General Science

Concept: Sublimation is the process in which a substance changes directly from a solid state to a gaseous state without passing through the liquid phase. This occurs when molecules of a solid gain enough energy to escape directly into the gas phase. Common examples include:

• Dry ice (solid )
• Camphor
• Naphthalene
• Iodine crystals

Step 1: Identify the phase change.
Solid → Gas without becoming liquid is called sublimation.
Step 2: Eliminate other options.

• Condensation is gas → liquid.
• Evaporation is liquid → gas.
• Deposition is gas → solid.

Thus, the direct change of a solid into gas is called sublimation.

Concept: Myopia (nearsightedness) is a vision defect in which a person can see nearby objects clearly but distant objects appear blurred. This occurs when the image of distant objects forms in front of the retina instead of on the retina. A concave lens (diverging lens) is used to correct this defect.
Step 1: Understanding how the concave lens corrects vision.
A concave lens diverges incoming light rays so that they appear to come from a point farther away. This adjustment allows the image to form correctly on the retina.
Step 2: Eliminating other options.

• Convex lens is used to correct hypermetropia (farsightedness).
• Cylindrical lens is used for astigmatism.
• Bifocal lenses correct both near and distant vision problems.

Thus, myopia is corrected using a concave lens.

Concept: The pH scale measures the acidity or basicity of a solution and ranges from 0 to 14.

• pH<7 → Acidic
• pH = 7 → Neutral
• pH>7 → Basic (alkaline)

Pure water at room temperature ( ) has equal concentrations of hydrogen ions ( ) and hydroxide ions ( ), making it neutral.
Step 1: Understand neutrality of pure water.
In pure water: This balance results in a neutral pH.
Step 2: Determine the neutral value on the pH scale.
The neutral point on the pH scale is 7. Therefore, pure water has a pH of 7.

Concept: Mars is commonly known as the "Red Planet" because of its reddish appearance when observed from Earth. The red color is caused by the presence of iron oxide (rust) on the planet’s surface. Iron minerals in Martian soil react with oxygen and form iron oxide, giving the planet its distinctive red color.
Step 1: Identify the planet with reddish appearance.
Mars appears red due to iron oxide dust covering much of its surface.
Step 2: Eliminate other options.

• Venus is known for its thick atmosphere and extreme temperature.
• Jupiter is the largest planet with prominent gas bands.
• Saturn is known for its extensive ring system.

Thus, the planet known as the Red Planet is Mars.

Concept: Photosynthesis is the process by which green plants convert light energy from the sun into chemical energy stored in glucose. This process takes place in the chloroplasts of plant cells using the pigment chlorophyll. The simplified photosynthesis equation is:
Step 1: Understand the energy conversion.
During photosynthesis, plants use sunlight to convert carbon dioxide and water into glucose and oxygen.
Step 2: Eliminate other options.

• Respiration releases energy from food.
• Transpiration is the loss of water vapor from plant leaves.
• Fermentation occurs in microorganisms under anaerobic conditions.

Thus, the process converting light energy into chemical energy is photosynthesis.

Concept: Newton’s Third Law of Motion states that: This means that whenever one object exerts a force on another object, the second object exerts an equal force in the opposite direction on the first object.
Step 1: Understanding the action–reaction principle.
If object A applies a force on object B, then object B simultaneously applies an equal force on object A in the opposite direction.
Step 2: Examples of Newton’s Third Law.

• A rocket moves upward because gases push downward.
• When walking, the foot pushes the ground backward and the ground pushes the body forward.

Step 3: Eliminating other options.

• Newton’s First Law explains inertia.
• Newton’s Second Law relates force, mass, and acceleration.
• Law of Gravitation explains gravitational attraction between masses.

Thus, the law stating that every action has an equal and opposite reaction is Newton’s Third Law of Motion.

Concept: Plant cells contain a rigid outer layer called the cell wall. It surrounds the cell membrane and provides structural support, protection, and shape to the cell. The cell wall is primarily composed of cellulose, a complex carbohydrate that gives the cell strength and rigidity.
Step 1: Understanding the function of the cell wall.
The cell wall helps the plant cell maintain its shape and protects it from mechanical damage and excessive water intake.
Step 2: Eliminating other options.

• Cell membrane controls the movement of substances in and out of the cell.
• Vacuole stores water and nutrients.
• Nucleus controls genetic activities of the cell.

Thus, the structure responsible for rigidity and protection in plant cells is the cell wall.

Concept: The greenhouse effect is a natural process in which certain gases in the Earth's atmosphere trap heat and keep the planet warm enough to sustain life. These gases are known as greenhouse gases. Major greenhouse gases include:

• Carbon dioxide ( )
• Methane ( )
• Nitrous oxide ( )
• Water vapor

Among these, carbon dioxide is the most significant contributor to the greenhouse effect due to its large quantity in the atmosphere and its release from human activities such as burning fossil fuels and deforestation.
Step 1: Identify the main greenhouse gas.
Carbon dioxide absorbs infrared radiation emitted by the Earth and traps heat within the atmosphere.
Step 2: Eliminate other options.

• Oxygen and Nitrogen make up most of the atmosphere but do not significantly trap heat.
• Hydrogen is not a greenhouse gas in Earth's atmosphere.

Thus, the gas primarily responsible for the greenhouse effect is Carbon dioxide.

Concept: Electrical resistance is the property of a material that opposes the flow of electric current through it. The SI unit of electrical resistance is the Ohm, represented by the symbol . Resistance is related to voltage and current through Ohm’s Law: where:

• = Voltage (Volts)
• = Current (Amperes)
• = Resistance (Ohms)

Step 1: Identify the SI unit for resistance.
The standard SI unit used to measure resistance is the Ohm ( ).
Step 2: Eliminate other options.

• Volt measures electrical potential difference.
• Ampere measures electric current.
• Watt measures electrical power.

Therefore, the correct unit for electrical resistance is the Ohm.