Melting and Boiling Point

What is the boiling point in chemistry?

Melting and boiling points of solids, liquids and gases

At these points, the change of state just starts to begin. So why is that? Well first, let's talk about the different states of matter: solid, liquid, and gas (shown below)

Fig.1 The different states of matter

In a solid, particles are held very close together and only have enough energy to vibrate in place. In a liquid, the particles are slightly farther apart and can change position with one another. Lastly, we have a gas, whose particles can move freely in their container.

The reason behind why each state has those particle characteristics are: temperature and attraction, and the interactions between particles.

• Particles/molecules have different forces that attract them together, they are the strongest in solids, weaker in liquids, and almost nonexistent in gases.
• Temperature is a measure of energy, so the greater the temperature, the more energy a species has.

So what does this mean for melting and boiling points? Well, these temperatures are the temperature marks at which the energy needed to overcome the interactions between particles is reached, so they can move farther apart (and be in the next state).

Typically, boiling points are significantly higher than melting points since you need to almost completely sever the forces between particles in gases, while liquids still have somewhat strong forces between them.

Melting and boiling processes

It's important to remember that these points mark the start of the melting/boiling process. It takes energy to move particles farther apart, and melting/boiling point marks when our system has just enough energy to start the task. As more energy is added to the system, the process continues until the system has completely changed state. Once that happens, the system can begin heating again.

For example, let's say you placed an ice cube in a glass on your kitchen counter. The ice cube will heat up until it begins to melt at 0°C. If you placed a thermometer in the water (while there is still some ice left), the water will still be at 0°C. This is because the energy added to the system (ice/water) is being used to change the ice into water, so it won't raise the temperature of the water.

Here is what this looks like in graph form:

Fig. 2. Melting and boiling point chart for water.

You'll see that the temperature stays stable for a while, then will increase as expected. The energy values listed are how much heat energy it takes for each process to complete.

Melting and boiling points of metals

As mentioned before, the interactions between particles play a huge part in how much energy it takes to change state. Because of this, metals have very high melting and boiling points.

Metals are large, crystalline lattice structures. This means the particles are in an ordered, grid-like pattern. It takes a lot of energy to keep structures organized like that, so likewise it takes a lot of energy to break them.

However, there are some exceptions like mercury, which has a melting point of -39ºC. For reference, copper has a melting point of 1085ºC.

Because of how strong the interactions in metals are, we often only talk about their melting points rather than also including their boiling points. This is because it is a lot less practical to boil most metals. For example, platinum has a boiling point of 3827ºC, while a blowtorch can reach about 1700ºC.

Boiling point elevation experiment

Let's do an experiment,

You take two beakers, each with 1 L of water. In the second pot, you add 116 grams of salt (NaCl) to the water. You place them each on a heating plate and put a thermometer in the water. After you turn on the heating plate to 120ºC, you watch each beaker. Once you notice the solution boiling (large bubbles rise from the bottom to the top), you mark the temperature on the thermometer. After three trials, you get these results:

Clearly, adding salt makes a difference, but why?

Liquids evaporate, meaning they can become gases without boiling. However, if there isn't enough volume, the gas will condense back into a liquid at the same rate it will evaporate. At this point, the pressure of the gas is called the vapour pressure.

When a liquid boils, the vapour pressure is equal to the pressure of the surrounding air (called atmospheric pressure). When a (solid) solute is added, it lowers the vapour pressure since the solute has 0 vapour pressure (i.e. it's a solid, so it can't evaporate). This means it takes more energy to increase the vapour pressure so boiling can occur.

Essentially, the solute "dilutes" the vapour pressure. This means that boiling point elevation is a colligative property, meaning it is based on the number of particles rather than their identity (i.e. salt versus sugar).

To get a better idea, let's look at the boiling point elevation formula:

$$\Delta T=i*K_b*m$$

Where \(\Delta T\) is the difference in boiling point (\(\Delta T=T_{sol}-T_{pure}\)), i is the Van't Hoff factor, K_b is a constant, and m is molality (mol of solute per kilogram of solvent).

The Van't Hoff factor tells us how many ions are formed when the solute dissolves. For example, NaCl has a Van't Hoff factor of 1.9 (usually simplified to 2), since it almost fully dissociates into Na⁺ and Cl^- in solution.

K_b is called the ebullioscopic constant. Its value is dependent on the identity of the solvent (for water, it is 0.512 K*kg/mol). It basically is a proportionality constant which "weighs" how important molality is to the boiling point elevation

Melting and boiling point examples

When we look at the melting/boiling points of elements, there are some general trends that emerge.

• Melting and boiling point increases as you move down a group
• Increasing atomic size \(\rightarrow\) increases forces between particles \(\rightarrow\) harder to melt/boil

Let's see another example:

All of these trends have to do with the metallic character of these elements. Typically, as the size of an element increases (going down a group), so does the metallic character, which makes these interactions stronger and harder to break (i.e. harder to melt).

However, as you go across the groups, we focus on the number of electrons. The strength of the interactions increases with the number of unpaired electrons. From group 3 to group 6, any added electrons are unpaired, so the interactions are strong. However, from group 7 onwards, the added electrons are paired up, so the interactions weaken. This is also why group 12 is different, since it has fully paired electrons.