Why does Ice float on water & stone doesn’t ?

Many of us at some point in our lives must have wondered: Why does ice float on water? Why doesn’t a stone? It’s common knowledge that solids are denser than their liquid counterparts. Then why? To understand better, lets break the question into two parts:

Why does an object float on water?

Something floats if it is comparatively less dense than the other component in the mixture. By dense we mean mass per unit volume (Density) For example, If you place a piece of paper on water, it floats. On the contrary, if you place a stone on water, it sinks.

In the first case, piece of paper which is less dense as compared to water, will float. In the second case, stone is more dense as compared to water. Hence it wont float. Whenever an object is immersed in a liquid, it displaces some of the liquid. For the object to be able to float, it has to displace a weight of fluid equal to its own weight. If the weight of the displaced liquid is less that the weight of the object, the object will sink. This explains the Archimedes Principle.

Archimedes Principle states that: “An object completely or partially submerged in a fluid (gas or liquid) at rest is acted upon by an upward force the magnitude of which is equal to the weight of the fluid displaced by the body.” The upward force is called Buoyant Force & the phenomenon is called Buoyancy.

Ice floats on Water

Why does ice float on water?

i. Most substances have maximum density in their solid (frozen) state than in their liquid state.‌ For water it’s not the case.

ii. Anomalous behavior of Water: Water achieves its maximum density at 4°C. As it cools further & freezes into ice, it expands & starts losing density. This happens because of hydrogen bonding.

iii. A water molecule is made up of two Hydrogen atoms & one Oxygen atom, strongly bound to each other via Covalent Bonds. Individual water molecules are also attracted to each other by comparatively weaker Hydrogen Bonds between the slightly positive Hydrogen atoms & slightly negative Oxygen atoms.

iv. In liquid water, hydrogen bonds connect each water molecule to approximately 3 to 4 other water molecules.

v. When water freezes into ice, it crystallizes into a rigid lattice (which we call Ice) with hydrogen bonds adjusting to keep the negatively charged oxygen atoms apart.

vi. Ice floats because it is about 9% less dense than liquid water. Its displaces water which is abut 9% more that the weight of the ice.

Effect of Anomalous Expansion of Water:

1. Marine life is preserved at freezing temperatures.

During winter, the water in lakes & rivers starts cooling downwards (temperature gradient is setup). On reaching 4°C, the surface water descends to the bottom as it is denser. On further cooling to freezing point, bottom layer changes to ice & rises up. The surface water comes down. Water & ice being poor conductors of heat, help maintaining the temperature of the water below the surface at 4°C. This is how marine life survive freezing winters.

2. Bursting of plumbing pipes in cold countries.

Its a common occurrence in cold countries. When temperatures go below 4°C, the water in the plumbings starts freezing & hence expanding. This increases the pressure inside the pipes. When the pipes can no longer contain it, they burst.


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