Explanation

In 1796, Edward Jenner successfully used material from cowpox to vaccinate against smallpox.

This was the first successful vaccination in medical history.

Explanation

Vitamin K is essential for the synthesis of clotting factors in the blood.

Its deficiency leads to prolonged bleeding or poor blood clotting.

Explanation

In the modern periodic table, there are 7 periods (horizontal rows) and 18 groups (vertical columns).

Explanation

NO₂ (Nitrogen dioxide) is a covalent compound, made of nonmetals (nitrogen and oxygen) sharing electrons.

The others like (NH₄)₂CO₃, HNO₃, and AgCl involve ionic bonding either fully or in part (e.g., NH₄⁺ acts ionically).

Explanation

Dative bond: This is a type of covalent bond where one atom donates both electrons to the bond, forming a coordinate covalent bond.

In this case, NH₃ (ammonia) (with a lone pair of electrons) acts as the electron donor, and BF₃ (boron trifluoride) (electron deficient) acts as the electron acceptor.

Explanation

John Dalton proposed the Atomic Theory in the early 1800s, stating that all elements are made of indivisible atoms.

He described atoms as indestructible and identical for each element, laying the foundation of modern chemistry.

Explanation

A reaction in which heat is absorbed is called an endothermic reaction.

These reactions lower the temperature of their surrounding area, thereby creating a cooling effect.

Explanation

Given y = 3:

Step 1: Calculate y³.

y³ = 3³ = 27

Step 2: Calculate y³ - y.

y³ - y = 27 - 3 = 24

Step 3: Calculate y³(y³ - y).

y³(y³ - y) = 27 × 24

= 648

Explanation

When a projectile moves upward, gravity acts downward.

This opposes the vertical motion, so the vertical component of velocity decreases until it becomes zero at the highest point.

Explanation

20 men can construct a building in 40 days.

Total work = 20 men × 40 days = 800 man-days.

Time = Total work / Number of men = 800 man-days / 10 men = 80 days.