class eight

CHAPTER 10
A. Answer the following question.
1. Define symbols
Ans:
A symbol is shortened form of name of an element. E.g. for representing hydrogen element it is capital letter “H”

2. What does a formula represent
Ans:
It tells us about the participating elements and their combining reaction. e.g. CO2 is the formula of carbon dioxide. Here the formula shows that there are two elements. I.e. carbon and oxygen and combination is 12:32.

3. Define Reaction
Ans:
Whenever a chemical change occur, a chemical reaction is said to happen e.g. iron combines will sulphur

Upon heating, we will represent the charge as
Fe + S FeS

4. What is a chemical equation?
Ans:
A balanced chemical reaction is called a chemical equation. Note the symbols of element and formula of compound are used for representing the chemical change.

e.g.

2H2 + O2 2H2O

5. What is an ionic bond?
Ans:
The bond formed by loss or gain of electron is called an ionic bond. E.g. in case of sodium chloride.

6. Describe covalent bond
Ans:
The bond formed by the sharing of
. E.g. In case of H2 molecule.

7. Write symbols of sodium, Oxygen, hydrogen, potassium and iron.
Ans:
Sr. No
Element
Symbols
1
Sodium
Na
2.
Oxygen
O
3.
Hydrogen
H
4.
Potassium
K
5.
Iron
Fe

8. Name the elements that following symbol represent.
Sr. No
Symbol
Element
1
K
Potassium
2.
Ca
Calcium
3.
Cu
Copper
4.
Ag
Silver

B. Fill in the blanks.
1. The symbol of O represents Oxygen element
2. The formula of NaCl represents compound of sodium chloride.
3. The balance equation has same number of atoms on both sides.
4. Sharing of electrons occurs in covalent bond.

C. Balance the following equation
Ans:
Balanced equations
1. H2 + Cl2 2HCl
2. 2C + O2 2CO
3. 2Mg + O2 2MgO
4. 2H2 + O2 2H2O
D. Write chemical equations for the following.
Ans:
1. N2 + 3H2 2NH3
2. H2 + Cl2 2HCl
3. 2H2 + O2 2H2O
4. I2 + Cl2 2ICl

E. Chose the correct answer.
1. An element is consist of
(i) One kind of atoms
(ii) Two kind of atoms
(iii) Varying number of atoms
2. A compound is a substance that always consists of
(i) Same elements
(ii) Different elements in varying ratio
(iii) Different elements in the same fixed ratio
3. The most common element of human body by weight is
(i) Carbon
(ii) Oxygen
(iii) Hydrogen


4. How many elements are found in nature
(i) 70
(ii) 90
(iii) 110

CHAPTER 11

A. Answer the following question.
1. Define Salts and state their properties
Ans:
A salt is formed when an acid read with a base.
Physical characteristics
(i) They are crystalline
(ii) Soluble in water
(iii) Some are colored
(iv) Some are hydrated

Chemical Characteristics
(i) Some give strong acidic solution in water. e.g. NH4Cl
(ii) Some give basic solution in water e.g. Na2CO3

2. Name three common acid
(i) Hydrochloric Acid (HCl)
(ii) Sulphuric Acid (H2SO4)
(iii) Nitric Acid (HNO3)

3. What are base. State their properties.
Ans:
Chemicals that give (OH-) ion in water called base. e.g.
NaOH Na+ + OH-
Properties
i. Bitter taste
ii. Soapy to touch
iii. Turn red litmus paper to blue.
iv. The reacts with acids and produce salt and water.
v. They react with ammonia compounds on heating and gives ammonia upon heating.
vi. They give OH- ion in water.

4. Name three bases
(i) Sodium Hydroxide (NaOH)
(ii) Potassium hydroxide (KOH)
(iii) Calcium hydroxide (Ca(OH)2)

5. Define Neutralization? State its use in daily life?
Ans:
Whenever acid reacts with base and neutral solution is obtain then such process is called neutralization. We use this process to get benefit as
1. Insect stings:
WASP: it injects alkaline liquid which is neutralized by vinegar (acid).
BEE: It injects acidic liquid and is neutralized by baking soda solution.
2. Indigestion: Indigestion if happens then it is neutralized by sodium bicarbonate.
3. Soil Treatment: Acidic or basic soil is neutralized by using (NH3) ammonia fertilizers and gypsum (CaSO4) to make it fit for cultivation.
4. Factory waste: it is after acidic and treated with slaked lime Ca (OH)2 to neutralize it.
B. Fill in the blanks
1. Solution of acid turns blue litmus paper red.
2. Acid reacts with base and form salt and water.
3. Bases are Soapy to touch
4. The sting of bee is cured by neutralization reaction.

