Understanding Water's Molecular Structure and Polarity

Water ($\text{H}_2\text{O}$) is a unique molecule with many interesting properties. These properties largely stem from its structure and the nature of the bonds within it. A water molecule consists of one oxygen atom bonded to two hydrogen atoms.

Let's look at the bonds:

• The bonds between oxygen and hydrogen atoms within a water molecule are covalent bonds.
• However, oxygen is much more electronegative than hydrogen. This means oxygen attracts the shared electrons more strongly than hydrogen does.
• This unequal sharing of electrons creates partial charges on the atoms. The oxygen atom gets a partial negative charge ($\delta^-$), and the hydrogen atoms get partial positive charges ($\delta^+$).

Because of this bent molecular geometry (not linear) and the presence of these partial charges, the overall water molecule has a positive end and a negative end. This makes water a polar molecule.

Analyzing the Statements about Water Properties

We need to determine which of the given statements about water is incorrect.

1. Hydrogen bonds are present in liquid water
   Hydrogen bonding is a special type of intermolecular force that occurs when a hydrogen atom bonded to a highly electronegative atom (like oxygen, nitrogen, or fluorine) is attracted to a lone pair of electrons on another electronegative atom in a different molecule. Since water is polar and has hydrogen atoms bonded to oxygen, hydrogen bonds form between adjacent water molecules. These strong intermolecular forces significantly influence water's properties. This statement is true.
2. Water has a high boiling point
   Boiling point is the temperature at which a liquid turns into a gas. For water to boil, enough energy must be supplied to overcome the intermolecular forces holding the liquid molecules together. Because of the strong hydrogen bonds between water molecules, a significant amount of energy is required to break these bonds and allow the molecules to escape into the gas phase. This results in water having an unusually high boiling point compared to other molecules of similar size (like $\text{H}_2\text{S}$, which has weak intermolecular forces and a much lower boiling point). This statement is true.
3. Water has a high heat of fusion
   Heat of fusion is the amount of energy required to convert a solid (ice) into a liquid (water) at its melting point. In ice, water molecules are held in a rigid structure by hydrogen bonds. Melting requires breaking many of these hydrogen bonds to allow the molecules to move freely in the liquid state. The presence of strong hydrogen bonds in ice means that a large amount of energy is needed to melt it, resulting in a high heat of fusion. This statement is true.
4. Water is a nonpolar molecule
   As explained earlier, the bent shape and the unequal sharing of electrons in the $\text{H}_2\text{O}$ molecule lead to the formation of partial positive and negative poles. Therefore, water is a polar molecule, not a nonpolar molecule. This statement is false.

Conclusion

Based on the analysis of each statement and understanding water's molecular structure and intermolecular forces, the statement that is not true about water is that water is a nonpolar molecule.

Revision Table: Key Properties of Water

Additional Information on Water's Nature

The polar nature of water is fundamental to many of its roles in chemistry and biology. Because it is polar, water can effectively dissolve many ionic compounds and other polar molecules. This ability to dissolve a wide range of substances makes water an excellent solvent, often referred to as the "universal solvent". Nonpolar substances, like oils and fats, do not dissolve well in water because they lack the partial charges needed to interact favorably with polar water molecules.

The strong hydrogen bonding in water is also responsible for other properties, such as its high surface tension and cohesive and adhesive forces, which are crucial for processes like capillary action in plants.