Explain the following :
(i) Icebergs floating in the sea are dangerous for the ship.
(ii) An egg sinks in freshwater but floats in a strong solution of salt.
(iii) An iron nail sinks in water but a ship made of iron floats.
(iv) A ship sinks to a great depth in river water than in sea water.
(v) Iceberg floats in sea. Why ?
(vi) It is easier for a man to swim in sea water than in river water.
(vii) A dead body floats, with its head immersed in water.
(viii) A hydrometer is made heavy near the bottom.
(i) It is found that an iceberg floats in sea water with 90% of its volume below water and 10% of its volume above sea water. Icebergs are extremely dangerous for shipping as underwater ice can hit the ship and sink it.
(ii) When salt is mixed with water it increases the density of water and, hence, its density becomes greater than that of the density of egg which now floats in it.
(iii) If we place an iron nail on the surface of water, it sinks. This is because the density of iron is greater than that of water, so the weight of the nail is more than the upthrust of water on it. On the other hand, a ship made of iron does not sink. This is because the ship is hollow and the empty space contains air which makes the average density of the ship less than that of water. Therefore, even with a small part of it submerged into water, the weight of the water displaced becomes equal to the total weight of the ship and hence the ship floats.
(iv) The density of sea water, due to the presence of impurities like salt, etc., is greater than that of river water. Therefore, lesser volume of ship will be immersed in sea water to balance its weight.
(v) The iceberg has smaller density (0.917 g cm^-3) than sea water at the same temperature. The density of sea water is about 1.018 g cm-3, hence an iceberg floats with nearly one-twelfth of its volume outside water.
(vi) Due to the presence of minerals, the density of sea water, is more than the density of river water, therefore, upthrust is large. Hence, with a smaller portion submerged in sea water, the weight of sea water displaced is equal to the total weight of the body, while to displace the same weight of river water a larger portion of the body will have to be submerged in water. Therefore, it is easier to swim in sea water than in river water.
(vii) Dead bodies always float on the surface of water, but the head stays within water. The reason is that when the dead body decays its volume increases. Thus, it becomes lighter than water and, hence, floats. However, head being heavy cannot displace water more than its own weight, hence, it re plains underwater.
(viii) A hydrometer is made heavy near its bottom so that it can float with the stem in the vertical position.