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All questions of Chemical changes for Class 8 Exam
A pure substance has a melting point which is always .......- a)Higher than its freezing point
- b)Higher than its boiling point
- c)Different
- d)The same
Correct answer is option 'D'. Can you explain this answer?
A pure substance has a melting point which is always .......
a)
Higher than its freezing point
b)
Higher than its boiling point
c)
Different
d)
The same
 | Kritika Nambiar answered |
Understanding Melting and Freezing Points
When discussing the properties of a pure substance, it's essential to understand the concepts of melting and freezing points. These two terms describe the same physical process but from different perspectives.
Definition of Melting Point
- The melting point of a substance is the temperature at which it transitions from a solid to a liquid state.
- At this temperature, the solid's particles gain enough energy to overcome the forces holding them together.
Definition of Freezing Point
- The freezing point is the temperature at which a liquid turns into a solid.
- It occurs when the particles lose energy and come close enough to form stable bonds.
Key Relationship
- For pure substances, the melting point and freezing point occur at the same temperature.
- This means that when a pure substance melts at a certain temperature, it will freeze at that exact temperature when cooled.
Implications of the Same Temperature
- This relationship is significant because it confirms that the phase transition is reversible.
- The energy changes during melting and freezing are equal in magnitude but opposite in direction.
Conclusion
Thus, the correct answer is option 'D': the melting point is the same as the freezing point for a pure substance. Understanding this concept is fundamental in the study of materials and their properties in science.
When discussing the properties of a pure substance, it's essential to understand the concepts of melting and freezing points. These two terms describe the same physical process but from different perspectives.
Definition of Melting Point
- The melting point of a substance is the temperature at which it transitions from a solid to a liquid state.
- At this temperature, the solid's particles gain enough energy to overcome the forces holding them together.
Definition of Freezing Point
- The freezing point is the temperature at which a liquid turns into a solid.
- It occurs when the particles lose energy and come close enough to form stable bonds.
Key Relationship
- For pure substances, the melting point and freezing point occur at the same temperature.
- This means that when a pure substance melts at a certain temperature, it will freeze at that exact temperature when cooled.
Implications of the Same Temperature
- This relationship is significant because it confirms that the phase transition is reversible.
- The energy changes during melting and freezing are equal in magnitude but opposite in direction.
Conclusion
Thus, the correct answer is option 'D': the melting point is the same as the freezing point for a pure substance. Understanding this concept is fundamental in the study of materials and their properties in science.
Which of the following is an example of a chemical change?- a)Melting ice
- b)Boiling water
- c)Burning wood
- d)Tearing paper
Correct answer is option 'C'. Can you explain this answer?
a)
Melting ice
b)
Boiling water
c)
Burning wood
d)
Tearing paper
 | Subset Academy answered |
Burning wood results in new substances being formed, which is a characteristic of a chemical change.
Which of the following best describes an endothermic reaction?- a)Heat is released
- b)Light is produced
- c)Heat is absorbed
- d)Sound is produced
Correct answer is option 'C'. Can you explain this answer?
a)
Heat is released
b)
Light is produced
c)
Heat is absorbed
d)
Sound is produced
| | Subset Academy answered |
An endothermic reaction absorbs heat from the surroundings.
What is the correct word equation for the combustion of methane?- a)Methane + Water → Carbon dioxide + Oxygen
- b)Methane + Oxygen → Carbon dioxide + Water
- c)Methane + Carbon dioxide → Water + Oxygen
- d)Methane + Oxygen → Water + Carbon monoxide
Correct answer is option 'B'. Can you explain this answer?
a)
Methane + Water → Carbon dioxide + Oxygen
b)
Methane + Oxygen → Carbon dioxide + Water
c)
Methane + Carbon dioxide → Water + Oxygen
d)
Methane + Oxygen → Water + Carbon monoxide
| | Subset Academy answered |
The word equation for the combustion of methane is Methane + Oxygen → Carbon dioxide + Water.