C. Tick (√) true or false.
1. Acid are sweet in taste (x)
2. Bases turn blue litmus paper red (x)
3. The acids react with bases and form neutral solutions. (√)
4. Lemon juice has an acid. (√)

D. Choose the correct answer
1. Which solution turns red litmus paper blue?
(i) Acidic
(ii) Basic
(iii) Salt

2. Which acid is present in vinegar
(i) Hydrochloric acid
(ii) Acetic acid
(iii) Tartaric acid

3. Which of the following is used in making soap
(i) Sodium chloride
(ii) Sodium hydroxide
(iii) Potash

4. The pH value of vinegar that is acidic will be
(i) Less than 7
(ii) 7
(iii) More than 7

5. Which of the following tells that an unlabelled substance is an alkali
(i) It is soluble in water
(ii) It has bitter taste
(iii) It turns red litmus paper blue


CHAPTER 12

A. Answer the following question
1. Describe occurrence of carbon in nature?
Ans:
It occur both in free as well as in combined state.
As free, it occurs as diamond and graphite.
As combined, it occurs as carbonates, hydrocarbons in proteins, carbohydrates and in all organic compounds.
2. Describe properties of carbon?
Ans:
Properties
i. It is not soluble in water.
ii. It has high melting point
iii. Its one form (diamond) is used as jewelry
iv. Graphite is used in lead pencils, in dry cells, glass and cement industries.
v. Graphite is a conductor while diamond do not conducts.
Chemical Properties:
It focus two important oxides namely (i) Carbon mono oxide (Co) (ii) Carbon dioxide (CO2)

3. List sources that produce carbon dioxide?
Ans:
i. By the action of acids on carbonate
ii. By burning wood in enough supply of oxygen
iii. By fermentation of glucose.
iv. In respiration exhalation.

4. What is dry ice and what is its use
Ans:
When carbon dioxide is cooled to -78 Co, it turns solid and is called dry ice.
Use: it is used as cooling agent, antiseptic.

5. State properties of carbon dioxide?
Ans:
i. It is colorless gas with no smell
ii. It is heavier then air
iii. It is soluble in water
iv. Upon cooling it freeze at -78 Co, forming dry ice.
v. Can be used as fire extinguisher because it does not support combustion.
vi. It reacts with base to give carbonates.

B. Fill in the blanks.

1. Diamond is a form off Carbon.
2. Hydrochloric acid and marble react to form Carbon dioxide, salt and water.
3. The vapours of wax burn in oxygen to produce flame.
4. Carbon dioxide turns lime water milky.

C. Tick (√) true or false
1. Graphite is hard carbon (√)
2. Carbon dioxide is not soluble in water (x)
3. Fuels when burn produce carbon dioxide (√)
4. Our body has carbon compounds. (√)

D. Chose the right answer.
1. In which of the following process carbon dioxide forms a compound?
i. Burning
ii. Respiration
iii. Photosynthesis

2. Form the following which process produces carbon dioxide.
i. Photosynthesis
ii. Burning
iii. Melting

3. What makes the drinks fizzy?
i. Oxygen
ii. Carbon dioxide
iii. Air

4. What is a dry ice?
i. CO
ii. CO2
iii. CO3

CHAPTER 13

A. Answer the following question.
1. Describe that pressure of liquids increases with depth.
Ans:
This can be explained by simple experiment.
Take a can or polystyrene cup and puncture three holes on one side at different levels. Take it to the sink and run water rapidly. We will observe that greater the depth, more will be the pressure of the water observed and the water will shoot out of the hole (Near the bottom)

2. Why liquids exert pressure?
Ans:
Liquid exceeds pressure because of the weight.

3. Explain that liquids tend to keep its level?
Ans:
Liquid tends to keep their level because the intermolecular forms constantly break and refers thus they get the ability to flow. Secondly they posses weight so whenever they get chance they try to get equilibrium. So they keep level.

