Melting and Boiling Point

Have you ever head of super heated water? When water boils, large air bubbles float to the surface and escape as water vapor. However, sometimes pure water might be able to be heated past it's boiling point because the bubbles have nowhere to form (called nucleation sites). When water is heated in a microwave, it might not be able to boil for this reason, However, if it bumped or another ingredient is added (like sugar), the water can begin to boil violently like an explosion!

Melting and Boiling Point Melting and Boiling Point

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Contents
Table of contents

    In this article, we will be learning about how substances boil (and melt) as well as the trends in boiling/melting point.

    • This article is about melting and boiling point
    • First, we will explain what melting and boiling are as we look at these processes
    • Next, we will look at the melting/boiling of metals
    • Then, we will look at boiling point elevation and see why adding a solid to a liquid solvent affects the boiling point
    • Lastly, we will look at the different trends in melting and boiling point

    What is the boiling point in chemistry?

    The boiling point is the temperature at which a liquid begins to convert to gas.

    As we discussed in the intro, boiling is when the liquid turns into a gas by forming bubbles that travel to the surface. Before we get ahead of ourselves, let's first talk about the processes of melting and boiling and what these points mean.

    Melting and boiling points of solids, liquids and gases

    The melting point is the temperature at which a solid begins to convert to a liquid.

    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)

    Melting and Boiling Point Different States of Matter StudySmarterFig.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:

    Melting and Boiling Point Water energy chart StudySmarterFig. 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.

    Metals have metallic bonding that holds them together.

    In a metal, there are delocalized electrons, meaning that there are electrons just floating around in the structure that do not "belong" to any one atom or molecule. The attraction between these electrons and the charged metal ions is incredibly strong, which is why they have high melting points.

    Boiling point elevation experiment

    Boiling point elevation refers to the fact that a liquid will boil at a higher temperature if a non-volatile solute (a species not easily vaporized) is dissolved in it.

    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:

    SolutionTrial 1Trial 2Trial 3Average
    Pure water100 °C100.2 °C99.7 °C100 °C
    Saltwater100.8 °C101.1 °C101.3 °C101.1 °C

    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, Kb 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.

    Kb 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.

    Groups 15, 16, and 17
    • Melting and boiling point increases as you move down a group
    • Increasing atomic size \(\rightarrow\) increases forces between particles \(\rightarrow\) harder to melt/boil
    Element (Group 16)Melting point (°C)Boiling Point (°C)
    O (Oxygen)-219-183
    S (Sulfur)113415
    Se (Selenium)221685

    Groups 1,2, 13, and 14

    • Melting and boiling point decreases as you move down the table
    • Increasing atomic size \(\rightarrow\) weakens metallic bonds \(\rightarrow\) easier to melt/boil

    Let's see another example:

    Element (Group 1)Melting point (°C)Boiling Point (°C)
    Li (Lithium)1811342
    Na (Sodium)99883
    K (Potassium)63759

    Transition metals are a bit trickier when it comes to trends:

    • Melting/Boiling point increases from group 3 to 6
    • It then decreases sharply at group 7
    • It increases at group 8, then continues to decrease
    • As you go down a group, the melting/boiling point increases, except for group 12 where it decreases

    Let's do a final example:

    ElementMelting point (°C)
    Ti1670
    V1910
    Cr1907
    Mn1246
    Fe1538
    Co1495

    For groups 5 and 6 elements, their melting/boiling points are approximately the same

    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.

    Melting and Boiling Point - Key takeaways

    • The melting point is the temperature at which a solid begins to convert to a liquid.
    • The boiling point is the temperature at which a liquid begins to convert to gas.
    • Metals have significantly higher melting points than other elements
    • Boiling point elevation refers to the fact that a liquid will boil at a higher temperature if a non-volatile solute (species not easily vaporized) is dissolved in it.
    • Melting/boiling point will increase when the forces between particles are stronger
    Frequently Asked Questions about Melting and Boiling Point

    What is the difference between melting and boiling point?

    The melting point is the temperature at which a solid starts becoming a liquid, and the boiling point is the temperature at which a liquid starts becoming a gas.

    What determines the melting and boiling points?

    The main factor that determines the melting and boiling points of a substance is the forces between its particles.

    What's meant by boiling point?

    The boiling point is the temperature at which a liquid begins to convert to gas. 

    What factors influence the boiling point?

    The boiling point of a liquid depends on the temperature, the atmospheric pressure (the pressure of the environment around the liquid) and the vapour pressure of the liquid. When the vapour pressure equals the atmospheric pressure, the liquid will reach its boiling point.

    Test your knowledge with multiple choice flashcards

    True or False: Once a system reaches the boiling point, all the liquid has become a gas

    True or False: The temperature of a system will change during melting/boiling

    Why does adding a solid solute to a liquid increase boiling point?

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