All questions of Achiever Section for Class 8 Exam

D is correct as frictional force is responsible for stopping of the car

Option d is correct answer for the given statements.

B) non-contact

Aneroid barometer is a direct reading instrument which is used for measuring atmospheric pressure. It is a type of barometer which uses a small, flexible metal box called an aneroid cell, which is made from an alloy of beryllium and copper. The cell is sealed and contains a vacuum. As the atmospheric pressure changes, the cell expands and contracts, which is then translated into a reading on the dial.

Both 1 and 2 answer

Kerosene (W) is used as a fuel in jet aircrafts. Coal tar (X) is a thick, black liquid which is used to obtain several compounds including naphthalene balls. CNG (Y) is a cleaner fuel as it does not cause any pollution and is used for power generation. Bitumen (Z) is used for road surfacing nowadays.

Explanation:
In order to determine which statement is not true, let's analyze each statement one by one.

1. Metals in the pure state possess lustre:
This statement is true. Metals in their pure state are shiny and possess lustre. This property is due to the presence of free electrons that can reflect light.

2. Metals have low tensile strength:
This statement is also true. Metals are known for their high tensile strength, which means they can withstand a large amount of pulling force without breaking. This property makes metals suitable for various applications, such as construction and manufacturing.

3. Metals are non-sonorous:
This statement is false. Metals are actually sonorous, meaning they can produce sound when struck. This property is due to the presence of free electrons that can vibrate and create sound waves.

Therefore, the statement that is not true is:

d) 2 and 3

Because statement 2 is true (metals have high tensile strength) and statement 3 is false (metals are sonorous).

The correct answer is option 'C' - disturbance.

Explanation:

A disturbance is the cause of sound production in a medium. When an object vibrates or moves, it creates a disturbance in the surrounding medium. This disturbance then travels through the medium in the form of waves, which we perceive as sound.

Sound waves are mechanical waves that require a medium to travel through, such as air, water, or solids. When an object vibrates, it sets molecules in the medium in motion. These molecules then collide with neighboring molecules, transferring the energy and causing them to vibrate as well. This creates a chain reaction, resulting in the propagation of sound waves through the medium.

The sound waves consist of compressions and rarefactions. In a compression, the molecules are closely packed together, while in a rarefaction, the molecules are spread apart. As the disturbance travels through the medium, the molecules oscillate back and forth, creating a pattern of compressions and rarefactions.

These compressions and rarefactions of molecules are responsible for the transfer of energy in the form of sound. When the sound waves reach our ears, they cause the eardrums to vibrate, which is then interpreted by our brain as sound.

In summary, the sound is produced due to the disturbance created by the vibrating object in the medium. This disturbance sets the molecules in the medium in motion, creating compressions and rarefactions that propagate as sound waves.

Explanation:
Incorrect Statement:

a) The string of pendulum Y is longer than that of pendulum X.
- This statement is incorrect because the length of the string of a pendulum is directly proportional to its time period. A longer string would result in a longer time period, not a shorter one as mentioned in the statement.
Correct Statements:
b) The angle of swing of release of pendulum changes its time period.
- The angle of release does affect the time period of a pendulum. A larger angle would result in a longer time period, while a smaller angle would result in a shorter time period.
c) Pendulum Y has a shorter period than pendulum X.
- This statement is correct based on the information given in the question. Pendulum Y completes fewer oscillations in a shorter time period compared to pendulum X, indicating that pendulum Y has a shorter period.
d) The higher mass of the bob of the pendulum corresponds to its lower time period.
- This statement is correct because the mass of the bob does affect the time period of a pendulum. A higher mass results in a lower time period, while a lower mass results in a higher time period.

Understanding the Groups
In the given problem, we have three groups of substances classified based on their chemical properties. Each group contains either acids or bases, and we need to identify the odd one out in each group.
Group I Analysis
- Substances: Nitric acid, Sulphuric acid, Citric acid
- Classification: All are acids, but:
- Odd One Out: Citric acid
- Reason: Nitric and sulphuric acids are strong acids, while citric acid is a weak organic acid.
Group II Analysis
- Substances: Ammonia, Baking soda, Caustic soda
- Classification: All are bases, but:
- Odd One Out: Caustic soda
- Reason: Ammonia is a weak base, while baking soda (sodium bicarbonate) is a mild base. Caustic soda (sodium hydroxide) is a strong base.
Group III Analysis
- Substances: Oxalic acid, Tartaric acid, Carbonic acid
- Classification: All are acids, but:
- Odd One Out: Carbonic acid
- Reason: Oxalic and tartaric acids are organic acids, while carbonic acid is a weak acid formed from carbon dioxide in water.
Conclusion
After analyzing the groups, we find that:
- Group I: Citric acid is the odd one out.
- Group II: Caustic soda is the odd one out.
- Group III: Carbonic acid is the odd one out.
Thus, the correct answer is option B: Citric acid, caustic soda, and carbonic acid.

Coal and natural gas are exhaustible natural resources. CNG is less polluting than petrol as it does not produce many harmful gases. I and IV are correct statements.

2 and 3

Understanding the Combustion Concepts
Combustion is a chemical reaction that involves the rapid oxidation of a substance, usually accompanied by heat and light. The types of combustion can be categorized based on the conditions and products formed.

Matching Column I with Column II
Let’s break down the matches based on the definitions provided:

P. Innermost zone of flame
- This zone is characterized by very high temperatures where complete combustion occurs, resulting in minimal soot and gases.
**Match:** (iii) Complete combustion

Q. Rusting of iron
- Rusting is a slow oxidation process where iron reacts with oxygen and moisture over time, leading to the formation of rust (iron oxide).
**Match:** (iv) Slow combustion

R. Burning of LPG
- Liquefied Petroleum Gas (LPG) burns rapidly, typically resulting in complete combustion when there is sufficient oxygen, producing carbon dioxide and water.
**Match:** (i) Rapid combustion

S. Non-luminous zone of flame
- This zone burns without producing light, which means it is a part of the flame where the combustion is efficient and complete.
**Match:** (ii) No combustion

Final Matching Summary
- **P** matches with **(iii)** (Complete combustion)
- **Q** matches with **(iv)** (Slow combustion)
- **R** matches with **(i)** (Rapid combustion)
- **S** matches with **(ii)** (No combustion)
Thus, the final matches are:
- P - (iii)
- Q - (iv)
- R - (i)
- S - (ii)

Correct Option
Hence, the correct answer is option **B**: P - (ii), Q - (iv), R - (i), S - (iii).

Like charges repel while unlike charges attract each other. A charged glass rod has a positive charge on its surface, whereas a charged plastic straw has a negative charge on its surface, resulting in attraction. Lightning conductors are used at the top of buildings to pass electric charges to the ground to save the building. Earthquake risk areas can be identified, but accurate prediction in advance is very difficult and rare.