4. How will you prove that water exert equal pressure in all direction.
Ans:
It can be prove by a simple experiment.
Experiment:
Take a can and punch boles around the sides near bottom, take the can to the sink and run water through it rapidly. We will observe that the water from all the holes comes out with equal shoot. It is clear that water exert equal pressure in all direction.

B. Fill in the blanks.
1. Water keep its own level.
2. Pressure varies with depth of water
3. Liquids exert equal pressure in all directions.


C. Tick (√) true or false
1. As we go down the sea pressure of water also increases. (√)
2. The surface of water in a lake is not uniform (√)
3. We can visit deep sea with naked body (x)
4. At same depth water has equal pressure (√)

D. Chose the correct answer.
1. What are the characteristics of a liquid?
a. Has weight
b. Exert pressure
c. Both a and b


2. How does the liquid exert pressure?
a. Up and down
b. Down
c. In all direction

3. If we dive in a tank full of water, how do we feel its pressure?
a. Greater pressure on the surface
b. Mild pressure in the bottom
c. Greater pressure in the bottom

4. If water is poured in three connected tubes at the bottom,
a. Water level will be uneven in three tubes.
b. Water level be even in all the tube
c. Both a and b.


CHAPTER 14

A. Answer the following question.
1. What is thermal expansion
Ans:
The expansion caused due to heat is called thermal expansion.

2. How would you prove that solids expand on heating?
Ans:
We can make our concept clear from the simple example that the metal lid of a glass jar some time get stuck tight but when putting into hot water the lid get expands and transit become to open.

3. Do the gases expand on heating? Explain with examples.
Ans:
(i) Take a balloon and a flat bottom flask and snap the balloon on its mouth. Heating the flask inflates the balloon while cooling, if deflates the balloon. So the conclusion we makes is that where heating the air inside the flask expands and tries to move out but in doing this inflates the balloon and when cooling the air get contracts. Thus the balloon deflates.
Example:
Other example is of formation of carbon dioxide cause rise of dough in cake.
4. What are the uses of thermal expansion in solids, liquids and gases in daily life?
Ans:
Uses of Solid thermal:
The black smith fits iron rim or metal rivets when the are red hot, when they cool, hold the parts tightly.

Uses of Liquid thermal:
We use thermometer and liquid mercury rises if come in contact with hot body so it gives temperature reading.

Uses of Gas thermal:
Expansion of gas causes rise in dough of cake and bread in bakery.

5. Suppose you observe a temperature of liquid as 45 oC. What will be the equivalent temperatures if you are using Kelvin scale and Fahrenheit scale?
Ans:
We know the formula is
K = C + 273
So K = 45 + 273
K = 318

And F = C x 1.8 + 32
F = 45 x 18 + 32
F = 113o


B. Fill in the blanks.
1. When we add heat energy to a material, it expands.
2. With same amount of heat liquid expand more than solid
3. On Fahrenheit scale there are 180 degrees between the freezing and boiling point of water.
4. Railway tracks have ability / resistance to accommodate their expansion.

C. Tick (√) true or false
1. Solid expand more than gases on heating. (x)
2. When heat energy is taken away from the material, it contracts. (√)
3. The doctors mostly use Celsius scale to note body temperature (x)
4. The freezing point of water on Kelvin scale is 272o K. (√)


D. Chose the correct Answer.
1. Which material will expand most on heating?
(i) Solid
(ii) Liquid
(iii) Gas
2. What will be the boiling point of water on Fahrenheit scale
(i) 100
(ii) 212
(iii) 372
3. The unit for measurement of temperature is
(i) Celsius
(ii) Fahrenheit
(iii) Degree
4. In which direction solids expand on heating.
(i) Lengthwise
(ii) Up and down
(iii) In all direction

E. Convert the following into scales as directed:
1. Convert 20oC, 35oC, 40oC and 80oC into Fahrenheit and Kelvin scales.
Ans:

Kelvin Scales
We know that K = C + 273

20oC = K = 20 + 273 = 293
35oC = K = 35 + 273 = 308
40oC = K = 40 + 273 = 313
80oC = K = 80 + 273 = 353




