Question - My 8 year old son has done a science fair project
entitled "Does an Ice Cube Melt Faster in Fresh Water or Salt Water?".We
have done the experiment several times and have concluded that an ice
cube melts faster in fresh water. Can you help me explain the
experiments results,so an 8 year old can better understand it.
Hi Franck!
So you have the correct answer, you did some experiments and
found correctly that an ice cube melts faster in fresh water...
Why?
Every pure substance has a definite temperature where it melts. It is
called melting point. (it is similar to the boiling point where
a pure substance boils). We can say that the temperatures in both
cases are related to the forces that hold the molecules together.
There is a physical constant called "molar heat of fusion" that
somehow controls the melting point temperature.
When you have salty water, the salts in the water make the molar
heat of fusion, (and the melting point) lower, and this means that
the ice inside this salted water will melt only at a lower temperature.
And the ice cubes will melt slowly...
That is the same principle one use when put some salt over
an ice pack to freeze some beer cans...The ice will stay
longer...
I hope, Dad, that the explanation will be satisfatory for your
8 year old son.
And tell others about NEWTON! It is a pleasure to
answer your questions!
Mabel
(Dr. Mabel Rodrigues)
entitled "Does an Ice Cube Melt Faster in Fresh Water or Salt Water?".We
have done the experiment several times and have concluded that an ice
cube melts faster in fresh water. Can you help me explain the
experiments results,so an 8 year old can better understand it.
Hi Franck!
So you have the correct answer, you did some experiments and
found correctly that an ice cube melts faster in fresh water...
Why?
Every pure substance has a definite temperature where it melts. It is
called melting point. (it is similar to the boiling point where
a pure substance boils). We can say that the temperatures in both
cases are related to the forces that hold the molecules together.
There is a physical constant called "molar heat of fusion" that
somehow controls the melting point temperature.
When you have salty water, the salts in the water make the molar
heat of fusion, (and the melting point) lower, and this means that
the ice inside this salted water will melt only at a lower temperature.
And the ice cubes will melt slowly...
That is the same principle one use when put some salt over
an ice pack to freeze some beer cans...The ice will stay
longer...
I hope, Dad, that the explanation will be satisfatory for your
8 year old son.
And tell others about NEWTON! It is a pleasure to
answer your questions!
Mabel
(Dr. Mabel Rodrigues)