Explanation: The spectrophotometer measures bacterial growth by determining the optical density (OD) or turbidity of a culture. As bacteria multiply, the suspension becomes more turbid and absorbs more light. The absorbance value is proportional to the number of bacterial cells present.

Explanation: The age of a tree is determined by counting the annual growth rings (also called annual rings) visible in a cross-section of its trunk. Each ring consists of a light band (spring wood) and a dark band (autumn wood), representing one year of growth. The study of these rings to determine a tree’s age and past climate conditions is known as dendrochronology.

Explanation: According to the Bureau of Indian Standards (BIS) – IS 10500:2012, the acceptable limit of total hardness in drinking water, expressed as CaCO₃, is 200 mg/l. The permissible limit in the absence of an alternate source is up to 600 mg/l. Excess hardness can cause scaling and affect the taste of water.

When an ant bites, it releases formic acid (HCOOH) into the skin. This acid causes a sharp burning sensation, redness, and irritation at the site of the bite. The name “formic” comes from the Latin word “formica”, meaning ant, as the acid was first extracted from ants. The pain can be neutralized by applying a mild base like baking soda (sodium bicarbonate).

Other options (for context)

• Hydrochloric Acid: Found in the human stomach, aids digestion.
• Acetic Acid: Main component of vinegar, not involved in ant bites.
• Phosphoric Acid: Used in fertilizers and soft drinks, not produced by ants.

Explanation: To read small letters, a convex lens (converging lens) with a short focal length such as 5 cm is used as a magnifying glass. It produces an enlarged, upright, and virtual image of the object, making tiny text appear bigger and clearer to the eye.

Explanation: Kala-azar, also known as visceral leishmaniasis, is caused by the protozoan parasite Leishmania donovani. It is transmitted through the bite of the female Phlebotomus sandfly. The disease primarily affects the liver, spleen, and bone marrow, leading to symptoms such as fever, anemia, weight loss, and darkening of the skin. The term “Kala-azar” literally means “black fever.”

Sputnik I was the first artificial satellite to orbit the Earth. It was launched by the Soviet Union on October 4, 1957. The successful launch of Sputnik I marked the beginning of the space age and initiated the space race between the USA and the USSR.

• Explorer I: First U.S. satellite, launched in 1958.
• Vanguard I: A U.S. satellite launched later in 1958.
• Discoverer I: Part of an American reconnaissance satellite program (1959).

Explanation: Acids do not turn red litmus to blue; they turn blue litmus paper red. Acids are sour in taste, corrosive in nature, and have a pH value less than 7. The statement that acids turn red litmus to blue is incorrect.