Q: When we sit at a camp fire, objects beyond the fire are seen swaying. Give reasons for it.
A: When we sit at a camp fire, heat is transmitted to the surroundings.
1. As a result of this transmitted heat, the apparent density of air changes and its refractive index changes accordingly.
2. Due to this changing density and refractive index of the surrounding air, the angles of refraction and deviation change.
3. This makes the objects behind the fire appear as if they are swaying.
Q: Can a virtual image be photographed by a camera?
A: 1. We can photograph the virtual image formed in a plane mirror.
2. It is possible to photograph a virtual image as our eye can see the virtual image.
3. Thus the camera can also take the photograph of a virtual image.
Q: How does a rainbow appear for an observer in an aeroplane flying high?
A: 1. An observer in an aeroplane observes the rainbow as a complete circle.
2. A rainbow is a three dimensional cone of dispersed white light of the Sun and it appears as a circle which is the base of the cone.
Q: Why lanthanoids and actinoids are placed separately at the bottom of the periodic table.
A: The ‘f’ block elements are called as lanthanoids and actinoids.
1. So they are shown separately at the bottom of the periodic table as these cannot be fitted in the general periodic table.
Q: How do the following properties change in a group and period? a) Ionization energy b) Electronegativity.
A: a) Ionization energy is the energy required to remove an electron from the outer most shell of a neutral gaseous atom.
1. In a group as the atomic radius increases from top to bottom, it becomes comparatively easy to remove an electron from the outer
shell. So ionization energy decreases in a group as we go down in a group.
2. In a period as we move from left to right more electrons join the outer shell and it becomes difficult to remove the electron from the outer shell. So ionization energy generally increases in a period as we move from left to right.
b) Electronegativity is the relative tendency of its atom to attract electrons towards itself when it is bounded to the atom of another
element.
3. Electronegativity decreases as we go down a group.
4. Electronegativity increases as we move from left to right in a period.
Q: Padma wears a spectacle. Her eyes began seem to be bigger in size.
a) What lens is there in her spectacle?
b) Which type of eye defect is she having?
Draw the ray diagram for this defect.
A: a) convex lens is there in her spectacle.
b) she is having hypermetropia eye defect.
H: Place where the object is present.
L: Least distance of distinct vision.
Q. Why did a glass bottle break when it is filled completely with water and put it in the deep freezer of a refrigerator?
A: We know the volume of water taken in the glass bottle is equal to the volume of the glass bottle.
1. When water freezes to ice, the volume of ice should be greater than the volume of water filled in the bottle.
2. As such the bottle fixed with the lid tightly breaks.
Q. Why does not temperature of water at 100 °C rise though heat is supplied continuously?
A: No rise in temparature is observed when heat is supplied continuously to water at 100 °C.
1. The heat energy supplied to water at 100°
C is used to change the state of water
from liquid to vapour (gas).
2. This is called latent heat of vapourization.
Q. Why do we sweat while doing work?
A: When we do work we spend our enrgy in the form of heat energy from our body.
1. As a result the temperature of the skin become higher and the water in the sweat glands start evaporating.
2. This evaporation cools our body.
Q. What is an alloy? Give examples.
A: A metallic substance made by mixing and fusing two or more metals or a metal and a non-metal, to obtain desirable qualities such
as hardness, lightness and strength is known as an alloy.
Eg: Brass, Bronze, Steel… etc
Q. Mention some chemical reactions you observe in your daily life.
A: Some chemical reactions we observe in our daily life:
1) Coal is burnt
2) Food gets digested in our body
3) Iron nail is exposed to humid atmosphere
for a long time.
4) We respire.
5) Milk is converted into curd.
6) Water is added to quick lime.
7) Crackers are burnt.
Q. Why is the angle of incidence equal to angle of reflection when a light ray gets reflected from a surface?
A: Light selects a path which takes least time to travel.
1. This principle was given by Pierre de Fermat.
2. It is also applicable to reflection of light. When light gets reflected from a surface it selects the path that takes least time.
3. That is why the angle of incidence is equal to the angle of reflection.
Q. A milk man adds a very small amount of baking soda to fresh milk. Why does this milk take a long time to settle as curd?
A: By adding a small amount of baking soda, the milkman shifts the pH of the fresh milk from 6 to slightly alkaline.
1. This keeps the milk in alkaline condition for a long time.
2. Acids produced in the milk due to bacterial action, will neutralize the alkali condition of milk. As this process takes a longer time, the said fresh milk takes a long time to settle as curd.
Q. What type of reaction takes place in stomach when an antacid tablet is consumed?
A: Our stomach produces hydrochloric acid which helps in digestion of the food without harming the stomach.
1. During indigestion the stomach produces too much acid and this causes pain and irritation.
2. To get rid of this pain, people use bases called antacids.
3. The antacids neutralize the excess of acid in the stomach. This is a neutralization reaction.
Q. Why are pickles never stored in Tin cans?
A: Pickles, Jams and Jellies are never stored in Tin Cans as all of them contain organic acids.
1. These acids react with the metals to form harmful salts.
2. Further these acids corrode the container. So these are stored in glass vessels or plastic vessels.
Practice Questions:
1. The speed of the light in a diamond is 1,24,000 km/s. Find the refractive index of diamond if the speed of light in air is 3,00,000 km/s.
2. The focal length of a converging lens is 20 cm. An object is 60 cm. from the lens. Where will the image be formed and what kind of image is it?
3. Draw the ray diagram for a convex lens for the image formed by it when an object is placed between focal point and optic centre.
4. In an atom the number of electrons in Mshell is equal to the number of electrons in the K and L shell. What is the atomic number of the element? Write its electronic configuration.