[DeltaSS]

Hello. I have some final doubts about my related question. So, for example, if I have a material which has a mass of 25g, which has density of solid state 1,7g/cm^3, and the melt density of the same material is 1,5g/cm^3, then the melt density would be:
1) 25*1,5/1,7= 22,06g?
or:
2)25/1.5=16,67g?

Thank you very much!

[chenbeier]

Which one can it only be. Check the units in your calculation.

[DeltaSS]

The units are correct. What I mean is that by dividing/multiplying it simplifies and what is left only grams. However, it's more of a logical question than mathematical in this case. Im almost certain it is the first situation ( 1) ) which is correct, but I want other people opinion too

[AWK]

What do you want to calculate? You have 25 grams before and after the melting of a solid. I think it's about volume?

[MNIO]

you wrote
  (1) mass = 25g
  (2) solid density = 1.7 g/cm³
  (3) melt density = 1.5 g/cm³

then
  "then the melt density would be"

and toss in these calcs
  25*1.5/1.7 = 22.06g
and
  25/1.5 = 16.67g

********
What exactly are you trying to calculate? 

See the image below where I've added units to your work.



For the first, you're basically calculating the mass of melt that would exist in the same volume as 25g of solid exists in.  For the second, you're calculating volume of 25g of melt.  Neither is the "melt density" which you were given in the first place as 1.5 g/cm³

[DeltaSS]

you wrote
  (1) mass = 25g
  (2) solid density = 1.7 g/cm³
  (3) melt density = 1.5 g/cm³

then
  "then the melt density would be"

and toss in these calcs
  25*1.5/1.7 = 22.06g
and
  25/1.5 = 16.67g

********
What exactly are you trying to calculate? 

See the image below where I've added units to your work.



For the first, you're basically calculating the mass of melt that would exist in the same volume as 25g of solid exists in.  For the second, you're calculating volume of 25g of melt.  Neither is the "melt density" which you were given in the first place as 1.5 g/cm³

Thank you very much for reply. To be more precise, I'm trying to calculate what is the volume of a melt, when the solid weighs 25g. So I guess first i have to transfer solid weight to its volume, that is:

Vsolid = 25g/(1,7g/cm^3)= 14,7cm^3 (solid)
Then the melt volume is:
Vmelt = (14,7cm^3*1,5g/cm^3)/(1,7g/cm^3)=12,97cm^3 (melt)

Is that correct?

[chenbeier]

No,
The mass is constant.
You can calculate the volume before Melting by using 1,5 g/ cm^3 and after melting by using 1,7 g/ cm^3
Before 16,6 cm^3 after 14,7 cm^3
A decrease of 1,9 cm^3 happens, by releasing air probably.

[DeltaSS]

No,
The mass is constant.
You can calculate the volume before Melting by using 1,5 g/ cm^3 and after melting by using 1,7 g/ cm^3
Before 16,6 cm^3 after 14,7 cm^3
A increase of 1,9 cm^3 happens, by releasing air probably.

All right. Understood. Thank you for a quick response!

[AWK]

... mass of 25g, which has density of solid state 1,7g/cm^3, and the melt density of the same material is 1,5g/cm^3, then the melt density would be:

VOLUME

No,
The mass is constant.
You can calculate the volume before Melting by using 1,5 g/ cm^3 and after melting by using 1,7 g/ cm^3
Before 16,6 cm^3 after 14,7 cm^3
A decrease of 1,9 cm^3 happens, by releasing air probably.

[MNIO]

if you want the volume of 25g of "melt" you did that in the second equation.. 25g * (1cm³ / 1.5g) = 16.7 cm³

The density of plastic decreases as it melts due to expansion of the plastic.  Has nothing to do with trapped air.

[chenbeier]

Who said it is plastic?  But you are right density increase after melting.

[DeltaSS]

I didn't mention, but MNIO is actually correct - I'm doing project that is correlated with a plastic. Just a fun fact Anyways, thank you all very much!