Why does ice float on water?

To first understand why ice floats on water we must understand the term “Density”. It is a property of matter that describes how much weight that a unit volume of the particular matter will possess. Mathematically it is represented as the ratio of weight to the volume occupied (i.e., Density = Weight/Volume). Density is constant for a given material irrespective of the amount of it taken. Hence it is called an intrinsic property of matter (a property of matter that does not depend upon the amount of substance taken.). In the 16thcentury, Archimedes defined the conditions for a substance to float on water by observing the weight of water displaced by that object. If the object displaces water lesser than its weight, the body floats and sinks when the weight of water displaced is more than the weight of the body.



Why does Ice float?When we throw a stone in deep water, we notice that it sinks to the bottom. This is observed since the stone has more density than water. The density of pure water is 1kg/Litre. But the density in its solid state (ice) is 9% lesser than the liquid form. Generally when liquids solidify the distance between its molecules decreases causing an increase in the density since more number of molecules will be present in the same volume increasing the total weight.It also causes a contraction in volume. But water being a peculiar liquid, expands on being solidified. The same weight of the water cooled now occupies a larger volume causing a fall in its density by 9% i.e. when 9 litres of water is solidified; it expands to 10 litres of solid ice. When submerged in water, ice displaces water equal to 9/10th of its weight and therefore 1/10th part of the ice floats above water.This is proved mathematically by applying the principle of buoyancy or “Archimedes’ Principle”.



A water molecule is chemically composed of one oxygen atom and two hydrogen atoms joined together by covalent bonds (bonds formed by mutual sharing of electrons). Oxygen is a highly electro-negative element meaning that it has more affinity to gain electrons. Due to this character of oxygen, a chemical bond phenomenon known as “Hydrogen Bonding” occurs between water molecules. This is caused when a hydrogen atom of one molecule forms a weak chemical bond with an electronegative element of the neighbouring molecule.When water is cooled to 4o Celsius, oxygen atoms are separated from their hydrogen bonds to form crystals of ice. Due to this the distance between the molecules in the crystal structure increases leading to an increase in volume while the weight remains constant.This causes the water to expand on freezing making it lighter than water.



A further examination on the behaviour of water in relating its solid and liquid state can be understood by referring the phase diagram of water. The phase diagram of water is a graph plotted between pressure and temperature by marking the values at which two states or even all the three states of water exists in an equilibrium with each other. The phase diagram is extremely useful in predicting the state at which water will exist at a given set of temperature and pressure. Analysis of the slopes of the graph also supports how ice floats on water by obtaining a relation between pressure and temperature. When the relation between pressure and temperature are obtained, it is correlated with density using various thermodynamic and fluidic equations.



This unique property of water is the reason for the existence of the continent of Antarctica which is just a humongous block of ice floating on the Antarctic Ocean. A small chemical property affecting at the molecular level causes changes in such an enormous level. This property also serves useful by enabling people to cross water frozen water bodies easily. But also these ice masses pose a high risk of accidents to aquatic vehicles such as ships and boats. An example of this was the accident of the famous “Titanic” which collided with an ice berg in the arctic region and lead to death of many people.

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