We know that the entropy of the universe increases for spontaneous processes. For example at T>0 ice melts and this increases the entropy. But at T<0 water freezes and this is also spontaneous so this should also increase entropy. We know that liquids have higher entropy than solids. So, when water freezes does the entropy decrease or increase?
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Entropy OF WHAT?
You know that thermal interactions are not isolated. You know that if the system cools due to heat transfer, the environment warms due to heat transfer.
Same is true with entropy. Although entropy is a little more interesting.
Upon melting ice, you add energy to the ice and simultaneously add entropy to the ice. The outside source of heat then decreases its energy and decreases its entropy in order to do so.
Upon freezing ice, you remove energy from the ice and simultaneously remove entropy from the ice. The outside source of heat then increases its energy and increases its entropy in order to do so.
Energy in any process is conserved. The exact same amount of energy that enters the ice to melt it will exit the source of heat (neglecting other unrelated losses). The exact same amount of energy that exits the ice to freeze it will enter the environment.
Entropy is a little different. In heat transfer processes, the colder object ALWAYS gains more entropy than the hotter object. Entropy is generated in the process itself of heat transfer. Net entropy of the universe increases, even though the hotter object's entropy decreases.
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Let's throw some numbers at it.
Ice has a heat of fusion of h=333 kJ/kg, and it undergoes its melting and freezing processes at Tm=273.15 K. I am strategically giving you temperatures in Kelvin.
Suppose we have an example of 1 kilogram of ice, at a temperature ready to melt, that we then expose to a background environment of Tbg=300 K. The ice completely melts, and we stop considering the process once it finishes melting (i.e. we don't consider the warming of the water).
You know that thermal interactions are not isolated. You know that if the system cools due to heat transfer, the environment warms due to heat transfer.
Same is true with entropy. Although entropy is a little more interesting.
Upon melting ice, you add energy to the ice and simultaneously add entropy to the ice. The outside source of heat then decreases its energy and decreases its entropy in order to do so.
Upon freezing ice, you remove energy from the ice and simultaneously remove entropy from the ice. The outside source of heat then increases its energy and increases its entropy in order to do so.
Energy in any process is conserved. The exact same amount of energy that enters the ice to melt it will exit the source of heat (neglecting other unrelated losses). The exact same amount of energy that exits the ice to freeze it will enter the environment.
Entropy is a little different. In heat transfer processes, the colder object ALWAYS gains more entropy than the hotter object. Entropy is generated in the process itself of heat transfer. Net entropy of the universe increases, even though the hotter object's entropy decreases.
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Let's throw some numbers at it.
Ice has a heat of fusion of h=333 kJ/kg, and it undergoes its melting and freezing processes at Tm=273.15 K. I am strategically giving you temperatures in Kelvin.
Suppose we have an example of 1 kilogram of ice, at a temperature ready to melt, that we then expose to a background environment of Tbg=300 K. The ice completely melts, and we stop considering the process once it finishes melting (i.e. we don't consider the warming of the water).
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