C902 Middle School Science: Content Knowledge
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Free C902 Middle School Science: Content Knowledge Questions
Which is a key characteristic of a scientific theory?
- It is an educated guess
- It explains a wide range of observations and is supported by evidence
- It can never be changed
- It is the same as a hypothesis
Explanation
A scientific theory is a well-substantiated explanation of some aspect of the natural world that is based on a body of evidence and has been repeatedly tested and confirmed through observation and experimentation. Unlike a hypothesis, which is a tentative idea or educated guess, a theory explains a wide range of phenomena and can be refined as new evidence emerges. The strength of a scientific theory lies in its ability to make accurate predictions and provide a coherent framework for understanding observations.
Correct Answer:
It explains a wide range of observations and is supported by evidence
In the carbon cycle, which process releases carbon dioxide into the atmosphere?
- Photosynthesis
- Respiration
- Transpiration
- Nitrogen fixation
Explanation
Respiration is the process by which organisms, including plants and animals, break down glucose to produce energy in the form of ATP. During this process, carbon dioxide is released as a byproduct and returned to the atmosphere. Photosynthesis, in contrast, removes carbon dioxide from the atmosphere to produce glucose. Transpiration involves the movement of water from plants to the atmosphere and does not release carbon, while nitrogen fixation converts atmospheric nitrogen into forms usable by plants, unrelated to carbon dioxide release.
Correct Answer:
Respiration
In a balanced ecosystem, which group is responsible for recycling nutrients back into the soil?
- Producers
- Primary consumers
- Secondary consumers
- Decomposers
Explanation
Decomposers, such as bacteria and fungi, break down dead plants and animals, returning essential nutrients to the soil. This process recycles matter, making nutrients available for producers and maintaining the balance of the ecosystem. Producers create food through photosynthesis, primary consumers eat producers, and secondary consumers eat primary consumers, but these groups do not perform nutrient recycling.
Correct Answer:
Decomposers
Sound travels fastest through:
- Air
- Water
- Steel
- Vacuum
Explanation
Sound is a mechanical wave that requires a medium to travel. It moves fastest through solids because the particles are closely packed, allowing vibrations to transfer more efficiently from particle to particle. Steel, being a dense solid, provides the closest particle spacing and strongest intermolecular forces compared to liquids like water or gases like air. In a vacuum, there are no particles to transmit sound, so it cannot travel at all.
Correct Answer:
Steel
Which weather instrument measures air pressure?
- Anemometer
- Barometer
- Hygrometer
- Psychrometer
Explanation
A barometer measures air pressure, which is essential for forecasting weather and understanding atmospheric conditions. An anemometer measures wind speed, a hygrometer measures humidity, and a psychrometer is used to determine relative humidity using wet- and dry-bulb thermometers.
Correct Answer:
Barometer
The apparent path of the Sun across the sky changes throughout the year due to:
- Earth’s rotation only
- Earth’s revolution and axial tilt
- Moon’s gravity
- Precession
Explanation
The Sun’s apparent path changes because of Earth’s revolution around the Sun combined with its 23.5° axial tilt. This tilt causes variations in the angle and duration of sunlight at different times of the year, producing seasons. Earth’s rotation affects day and night but not the seasonal path, the Moon’s gravity affects tides, and precession is a slow wobble of Earth’s axis that affects long-term cycles, not annual changes in the Sun’s path.
Correct Answer:
Earth’s revolution and axial tilt
Science and technology have improved public health in Kenya through:
- Vaccination programs
- Mobile clinics using apps
- Both a and b
- None
Explanation
Science and technology have contributed significantly to public health improvements in Kenya. Vaccination programs prevent the spread of infectious diseases, while mobile clinics and health apps provide better access to healthcare services, especially in remote areas. Combining both approaches enhances the reach, efficiency, and effectiveness of healthcare delivery. Therefore, the most accurate answer reflects the combined impact of these innovations.
Correct Answer:
Both a and b
The asthenosphere allows tectonic plates to move because it is:
- Solid and rigid
- Partially molten and plastic
- Completely liquid
- Composed of granite
Explanation
The asthenosphere is a layer of Earth’s mantle that is partially molten and behaves plastically, allowing tectonic plates of the lithosphere to move over it. Its semi-fluid nature provides the flexibility necessary for plate tectonics. The lithosphere above is solid and rigid, the outer core is completely liquid, and granite is a rock found in the crust, not the mantle.
Correct Answer:
Partially molten and plastic
The Hardy-Weinberg principle predicts that allele frequencies will remain constant if which condition is violated?
- Random mating
- No natural selection
- Large population
- Migration
Explanation
The Hardy-Weinberg principle states that allele frequencies in a population remain constant from generation to generation only if certain conditions are met, including random mating, no natural selection, large population size, no migration, and no mutation. If any of these conditions are violated, such as non-random mating, natural selection, or migration, allele frequencies can change over time, leading to evolution.
Correct Answer:
Random mating
A transformer is used in the U.S. power grid primarily to:
- Increase frequency
- Change AC to DC
- Step voltage up or down for efficient transmission
- Store electrical energy
Explanation
Transformers are electrical devices used to change the voltage level of alternating current (AC) electricity without altering its frequency. In the U.S. power grid, transformers step up the voltage for long-distance transmission to reduce energy loss and step it down near homes and businesses for safe usage. They do not increase frequency, convert AC to DC, or store electrical energy. Their primary purpose is to improve transmission efficiency by adjusting voltage levels.
Correct Answer:
Step voltage up or down for efficient transmission
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