffer from anaerobism, water logging and nutrient lock-up and, ultimately, plants will die!

e) What are the various uses of soil?
Answer : Uses of Soil:

  1. Plant conservation : To help plants grow. 
  2. Water conservation : Soil holds water. As a result, by means of bunds and lakes, we can get water for use throughout the year. 
  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  articles of a variety  of shapes. These articles  can be baked to make them hard. Water storage earthen pots, earthen lamps, idols, bricks, etc. are articles made from soil.
  4. Soil is used in agriculture, where it serves as the primary nutrient base for the plants. Soil resources are critical to the environment, as well as to food and fiber production. 
  5. Soil is used in constructions and arts. Soil material is a critical component in the mining and construction industries. 
  6. Soil serves as a foundation for most construction projects. Soil play an important role in filtrating and purifying water. 
  7. After coming down as precipitation, much of the rain water is percolated through the many horizons of a soil profile and renamed as groundwater. 
  8. Waste management often has a soil component. Landfills use soil for daily cover.  
  9. Septic drain fields treat septic tank effluent using aerobic soil processes.
  10. Organic soils, especially peat, serve as a significant fuel resource.



f) What is the need and importance of soil testing from the point of view of farmers?
Answer : 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. The soil test analyses the levels of macro nutrients, micro nutrients and pH present in the soil and balance them with crop’s nutrient demands. It also vital for observe the levels of degradation of land. Soil testing analyses is important for farmers to better understand their soil types and deficient of nutrients and it help to minimise the quantity and different types of fertilisers, it become cost benefit to peasants.

g) What is the importance of air in transmission of sound?
Answer: The air is the medium which is necessary for the transmission of sound. The sound needs a medium to propagate and air provides that medium. Sound waves are created by help of the disturbances in the medium.

h) Why should a glass bottle completely filled with water never be kept in a freezer?
Answer : The expansion of water takes place as it is cooled from 4°C to 0°C. This behavior is unusual because most substances contract when they are cooled. Due to this expansion a glass bottle completely filled with water and tightly closed at room temperature is likely to burst when kept in a freezer of a refrigerator.

# let's recall 
1. Which  are the  gases present in air? 
Answer : By volume, dry air contains 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, and small amounts of other gases. Air also contains a variable amount of water vapor, on average around 1% at sea level, and 0.4% over the entire atmosphere.

2. Why is air called a homogeneous mixture ? What are the uses of the various gases in air ?
Answer : Air is said to be Homogeneous mixture of different gases because it contains different gases like Nitrogen ,oxygen, carbon dioxide, argon, etc. This is said to be homogeneous mixture because the constituent cannot be distinguished unlike heterogeneous mixture.

# Use your brain
1. What is the effect of increased temperature on the pressure of air ? 
Answer : Gay-Lussac's Law is one part of the ideal gas law and so explains how gases change when volume is held constant. As the temperature increases, the molecules in the gas move faster, impacting the gas's container more frequently and exerting a greater force. This increases the pressure.

2. What are the states in which water is found ?
Answer : 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.

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