# Heating water and air in a constant volume

• liquidFuzz
In summary, heating water and air in a constant volume refers to the process of increasing the temperature of these substances without changing their volume. This can be achieved through various methods such as using heat exchangers, combustion, or electric heating elements. This process is commonly used in household appliances like water heaters and central heating systems, as well as in industrial processes for heating fluids and gases. By maintaining a constant volume, the energy input is solely focused on increasing the temperature, resulting in more efficient heating.
liquidFuzz
I found a simulation tool on wolfram alpha where the pressure of a constant volume is calculated. The pressure is affected by the heating of a mixture of water and air inside the container. There is an explanation to how the pressure is calculated which seems pretty straight forward. But there is one thing... in the beginning of the Detail section the final volume is calculated. Is this the Specific volume of water, or something else..?

The simulation tool with explanation: http://demonstrations.wolfram.com/HeatingWaterAndAirInASealedContainer/

No, it's the actual volume of water in the container.

Do you know how to calculate it? I can't find the equation, or more precise the constants they use in the equation in the explanation.

I assume they're embedded in the software. As the aim is to demonstrate the software, not enable you to do it yourself, they don't tell you.

Yeah maybe..?

Anyhow, I have an approximation i set yp myself that seems to be pretty close to empirical values i found on the e-net. Not fancy pancy as the wolfram alpha stuff though.

liquidFuzz said:
Do you know how to calculate it? I can't find the equation, or more precise the constants they use in the equation in the explanation.
What are your thoughts on how to calculate this? What physical property data do you think is needed?

liquidFuzz said:
I can't find the equation, or more precise the constants they use in the equation in the explanation
If you pick up the player and download the source ("author code") you can open the .nb file and see the expressions, e.g. :

vol2 = (-1*^-8*T^3 + 6*^-6*T^2 - 2*^-5*T + 0.99695)*vol1;​

Much more legible than what's in the .nb file itself:
Code:
      RowBox[{"vol2", "=",
RowBox[{
RowBox[{"(",
RowBox[{
RowBox[{
RowBox[{"-", "1*^-8"}], "*",
SuperscriptBox["T", "3"]}], "+",
RowBox[{"6*^-6", "*",
SuperscriptBox["T", "2"]}], "-",
RowBox[{"2*^-5", "*", "T"}], "+", "0.99695"}], ")"}], "*",
"vol1"}]}], ";", "\[IndentingNewLine]", "\[IndentingNewLine]",

btw, I was surprised the stuff expands that much. Then again, 250 C is pretty hot. Nice eye-opener.

BvU said:
vol2 = (-1*^-8*T^3 + 6*^-6*T^2 - 2*^-5*T + 0.99695)*vol1;​

How do you parse that?
For example, what is A? "-1*^-8"??

I presume it's a polynomial fit to some experimental data set. I looked at the NIST chemistry webbook and it puts the volume increase from 25 to 250°C at just under 25%, which would very nearly fill the container in the simulation.

I can describe how I would approach this problem, first (a) without dissolution of N2 and O2 in the liquid water and then (b) with dissolution. But I would first like to hear from the OP about his articulation of the physical mechanisms involved.

JT Smith said:
How do you parse that?
Come on ! $$v_2 = (-1*10^{-8}*T^3 + 6*10^{-6}*T^2 - 2*10^{-5}*T + 0.99695)*v_1$$As Clooney would say: What else ?

The missing "10". It seems so obvious now. Thanks!

Chestermiller said:
But I would first like to hear from the OP about his articulation of the physical mechanisms involved
Wholeheartedly subscribe to this. Expressions and equations are useless if not applied properly. Having a tool at your disposal is not equivalent to knowing what goes on !

My first approximation was ## P_{tot} = P_{air} + P_{water} ##

I use the ideal gas law for the air pressure and Antoine's equation to calculate the water pressure. Given the simulation tool on wolfram alpha I use some approximation they don't use. In addition to this, I always get a tad nervous when I'm getting closer to the world of chemistry.

Edit, thanks for all help. I'll try to set up a model based on the polynomial fit used in the wolfram alpha tool.

liquidFuzz said:
My first approximation was ## P_{tot} = P_{air} + P_{water} ##

I use the ideal gas law for the air pressure and Antoine's equation to calculate the water pressure. Given the simulation tool on wolfram alpha I use some approximation they don't use. In addition to this, I always get a tad nervous when I'm getting closer to the world of chemistry.

Edit, thanks for all help. I'll try to set up a model based on the polynomial fit used in the wolfram alpha tool.
My approach to this would be similar to yours, but a little more fleshed out. Let:

##\rho_L(T)## represent the density of liquid water at temperature T
##P(T)## represent the equilibrium vapor pressure of water at temperature T
##V_0## represent the volume of liquid water in the container at 25 C
##p_0## represent the pressure in the container at 25 C (in this case 1 atm)

Then the volume of gas in the container at 25 C is (V-V_0), where V is 1 liter. The mass of air in the container at 25 C (as well as all other temperatures) is then $$m_a=\frac{[p_0-P(25)]M_a}{298R}(V-V_0)$$where ##M_a## is the molecular weight of air. The mass of water in the container at 25 C (as well as all other temperatures) is: $$m_w=V_0\rho_L(25)+\frac{P(25)M_w}{298R}(V-V_0)$$where ##M_w## is the molecular weight of water. The first term on the right hand side of this equation is the mass of liquid water and the second term is the mass of water vapor.

