Specific heat (I'm not sure if it's capacity or latent)

In summary, based on the information given, it would take about 201 seconds for a 500 watt kettle to boil water from 20 degrees Celsius to boiling point. Once the water is boiling, all of the energy supplied is used to convert liquid water into water vapor (steam).
  • #1
gabloammar
73
0

Homework Statement


Part a. A 500 W kettle contains 300g of water at 20°C. Calculate the time it would take to raise the temperature of the water to boiling point.

Part b. The kettle is allowed to boil for 2 minutes. Calculate the mass of water that would remain in the kettle.

State any assumptions you make.

(Specific heat capacity of water = 4.18x103 J kg-1 °C-1, specific latent heat of vaporisation of water = 2.26x106 J kg-1.)


2. The attempt at a solution
I have the answer for part a. It's (rounded) about 201 seconds.

Problem: I don't know where to start from for part b. Someone help.
 
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  • #2
Oh and I'm making two assumptions.
1. There's no energy lost to the surroundings.
2. All the vapour formed escapes the kettle.
 
  • #3
I agree with your time for part (a)
Once the water is boiling there is no increase in temperature ( boils at 100C) so all of the energy supplied ( in 2 minutes) is used to convert liquid water into water vapor ( steam)
The usual assumptions about no heat loss etc.
 
  • #4
Yep yep I can think about that. Fine. But, the only formula I've got in my head is E=mL, and the only way I'm thinking of finding the energy change [E in the equation] is power equals work done by time taken. Using that, power being 500 W and time taken being 120 seconds, the work equals 60kJ [I'm not even sure if I should be doing that or not].

So I've got energy and I've got specific latent heat. What next? Put 'em in the formula?

mass equals energy by specific latent heat, and I get 37.7 [is this kg or grams?].

Seriously, I'm COMPLETELY at sea with this.
 
  • #5
Oh btw the answer is supposed to be 273g, rounded off to 270g. No idea where it comes from though.
 
  • #6
You are doing the right thing.
60kJ in 2 mins is correct so the mass of steam = ( 60 x 10^3)/2.26 x 10^6
which is 0.027 kg
(where did you get the answer to be 273g?)
 
Last edited:
  • #7
I could've sworn I was doing the same thing over and over again and not getting that value! Or maybe I was looking at the energy in a wrong way [I have the value 59892.5 J written on the notebook, no clue where I got it from though]. But thanks! Question solved :D
 
  • #8
great...don't you wish all problems were so easy:wink:
 
  • #9
Of course :p
 

1. What is specific heat capacity?

Specific heat capacity is the amount of energy required to raise the temperature of one gram of a substance by one degree Celsius. It is a measure of the substance's ability to store thermal energy.

2. How is specific heat capacity measured?

Specific heat capacity is typically measured using a calorimeter, which is a device that measures the heat transfer between a substance and its surroundings. The change in temperature of the substance, along with its mass and the amount of heat transferred, is used to calculate its specific heat capacity.

3. What is the difference between specific heat capacity and latent heat?

Specific heat capacity and latent heat are both measures of a substance's ability to store thermal energy, but they are different in how they are measured and used. Specific heat capacity is the amount of energy required to change the temperature of a substance, while latent heat is the amount of energy required to change the phase of a substance (i.e. from solid to liquid or liquid to gas) at a constant temperature.

4. How does specific heat capacity affect the properties of a substance?

The specific heat capacity of a substance can affect its thermal conductivity, which is the rate at which it can transfer heat. It can also affect its heat capacity, which is the total amount of heat energy a substance can store. Additionally, substances with a high specific heat capacity tend to have a more stable temperature and are less likely to experience drastic changes in temperature.

5. What factors can affect the specific heat capacity of a substance?

The specific heat capacity of a substance can be affected by its molecular structure, density, and temperature. It can also be influenced by external factors such as pressure and the presence of impurities. In general, substances with more complex molecular structures or higher densities tend to have higher specific heat capacities.

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