Fahrenheit
We know that F= C x 1.8 + 32
20oC = 20 x 1.8 + 32
= 68

35oC = 35 x 1.8 + 32
= 95

40oC = 40 x 1.8 + 32
= 104

80oC = 80 x 1.8 + 32
= 176

2. Convert 81 0F, 90 0F and 122 0F into Celsius and Kelvin scales.
Ans:

We know that: C = F-32 / 1.8

(i) C = 81 – 32 / 1.8
= 27.22

(ii) C = 90 – 32 / 1.8
= 32.22

(iii) C = 122 – 31 / 1.8
= 50


3. Convert 290oK, 315oK and 367oK into Celsius scale.
As we know that
K = C + 273 or K-273=Co

(i) CO = 290 – 273
= 17

(ii) CO = 315 – 273
= 42

(iii) CO = 367 – 273
= 94





CHAPTER 15

A. Answer the following question?
1. Describe the types of lenses?
Ans:
1. Convex lens or converging lenses
2. Concave lens or diverging lenses
2. How light rays behave when they pass through convex and concave lens.
Ans:
The light rays converge on passing through convex lens while they diverge on passing through concave lens.

3. Describe image formation in convex lenses?
Ans:
Object
Image As shown from the diagram the image formed by convex lens is real, small and inverted.








4. What are the similarities between the working of our eye and lens camera?
Ans:
a. Both have convex lenses
b. Both can control the light i.e. in camera aperture controls excess light while in human eye pupil limits the light passing through the lens.

5. Name three instruments in which lens are used?
a. Camera
b. Microscope
c. Telescope


B. Fill in the blanks:
1. Lens is a transparent piece of glass
2. A lens thicker in the middle is called convex
3. In magnifying glass convex lens is used
4. The concave lens form image on same side of the lens
5. The convex lens converge beam of light.


C. Tick (√) true or false.
1. Lenses are used to refract light (√)
2. In case of convex lens an image is formed on (x)
3. Concave lens is thicker in the middle (x)
4. Camera has convex lens (√)
5. In our eye an inverted image is formed. (√)


D. Chose the correct answer
1. In which of the following the beam of light spreads out ward
a. Convex
b. Concave
c. Mirror

2. What type of lens is used in human eye?
a. Convex
b. Concave
c. Mirror

3. Image of which lens can be detected on screen
a. Convex
b. Concave
c. Mirror

4. In which of the following appliance lens is used
a. T.V.
b. Bulb
c. Binocular

5. What type of image is formed by a magnifying glass
a. Inverted
b. Not real
c. Real and inverted.
CHAPTER 16

A. Answer the following questions
1. Describe potential difference?
Ans:
It is the difference if electric current between two points in a circuit.

2. What is the relationship of electric current and potential difference?
Ans:
The potential difference determines the size of current.

3. State different ways of producing electricity?
Ans:
Different ways are adopted for production of electricity.
1. Hydroelectric power
2. Thermal power stations
3. Wind power stations.
4. Nuclear Power stations.
5. Solar power panel
6. Tidal power plants
7. Geothermal power stations
8. Coal power generators
9. Gas electric power generators.

4. Explain electromagnetic induction?
Ans:
Mechanical energy of motion is changed to electrical energy by mean of magnetism is called electromagnetic induction.

5. What is kWh?
Ans:
It mean 1000 watt per hour, related to consumption of electricity.

B. Fill in the blanks
1. Potential difference cause the flow of electric current
2. The electric current varies with a change in potential difference.
3. Wind power can be used to produce electric energy
4. The watt is the unit for electric power
5. The generator produces electricity from mechanical energy.



C. Tick true of False
1. Potential difference determines the flow of electric current. (√)
2. The volt is a unit of potential difference. (√)
3. When a coil of wire is moved up and down the magnet, an electric current
is produced. (√)
4. Kilo-watt-hour determines the consumption of electricity. (√)

D. Chose the correct answer
1. How long a coil retains magnetic effect after the magnetic movement is stopped?
a. Many days
b. Many hours
c. Disappears immediately

2. What is the unit of determining potential difference?
a. Watt
b. Volt
c. Ampere

3. How does the electric current flow in a wire?
a. From higher potential to lower
b. From lower potential to higher
c. Both a and b

4. What is the potential difference between two wires that light our bulb at home?
a. 9 volts
b. 110 Volts
c. 220 Volts

E. A person uses a machine of 4 kW for thirty minutes. Calculate kilowatt-hour and cost of electricity if cost of each unit is Rs. 1.50.
Ans:
Sol:
Electricity used = Power x time
= 4 kW x 30/60
= 2 kWh
Cost of electricity = 2 X 1.50
= 3 Rs.