Water

The absolute necessity of Water in our lives and on our planet needs no explanation as such. However, we need to understand the structure and the properties of water to understand its chemical behaviour and its importance. Let us take a look at these.

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Physical Properties of Water

Water has some very important uses in our world. It is the universal liquid and will find a use in 99% of life’s functions. This unique ability of water to be so versatile and ubiquitous is due to some of its very impressive properties. These properties of water are often overlooked, but are the main reason for what makes water such a valuable compound,

• Chemical Formula: Chemical Formula of water as we are all aware is H2O. The molecule of water has covalent bonding between Hydrogen and Oxygen atoms. Two hydrogen atoms form a bond with a single atom of oxygen.
• Appearance: Water is colorless, odorless and tasteless liquid in its natural state.
• Boiling Point: As we know, water has a boiling point of 100 C. But this relatively high boiling point of water defies the trend in the periodic table. In comparison to Hydrogen Telluride and Hydrogen Sulphide, both of which have low boiling points, hydrogen (the next hydride) has a very high boiling point, This is because the hydrogen bonds in the water molecule are very strong, They require a huge amount of energy to break and start boiling.
• Freezing Point: The same concept applies to the freezing point of water as well. The freezing point of water is 0 C. The fact that water is very slow to freeze (or even boil for that matter) is very important for our ecosystem and our survival.
• Density: One unique property of water is that in the sold state, it is lense dense. Up to 4°C water’s density does increase on cooling. But after that point water becomes less dense. This is why ice floats in water,
• Viscosity: Water has high viscosity due to very strong intermolecular interactions
• Solvency: Water is an excellent solvent. In fact, it is known as a Universal Solvent. Due to a water molecule’s polarity, it can dissolve almost any substance.

Chemical Properties of Water

• Amphoteric Nature: One of the unique qualities of Water is its amphoteric nature. An amphoteric substance is one which can act as an acid or a base. While Water is neither acidic or basic it acts as both. This is because of its ability to both donate and accept protons. For acids stronger than water it acts as a base. And it acts like a acid to bases stronger than itself. These two following reactions show this amphoteric nature.

H₂O (l) + HCl (aq) ⇌ H₃O⁺⁺ Cl^– 

H₂O (l) + NH₃ (aq) ⇌ NH₄⁺ + OH^–

• Hydrolysis Reaction: Water has a very high dielectric constant. This results in it having a strong hydrating tendency. Water has strong reactions with ions of salts and creates hydrating shells around them.

SiCl4 + 2H2O → SiO2 + 4HCl

• Redox Reactions: Water is a great source to obtain dihydrogen since it can be reduced by reacting it with highly electropositive metals such as Sodium.

H2O + Na → 2NaOH + H2

Structure of Water

Closely observe the structure of a water molecule. You will see one atom of oxygen and two atoms of hydrogen. Each atom of hydrogen bonds covalently with the atom of oxygen. So both atoms of hydrogen share one pair of electrons with the oxygen atom.

Oxygen is a more electronegative element in comparison to water. This results in an uneven distribution of electron density. This gives the water molecule an angular bent structure. The H-O-H bond has a slight angle of about 104.5°. So it can be said that water molecule is polar. It has a slight negative polarity near the oxygen atom and slight positive charge near both the hydrogen atoms.

Structure of Ice

The structure of the molecules of water in its frozen form i.e. ice is very unique. It forms a Lattice Structure that does not generally occur naturally in any other substance other than ice.

When water reaches its freezing point its atoms rearrange themselves in a very specific three-dimensional pattern. The oxygen atom is surrounded by four hydrogen atoms. Two of these form O-H bonds normally seen in water molecules. The other two form a hydrogen bond.

This very special hexagonal shape is what gives ice the unique property of being less dense than water. Since in the structure of ice there are empty spaces between the hexagonal structure, its density is less than that of water in its liquid state. This is why ice floats on water.

Solved Question for You

Q: Which of the following are properties of water?
I. It has a permanent dipole moment attributed to its molecular structure.
II. It is a very good conductor of electricity.
III. It has its polar covalent bonds with hydrogen on opposite sides of the oxygen atom so that the molecule is linear.

1. I only
2. II only
3. Both I and II
4. Both II and III

Ans: The correct answer is “A”. The property I only can be assigned to water.

1. It has a permanent dipole moment attributed to its molecular structure. Water is polar in nature due to electronegativity difference between H atom and O atom. The individual bond dipoles do not cancel each other as water has a bent shape.
2. It is a non-conductor of electricity as it lacks ions.
3. It has its polar covalent bond between hydrogen and oxygen which forms V shape for the molecule so that the molecule is non-linear or bent.