About Amy Expertise I love understanding how things work, especially when it comes to living
things. I`ve spent most of my life studying science and love passing on
this knowledge to kids. I currently work in a children`s science museum
where I`ve designed science exhibits on all sorts of topics; including
predators, music, weather, water, technology and much, much more! I`m
happy to answer questions about anything you have problems with at
school or help you with science projects. Also, I can help teachers
with ideas for classroom activities. Please note that I don`t answer
homework questions.
Experience I have an honours degree in Biochemistry and work as a science communicator for the National Science and Technology Centre in Australia.
Question Salt added to ice, melts the ice. I think this is because it lowers the temperature water needs to get to before it freezes. How does adding salt to water affect the boiling point of water?
Answer Hi Dave,
As soon as any of the salt dissolves in the water, the boiling point of the water will begin to rise -- by about one half degree Celsius for every 58 grams
of salt dissolved per kilogram of water. In fact, any non-volatile soluble substance will
raise the boiling point of water.
So why does the salt affect the boiling point?
Here's how boiling works
The boiling point is defined as the temperature at which the saturated vapor pressure of a liquid is equal to the surrounding atmospheric pressure. For water, the vapor pressure reaches the standard sea level atmospheric pressure of 760 mmHg at 100°C. Since the vapor pressure increases with temperature, it follows that for pressure greater than 760 mmHg (e.g., in a pressure cooker), the boiling point is above 100°C and for pressure less than 760 mmHg (e.g., at altitudes above sea level), the boiling point will be lower than 100°C. As long as a vessel of water is boiling at 760 mmHg, it will remain at 100°C until the phase change is complete.
Here's what salt does
Part of the pressure in the liquid comes from the salt, not the water. So the pressure due to the water alone is reduced compared to that of pure water at the same temperature. The vapour pressure, that is, the pressure of water vapour that would stay in equilibrium with the liquid, is reduced by the same amount because of the salt.
I hope this helps,
PLease let me know if you have any more questions
