Properties of Water

Structure of Water

The official name for water is dihydrogen monoxide. Looking more closely at this name gives us an idea of its structure. -hydrogen tells us that it contains hydrogen atoms, and di- indicates that it has two. -oxide refers to oxygen atoms, and mono- tells us that it has just one. Put this all together and we’re left with water: H₂O. Here it is, shown below:

Fig. 1 - A water molecule

Water consists of two hydrogen atoms joined to a central oxygen atom by single covalent bonds. The oxygen atom has two lone pairs of electrons. These squeeze the two covalent bonds tightly together, reducing the bond angle to 104.5° and making water a v-shaped molecule.

Fig. 2 - The bond angle in water

Bonding in Water

Let’s now look at how water’s structure affects its bonding.

Hydrogen bonds are a type of intermolecular force. They occur due to the difference in electronegativity between hydrogen and an extremely electronegative atom, such as oxygen.

As we know, water contains two hydrogen atoms bonded to a central oxygen atom by covalent bonds. Due to this, you’ll find hydrogen bonding between adjacent water molecules.

In the case of water, oxygen is a lot more electronegative than hydrogen. This means that oxygen pulls the bonded pair of electrons found in each of the oxygen-hydrogen bonds towards itself and away from hydrogen. The hydrogen becomes electron-deficient and we say that overall, the molecule is polar.

Because electrons have a negative charge, the oxygen is now slightly negatively charged and hydrogen slightly positively charged. We represent these partial charges with the delta symbol, δ.

Fig. 3 - The polarity of water

But how does this lead to the formation of hydrogen bonds? Well, hydrogen is a small atom. In fact, it is the smallest atom in the whole of the periodic table! This means that its partial positive charge is densely packed into one tiny space. We say that it has a high charge density. Because it is so positively charged, it is particularly attracted to negatively charged particles, such as other electrons.

What do we know about the oxygen atom in water? It contains two lone pairs of electrons! This means that hydrogen atoms in water molecules are attracted to the lone pairs of electrons in oxygen atoms in other water molecules.

Fig. 4 - Hydrogen bonding between water molecules

To summarise, we find hydrogen bonding when we have a hydrogen atom covalently bonded to an extremely electronegative atom with a lone pair of electrons. The hydrogen atom becomes electron-deficient and is attracted to the other atom’s lone pair of electrons. This is a hydrogen bond.

Physical Properties of Water

Now that we’ve covered the structure and bonding of water, we can explore how this affects its physical properties. In this next section, we’ll look at the following properties:

• Cohesion
• Adhesion
• Surface tension
• Specific heat capacity
• Melting and boiling points
• Density
• Ability as a solvent

Cohesive Properties of Water

If you splash a small amount of water across a surface, you’ll notice that it forms droplets. This is an example of cohesion. Instead of spreading out uniformly, water molecules stick to each other in clusters. This is due to the hydrogen bonding between neighbouring water molecules.

Adhesive Properties of Water

When you pour water into a test tube, you’ll notice that the water appears to climb up the edges of the vessel. It forms what is known as a meniscus. When you measure the volume of the water, you have to measure from the bottom of the meniscus in order for your measurements to be completely accurate. This is an example of adhesion. It occurs when water forms hydrogen bonds with another substance, such as the sides of the test tube in this case.

Fig. 5 - A meniscus

Surface Tension of Water

Have you ever wondered how insects are able to walk across the surface of puddles and lakes? It is due to surface tension.

This is where the particles on the surface of a liquid are strongly attracted to the other particles in the liquid. These outer particles are pulled into the bulk of the liquid, making the liquid take the shape with the least surface area possible. Due to this attraction, the surface of the liquid is able to withstand external forces, such as the weight of an insect. Water has a particularly high surface tension due to hydrogen bonding between its molecules. This is another example of the cohesive nature of water.

Specific Heat Capacity of Water

Remember that a change of one degree Kelvin is the same as a change of one degree Celsius.

Changing the temperature of a substance involves breaking some of the bonds within it. Hydrogen bonds between water molecules are very strong and so require a lot of energy to break. This means that water has a high specific heat capacity.

Melting and boiling points of water

Water has high melting and boiling points due to the strong hydrogen bonds between its molecules, which require a lot of energy to overcome. This becomes apparent when you compare water to similar-sized molecules that don’t experience hydrogen bonds. For example, methane (CH₄) has a molecular mass of 16 and a boiling point of -161.5 ℃, whereas water has a similar molecular mass of 18, but a much higher boiling point of exactly 100.0 ℃!

Density of water

You might know that most solids are denser than their respective liquids. However, water is a bit unusual - it is the other way round. Solid ice is a lot less dense than liquid water, which is why icebergs float at the top of the sea instead of sinking to the ocean floor. To understand why, we need to look more closely at water’s structure in the two states.

Liquid water

As a liquid, water molecules are constantly moving about. This means that the hydrogen bonds between the molecules are constantly being broken and reformed again. Some of the water molecules are very close together whilst others are further apart.

Solid ice

As a solid, water molecules are fixed into position. Each water molecule is bonded to four adjacent water molecules by hydrogen bonds, holding it in a lattice structure. The four hydrogen bonds mean that the water molecules are held a fixed distance from each other. In fact, in this solid state, they are held further apart than in their liquid form. This makes solid ice less dense than liquid water.

Fig. 6 - An ice lattice

Water as a solvent

The final physical property that we’ll look at today is water’s ability as a solvent.

Water is often referred to as the universal solvent. This is because it can dissolve a wide range of different substances. In fact, almost all polar substances dissolve in water. This is because water molecules are also polar. Substances dissolve when the attraction between them and a solvent is stronger than the attraction between solvent molecule and solvent molecule, and solute molecule and solute molecule.

In the case of water, the negative oxygen atom is attracted to any positively charged solute molecules, and the positive hydrogen atoms are attracted to any negatively charged solute molecules. This attraction is stronger than the forces holding the solute together, so the solute dissolves.

Chemical Properties of Water

All the ideas we explored above were examples of physical properties. These are properties that can be observed and measured without changing the chemical composition of the substance. For example, the water molecules in steam have the exact same chemical identity as the water molecules in ice - the only difference is their state of matter. However, chemical properties are properties that we see when a substance undergoes a chemical reaction. We’re going to focus on two of water’s chemical properties in particular.

• Ability to self-ionise
• Amphoteric nature

Self-ionisation of water

As a liquid, water exists in an equilibrium. Most of its molecules are found as neutral H₂O molecules, but some ionise into hydronium ions, H₃O⁺, and hydroxide ions, OH^-. The molecules are constantly switching backwards and forwards between these two states, as shown by the equation below:

2H₂O ⇋ H₃O⁺ + OH^-

This is known as self-ionisation. Water does this all by itself - it doesn’t need another substance to react with.

Amphoteric Nature of Water

Because water self-ionises, as we saw above, it can act amphoterically.

How does water do this? Well, look at the ions it forms when it self-ionises: H₃O⁺ and OH^- . The hydronium ion, H₃O⁺, can act as an acid by losing a proton to form H₂O and H⁺. The hydroxide ion, OH^-, can act as a base by accepting a proton, forming H₂O once again.

H₃O⁺ → H₂O + H⁺

OH^- + H⁺ → H₂O

If water reacts with other bases, it acts as an acid by donating a proton. If it reacts with other acids, it acts as a base by accepting a proton. You could say that water isn’t fussy - it just wants to react with everyone!