Explanation: The speed of light is maximum in a vacuum (approximately 3 × 10⁸ m/s) and decreases as it passes through denser media. Among the given options — vacuum, air, water, and glass — light travels slowest in glass because glass has the highest refractive index (around 1.5). The greater the refractive index, the slower the light travels. This slowing down of light causes refraction, or bending of light, when it passes from one medium to another.

Explanation: Dissolved oxygen (DO) is the amount of oxygen available in water for aquatic organisms to breathe. For the healthy survival of most aquatic life, the DO level should be between 4 and 6 mg/L. Levels below 3 mg/L can lead to stress or suffocation, especially for fish and aerobic microorganisms. Higher DO levels are typical of clean, cold, and well-aerated waters, while polluted or stagnant waters often have lower oxygen levels due to organic decay.

Factors Affecting DO Levels:

• Temperature (higher temperature = lower DO)
• Organic pollution and bacterial activity
• Photosynthesis by aquatic plants and algae
• Water turbulence and aeration

Explanation: The Indian Space Research Organisation (ISRO) is headquartered in Bangalore (Bengaluru), Karnataka. It was established in 1969 under the guidance of Dr. Vikram Sarabhai, who is known as the father of the Indian space program. ISRO is India’s national space agency, responsible for space research, satellite launches, and planetary exploration. It developed major missions such as Chandrayaan (Moon mission), Mangalyaan (Mars Orbiter Mission), and PSLV & GSLV launch vehicles.

Explanation: The Sun is seen a few minutes before actual sunrise and after actual sunset due to the refraction of light by the Earth's atmosphere. The atmosphere bends the Sun’s rays towards the Earth, making the Sun appear slightly above the horizon even when it is below it.

Sodium bicarbonate (NaHCO₃), also known as baking soda, is used in fire extinguishers. When heated, it decomposes to produce carbon dioxide (CO₂), which displaces oxygen and helps smother the fire. It is especially effective for extinguishing electrical and small grease fires.

• Sodium carbonate: Used in glass and soap making, not for fire control.
• Sodium nitrate: An oxidizing agent, not suitable for extinguishing fires.
• Sodium sulphate: Used in detergents and paper making, not in fire extinguishers.

Explanation: The longest day in the Southern Hemisphere occurs on 22nd December, which marks the Summer Solstice in that hemisphere. On this date, the South Pole is tilted toward the Sun, resulting in longer daylight hours and shorter nights. Conversely, at the same time, the Northern Hemisphere experiences its winter solstice—the shortest day and longest night of the year. After 22nd December, the days in the Southern Hemisphere gradually begin to shorten.

Explanation: Plants absorb nitrogen from the soil mainly in the form of nitrates (NO₃⁻). Inside the plant, nitrates are reduced to nitrites (NO₂⁻) and then to ammonia (NH₃) through the action of specific reductase enzymes. The ammonia thus formed is then used to synthesize amino acids, proteins, nucleic acids, and other nitrogen-containing compounds necessary for plant growth.

Simplified Pathway:

NO₃⁻ → NO₂⁻ → NH₃ → Amino acids → Proteins

Explanation: Normal human blood has a slightly alkaline pH of 7.35 to 7.45. This pH is maintained by buffer systems such as the bicarbonate buffer system and controlled by the lungs (CO₂ regulation) and kidneys (acid-base balance). A deviation from this range can be life-threatening — acidosis occurs if pH falls below 7.35, and alkalosis occurs if it rises above 7.45.

Explanation: A barometer is an instrument used primarily for measuring atmospheric pressure. It helps in weather forecasting and determining altitudes (since atmospheric pressure decreases with height). A mercury barometer consists of a glass tube filled with mercury inverted in a mercury reservoir. When air pressure changes, the height of the mercury column changes accordingly. However, a barometer cannot measure the density of a liquid — that function is performed by a hydrometer.