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Download 8th Class General Science Notes (Unit # 9) for Islamabad Model Schools & Colleges

Download 8th Class General Science Notes (Unit # 9) for Islamabad Model Schools & Colleges

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Download 8th Class General Science Notes (Unit # 9) for Islamabad Model Schools & Colleges

SHORT QUESTIONS
Explain the following in your own words
Expansion:
Increase in size (length, width or volume) of an object on heating is called expansion.
Contraction:
When the temperature of materials falls down they contract and show thermal contraction.
Temperature:
Temperature is the degree of hotness and coldness.
Thermometer:
Thermometer is a device to measure the temperature of an object or body.
Degrees Celsius:
Celsius is the temperature measuring standard scale.
Bimetallic strip:
A bimetallic strip may be used in a fire alarm and as a thermostat (temperature regulator) in an electric iron, air conditioners, rice cookers and refrigerators
List the sources of heat.
  • Sun
  • Friction
  • Electricity
  • Chemical reactions
  • Burning of Fossil Fuels
List the effects of heat.
·        Heat changes states of matter.
·        Heat can produce chemical changes.
·        Heat causes expansion of matters i.e., solid, liquid and gases.
·        Heat produces an increase in temperature which causes, wind, rain, waves in sea etc.
Why does a solid expand on heating?
When a solid is heated it absorbs energy. This causes the particles from which it is made to vibrate more vigorously. As a result of this 'extra' movement, the particles are pushed apart a little and so the solids expand in all directions.

Why the metal lid on a glass jar can be unscrewed easily if the jar is inverted for a few seconds with the lid in very hot water?
Because all solids or metals expand on heating. Therefore when jar is inverted for a few seconds with the lid in very hot water can unscrewed easily.

Why liquids expand more than solids?
Liquids expand more than solids on heating. So the particles in liquid are both vibrating and moving from place to other place i.e., particles can change position.

What is riveting?
Riveting is a professional fastening method in which 2 work pieces are joined (riveted) together.
What is thermal expansion?
Heat causes expansion of solids, liquids and gases. This is known as thermal expansion.

How an axle of a wheel is fixed?
Axle of a wheel is fixed by riveting. Riveting is a professional fastening method in which 2 work pieces are joined (riveted) together.

Draw two diagrams to show what happens to the same bimetallic strip when it is: (a) heated in a flame (b) cooled in a freezer.
What is the use of bimetallic strip in fire alarm and electric iron?
·        A bimetallic strip may be used in a fire alarm. If there is a fire and the bimetallic strip becomes hot it bends, completes the circuit and the fire alarm bell rings.
·        When electric iron becomes very hot the bimetallic strip bends and disconnect the circuit to save energy

Why the rails in railway lines have tapered ends?
·        The long rails have tapered ends, which overlap each other.
·        This arrangement allows the ends to slide past each other when the metal expands.
·        These gaps provide space for thermal expansion without affecting the safety of the train.

Why the concrete roads are laid in sections with pitch between them.
·        During summer the concrete surfaces absorbs heat and expand.
·        So to avoid damage of the concrete road, surfaces are laid in sections with small gaps between the sections.

Why the ends of bridges often rest on rollers and have small gaps?
·        Large structures such as bridges may expand and contract by considerable amounts as their temperature changes.
·        To allow for this, the ends of bridges often rest on rollers and small gaps permit a small amount of expansion.

What is an expansion joint?
·        Large bridges have expansion joints to accommodate the expansion and contraction.
·        These joints are like two metal combs facing one another.
·        On hot day, the two sides of the expansion joint move towards one another.
·        As the bridge cools down, they gradually begin to retract.
In what way the expansion of water is different from that of solid?
Liquids (i.e. Water) expand more than solids on heating. So the particles in water are both vibrating and moving from place to other place i.e., particles can change position.

At what temperature does water have its maximum density?
Water has maximum density at 4oC.

When a quantity of water freezes what happens to its volume?
Volume will be increased when a quantity of water freezes.

If a gas is heated how does it affect the motion of its particles?
Motion of its particles will be increased (so vigorous) if a gas is heated.

Explain the following:
(a) A metal bar expands when heated
When a metal bar is heated, its particles vibrate more rapidly. As a result of this 'extra' movement, the particles are pushed apart a little and metal bar expands
(b) Overhead cables are hung with plenty of slack in them
When overhead cables are suspended from poles, they are left slack, partly to allow for the contraction that would happen on a very cold day.
(c) A bimetallic strip bends when heated
A bimetallic strip is made up of brass and iron. When heated brass expands more than iron and allows this strip bends with copper on the outside
(d) Water expands when it freezes.
In water, the molecules are close together. In ice the water molecules link up in a very open structure that actually takes more space than liquid.

Whether each of the following describes a solid, a liquid or a gas.
(a) Particles move about freely at high speed
Gas
(b) Particles vibrate and cannot change positions.
Solids
(c) Fixed shape or volume
Solids
(d) Particles vibrate but can change positions
Liquids
(e) No fixed shape or volume
Gas
(f) Fixed volume but no fixed shape
Liquid
(g) Virtually no attraction between particles.
Gas



What are the sources of heat?
  • Sun
  • Friction
  • Electricity
  • Chemical reactions
  • Burning of Fossil Fuels

Why will a hot thick glass beaker shatter if plunged into cold water?
Hot glass can break in cold water because glass is a poor conductor of heat. When part of the glass is in cold water and part is not, the part of the glass in cold water will shrink much faster than the part not in cold water. When one part suddenly becomes much smaller than the other part, the glass breaks

What will happen if two glasses one full of water and other full of milk up to neck are placed in a freezer for 24 hours?
The volume of water will be increased and some water will be oozes out from glass while the milk will keep its volume intact.



LONG / EXTENSIVE QUESTIONS


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Download Class Notes: Download 8th Class General Science Notes (Unit # 9) for Islamabad Model Schools & Colleges
Download 8th Class General Science Notes (Unit # 9) for Islamabad Model Schools & Colleges
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