# Why specific latent heat of vaporisation > fusion?

• Janiceleong26
In summary, the conversation discusses the first law of thermodynamics and the relationship between changes in internal energy, volume, and pressure. The main point is that when a liquid evaporates, the increase in volume and potential energy is greater than when a solid melts. There is also a discussion about the correct sign for work done on the system. The conclusion is that the work done should be negative since it is done by the system.
Janiceleong26
1. Homework Statement

## Homework Equations

First law of thermodynamics, ΔU=q+W

## The Attempt at a Solution

Ok so, I know that when liquid evaporates, the change in volume is much greater than that when solid melts. And for both cases, distance of separation of atoms increases too, so PE increases, and hence, internal energy increases.
So, ΔU=mL+ pΔV
L=(ΔU-pΔV) /m
But how do we know that the term (ΔU-pΔV) is greater for vaporisation than that of fusion?

Janiceleong26 said:

Ok so, I know that when liquid evaporates, the change in volume is much greater than that when solid melts.
Well, that is in fact the main essence of the answer.

Janiceleong26 said:
So, ΔU=mL+ pΔV
There is a sign problem here. The work done on the system ##W## should be ## - p \Delta V## - do you see why?

Fightfish said:
Well, that is in fact the main essence of the answer.There is a sign problem here. The work done on the system ##W## should be ## - p \Delta V## - do you see why?
Ahh I see why now. Yup, because it's work done by system.
Thanks !

## 1. Why is the specific latent heat of vaporisation greater than the specific latent heat of fusion?

The specific latent heat of vaporisation is greater than the specific latent heat of fusion because it takes more energy to convert a substance from its liquid state to its gaseous state than it does to convert it from its solid state to its liquid state. This is because in the process of vaporisation, the molecules of the substance have to overcome the intermolecular forces that hold them together in the liquid state, while in the process of fusion, the molecules only have to overcome the weaker intermolecular forces that hold them together in the solid state.

## 2. What is the significance of the specific latent heat of vaporisation being higher than the specific latent heat of fusion?

The higher specific latent heat of vaporisation means that it takes more energy to vaporise a substance than it does to melt it. This is an important factor in many natural processes, such as the water cycle, where the high specific latent heat of vaporisation of water allows it to absorb a large amount of heat from the sun, resulting in the formation of clouds and precipitation.

## 3. How does the specific latent heat of vaporisation affect the boiling point of a substance?

The specific latent heat of vaporisation is directly related to the boiling point of a substance. The higher the specific latent heat of vaporisation, the higher the boiling point of the substance will be. This is because a higher amount of energy is required to break the intermolecular forces and convert the substance from its liquid to its gaseous state.

## 4. Why is it important to consider the specific latent heat of vaporisation in industrial processes?

The specific latent heat of vaporisation plays a crucial role in many industrial processes where vaporisation is involved, such as distillation, sterilization, and evaporation. Understanding the specific latent heat of vaporisation of a substance is important in determining the amount of energy needed for these processes and can help in optimizing efficiency and reducing costs.

## 5. Can the specific latent heat of vaporisation of a substance change?

The specific latent heat of vaporisation of a substance is a physical property and is constant for a given substance at a specific temperature and pressure. However, it can vary slightly with changes in temperature and pressure. For example, as the pressure increases, the specific latent heat of vaporisation also increases, meaning more energy is required to vaporise the substance. Similarly, as the temperature increases, the specific latent heat of vaporisation decreases, as the molecules have more thermal energy and are easier to break apart.

• Introductory Physics Homework Help
Replies
3
Views
958
• Introductory Physics Homework Help
Replies
7
Views
3K
• Introductory Physics Homework Help
Replies
1
Views
2K
• Introductory Physics Homework Help
Replies
12
Views
5K
• Introductory Physics Homework Help
Replies
4
Views
2K
• Introductory Physics Homework Help
Replies
2
Views
1K
• Introductory Physics Homework Help
Replies
4
Views
3K
• Introductory Physics Homework Help
Replies
1
Views
615
• Introductory Physics Homework Help
Replies
2
Views
1K
• Introductory Physics Homework Help
Replies
3
Views
1K