Now for temperature T. Let f represent the fraction of the water that is liquid at temperature T. Then the mass of water in the gas phase must be ##m_a(1-f)##. And the total volume of liquid and gas must match the volume of the container V. This means that:
$$\frac{m_wf}{\rho_L(T)}+m_w(1-f)\frac{RT}{P(T)M_w}=V$$The first term on the left hand side of the equation is the volume of liquid water, and the second term is the volume of gas. This equation allows us to solve for f. Once f is known, we can solve for the partial pressure of air at temperature T as: $$p_{air}=\frac{m_aRT}{M_a\left[V-\frac{m_wf}{\rho_L(T)}\right]}$$Total pressure in the container at temperature T is then $$p=p_{air}+P(T)$$

Last edited:
I think you meant ##m_w## instead of ##m_a## in a few places.

Your approach will result in a higher total pressure than in the simulation. On the other hand, whereas you ignore the air going in and out of solution the simulation assumes that it reaches equilibrium effectively instantaneously. If the animation is supposed to be real-time I think that dissolution rates would have to be considered. Am I wrong? Would it actually happen quickly enough as the pressure/temperature rises?

JT Smith said:
I think you meant ##m_w## instead of ##m_a## in a few places.
Really? Like where?
Your approach will result in a higher total pressure than in the simulation. On the other hand, whereas you ignore the air going in and out of solution the simulation assumes that it reaches equilibrium effectively instantaneously. If the animation is supposed to be real-time I think that dissolution rates would have to be considered. Am I wrong?
Well, certainly, the dissolution of air is neglected in my analysis. But this can be included rather readily. As far as the rate effect is concerned, I'm not convinced that the animation describes the variations in real time. Is time included as a parameter in their equations?
Would it actually happen quickly enough as the pressure/temperature rises?
This would have to be quantified with a transient analysis.

Chestermiller said:
Really? Like where?

Here and in the denominator of the equation that followed this one:

$$\frac{m_af}{\rho_L(T)}+m_a(1-f)\frac{RT}{P(T)M_w}=V$$I don't remember the simulation mentioning time. It wasn't in the accompanying equations.

Chestermiller
JT Smith said:
Here and in the denominator of the equation that followed this one:

$$\frac{m_af}{\rho_L(T)}+m_a(1-f)\frac{RT}{P(T)M_w}=V$$I don't remember the simulation mentioning time. It wasn't in the accompanying equations.
Ah. Thanks. I'm going back and changing that.

... and in the sentence preceding the equation. :-)

Chestermiller said:
I can describe how I would approach this problem, first (a) without dissolution of N2 and O2 in the liquid water and then (b) with dissolution. But I would first like to hear from the OP about his articulation of the physical mechanisms involved.

Hi, I saw this now. I'm sorry I missed it and that I didn't notice your contribution in the thread. I'll walk through your ideas for solving it, when I have the time. Well now there's tad to much... life. :-)

## 1. How does heating water and air in a constant volume affect temperature?

When heating water and air in a constant volume, the temperature will increase as the molecules gain more kinetic energy and move faster. This increase in temperature is due to the thermal energy being transferred to the molecules, causing them to vibrate and collide more frequently.

## 2. Why is a constant volume necessary for heating water and air?

A constant volume is necessary for heating water and air because it allows for a controlled environment where changes in temperature can be accurately measured and compared. In a constant volume, the pressure remains constant, ensuring that the volume of the substance does not change during the heating process.

## 3. What is the relationship between temperature and pressure when heating water and air in a constant volume?

In a constant volume, the pressure and temperature of a gas are directly proportional. This means that as the temperature increases, the pressure will also increase, and vice versa. This relationship is known as Gay-Lussac's Law.

## 4. How does heating water and air in a constant volume affect the density of the substances?

As the temperature of water and air increases, the molecules will spread out and become less dense. This is because the increase in temperature causes the molecules to move faster, taking up more space and decreasing the density of the substance.

## 5. Can heating water and air in a constant volume cause a change in state?

Heating water and air in a constant volume does not necessarily cause a change in state, as this depends on the initial temperature and pressure of the substances. However, if the temperature and pressure reach the boiling point, the water can change from a liquid to a gas (steam) and the air can expand to fill a larger volume.

• Thermodynamics
Replies
11
Views
1K
• Thermodynamics
Replies
5
Views
1K
• Thermodynamics
Replies
5
Views
870
• Thermodynamics
Replies
2
Views
1K
• Thermodynamics
Replies
4
Views
1K
• Mechanics
Replies
2
Views
90
• General Engineering
Replies
25
Views
2K
• Introductory Physics Homework Help
Replies
8
Views
788
• Introductory Physics Homework Help
Replies
12
Views
889
• Thermodynamics
Replies
11
Views
2K