Which of the following is not evidence of a chemical change?- a)Production of a gas
- b)Change in color
- c)Change in state (solid to liquid)
- d)Formation of a precipitate
Correct answer is option 'C'. Can you explain this answer?
Which of the following is not evidence of a chemical change?
a)
Production of a gas
b)
Change in color
c)
Change in state (solid to liquid)
d)
Formation of a precipitate
 | Torcia Education answered |
A change in state (solid to liquid) (option C) is a physical change because the substance remains chemically the same (ice melting to water, for example). Options A, B, and D are evidence of chemical changes: production of a gas (option A) indicates a chemical reaction, change in color (option B) can indicate a change in chemical composition, and formation of a precipitate (option D) suggests a chemical reaction has occurred.
How can you tell if a process is exothermic?- a)The temperature of the surroundings decreases
- b)The temperature of the surroundings increases
- c)There is no temperature change
- d)The substance changes color
Correct answer is option 'B'. Can you explain this answer?
a)
The temperature of the surroundings decreases
b)
The temperature of the surroundings increases
c)
There is no temperature change
d)
The substance changes color
| | Subset Academy answered |
An exothermic process releases heat, causing the temperature of the surroundings to increase.
Which of the following statements best describes a chemical change?- a)It can be easily reversed by physical means.
- b)It involves a change in physical state.
- c)It results in the formation of new substances.
- d)It only affects the appearance of a substance.
Correct answer is option 'C'. Can you explain this answer?
Which of the following statements best describes a chemical change?
a)
It can be easily reversed by physical means.
b)
It involves a change in physical state.
c)
It results in the formation of new substances.
d)
It only affects the appearance of a substance.
 | Tutorpedia Coaching answered |
A chemical change (option C) involves a chemical reaction where one or more substances are converted into different substances with different properties. Options A, B, and D describe characteristics of physical changes rather than chemical changes: option A describes reversibility (physical change), option B describes a change in state (physical change), and option D describes a change in appearance (physical change).
What is the balanced chemical equation for the combustion of methane?- a)CH4 + O2 → CO2 + H2
- b)CH4 + 2O2 → CO2 + 2H2O
- c)CH4 + CO2 → H2O + O2
- d)CH4 + H2O → CO2 + O2
Correct answer is option 'B'. Can you explain this answer?
What is the balanced chemical equation for the combustion of methane?
a)
CH4 + O2 → CO2 + H2
b)
CH4 + 2O2 → CO2 + 2H2O
c)
CH4 + CO2 → H2O + O2
d)
CH4 + H2O → CO2 + O2
 | Trisha Vashisht answered |
The combustion of methane is represented as CH4 + 2O2 → CO2 + 2H2O.
Which mixture is an example of a homogeneous mixture?- a)Sand and iron filings
- b)Oil and water
- c)Salt dissolved in water
- d)Rice and beans
Correct answer is option 'C'. Can you explain this answer?
a)
Sand and iron filings
b)
Oil and water
c)
Salt dissolved in water
d)
Rice and beans
| | Subset Academy answered |
A homogeneous mixture has a uniform composition throughout, like salt dissolved in water.
Which of the following is an example of a physical change?- a)Rusting of iron
- b)Burning wood
- c)Melting ice
- d)Combustion of methane
Correct answer is option 'C'. Can you explain this answer?
Which of the following is an example of a physical change?
a)
Rusting of iron
b)
Burning wood
c)
Melting ice
d)
Combustion of methane
| | Trisha Vashisht answered |
Melting ice is a physical change because only the state of water changes, while its chemical composition remains the same. Options A, B, and D are chemical changes.
Which of the following is a pure substance?- a)Saltwater
- b)Air
- c)Carbon dioxide
- d)Milk
Correct answer is option 'C'. Can you explain this answer?
a)
Saltwater
b)
Air
c)
Carbon dioxide
d)
Milk
| | Subset Academy answered |
Carbon dioxide is a compound and is considered a pure substance, whereas the others are mixtures.
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