Everyday Science

Water reaches its maximum density at 4°C due to the unique hydrogen bonding in water molecules, which causes it to expand when cooled below this temperature, unlike most substances.

The Moon has an extremely thin and negligible atmosphere, often called an exosphere, composed mostly of trace amounts of gases like helium, neon, and hydrogen, but not in significant percentages like those listed. Therefore, none of the given compositions are correct, making option D the correct choice.

A raw apple (about 100 g) contains approximately 6 mg of calcium, 0.12 mg of iron, 11 mg of phosphorus, and only trace amounts of fluorine. While apples are not particularly rich in any mineral compared to other foods, among the options given, calcium is present in a higher amount than iron or fluorine. Phosphorus is also present but in a moderate amount. Therefore, calcium is the best answer among the options provided, though apples are not a rich source overall.

Fuse wires are typically made from an alloy of 63% lead and 37% tin because this composition has a low melting point suitable for protecting electrical circuits by melting quickly under excess current.

A chronometer is an instrument designed to measure time very precisely, often used in navigation and scientific experiments. Therefore, the correct answer is 'Time'.

Nicolaus Copernicus first proposed the heliocentric model placing the Sun at the center of the solar system in the early 16th century, but he did not provide definitive proof. Galileo Galilei provided strong observational evidence supporting heliocentrism through his telescopic discoveries, such as the phases of Venus and moons orbiting Jupiter, which challenged the geocentric model. However, it was Johannes Kepler who was the first scientist to mathematically prove that planets move around the Sun by formulating his three laws of planetary motion, demonstrating that planets follow elliptical orbits with the Sun at one focus. Kepler's laws provided the rigorous proof that firmly established the heliocentric model as scientifically valid.

The waterproof properties of materials used in rain-proof coats and tents primarily arise from their ability to repel water on the surface, which is largely due to surface tension effects. Water forms droplets on the surface instead of soaking in, preventing penetration. Additionally, waterproof fabrics often have tightly woven fibers, special coatings like polyurethane or durable water repellent (DWR), and sealed seams to enhance waterproofing. However, the fundamental principle that prevents water from soaking in initially is related to surface tension, which causes water to bead up and roll off the material.

Iapetus, one of Saturn’s moons, appears brightest when it is positioned west of Saturn. This happens because its brighter hemisphere faces Earth more directly in that position, making it easier to observe.

An atom is composed of three main kinds of subatomic particles: protons, neutrons, and electrons. These particles make up the structure of the atom.

Sailors' disease, also known as scurvy, is caused by a deficiency of Vitamin C. This vitamin is essential for collagen synthesis, and its lack leads to symptoms such as bleeding gums and weakness.