Boiling Point Elevation

Solutions

Solutions are homogeneous mixtures of a solute and a solvent. More simply put, a solution is when one substance mixes with another one so well that there are no clumps where molecules of the dissolved substance are stuck together. One example is sea water, which is a complex mixture of salts dissolved in water. Also, when you add sugar to your tea you are making a solution.

A solution at the molecular level.

A glass of salt water looks just like a glass of plain water. At the molecular level, the salt particles are mixed thoroughly with the water molecules.

Boiling

When a liquid boils it means that the pressure of the vapor produced by evaporation of the liquid is equal to the external pressure.

Let’s back up a little. You may have noticed that water left out in a dish or glass eventually evaporates away. This happens because the water molecules cease to belong to a liquid droplet and instead separate and join the gas phase. This evaporation happens all the time and the rate of evaporation depends on the temperature. A hot surface that is made wet will dry off sooner than a cold one.

Chemists describe this rate in a slightly weird way. Instead of talking about how many molecules evaporate per second, we talk about the pressure of the vapor. Air pressure is due to trillions of collisions between molecules and their container. Water vapor can make a contribution to this overall pressure. When liquid water is cool, it has a low vapor pressure and not many water molecules are escaping into the vapor phase. When liquid water is warm more molecules have enough energy to break the bonds to their neighbors and fly away as yet another particle in the air.

To get a liquid to boil it’s necessary to cause bubbles of its vapor to form within the body of the liquid. That’s right, bubbles in boiling water are full of water molecules in the gas phase: water vapor. These bubbles have a gas pressure and in order to avoid collapsing or simply exploding that pressure has to equal the pressure of the surrounding liquid. And that pressure is usually nearly equal to the pressure of the air above its surface.

Bubbles in boiling water

When water boils it is because it is hot enough to form bubbles of its own vapor that have enough pressure to avoid collapse. The bubbles are full of water molecules in the gas phase.

Boiling Solutions

A solution of a solid dissolved in water boils at a higher temperature than the pure water does. Water alone boils at 100°C (212°F). A sugar solution, in which the sugar makes up 80% of the mass of the solution, boils at about 110°C (230°F). Why should this be so?

It has to do with vapor pressure. When a solid is dissolved in water the relative number of water molecules decreases compared to the same volume of pure water. Here is an oversimplified example: Say there is enough room for a thousand molecules in a container. If it is full of water, there are 1,000 water molecules in the container. If it is instead full of sugar-water then there are still 1,000 total molecules in the container but now perhaps only 800 of them are water molecules.

With fewer overall water molecules available to evaporate from this tiny container, the evaporation rate decreases. In other words, the vapor pressure of a solution is less than the vapor pressure of the pure liquid at the same temperature.

When pure water gets to 100°C (212°F) it boils because the vapor pressure equals the atmospheric pressure. At 100°C (212°F) a solution has a lower vapor pressure. Bubbles cannot form and the solution does not boil. In order to boil, it has to reach a higher temperature so that its vapor pressure is high enough to allow bubbles to form.

The boiling point of a solution is higher than the boiling point of a pure liquid.

Pure water has a higher vapor pressure than a solution.

Pure water has a natural rate of evaporation that changes with temperature.

A solution has a lower vapor pressure than pure water.

A solution has a lower vapor pressure because fewer molecules are able to evaporate.

Relevance

When making candy the boiling point of the sugar solution is what determines the type of candy you make. This is because the boiling point of a solution is different for different concentrations.

When mixing water and sugar together it can be done in different proportions. If you add a teaspoon (about 5 grams) of sugar to a mug of coffee then you have made a solution with a concentration of about 1.3% by mass. (This is based on the following calculation: 5 grams sugar divided by a mass equal to the sum of the water (355 g for a mug-full) and the sugar. That is, 5/360 x 100% = 1.3%.

For making candy, this is far too low a concentration. To make fudge requires that the solution be 85% sugar by mass. To make hard candy it has to be 99% sugar.

By boiling a sugar-water solution, it is possible to change its concentration. The sugar cannot evaporate but the water can. So as the water boils it leaves the pot and the solution becomes more and more concentrated. The longer the pot is left on the heat, the more concentrated the sugar gets. And the more concentrated the sugar, the higher the boiling point of the solution. Once all the water is gone, the pot has just melted sugar in it. If you continue heating it you will cause it to undergo chemical changes, such as caramelization.

By closely monitoring the temperature it is possible to have a good idea of the concentration of the sugar. And since the concentration of the sugar is the main determinant of the texture of the candy, this is very useful. In the table below you can see the usual stages of candy making along with their respective temperatures and sugar concentrations. These temperatures are only valid at sea level as the boiling point changes based on external air pressure and at higher elevation all of these mixtures would boil at a lower temperature.

Stage Temperature Sugar concentration
thread (e.g., syrup) 110 to 112 °C (230 to 234 °F) 80%
soft ball (e.g., fudge) 112 to 116 °C (234 to 241 °F) 85%
firm ball (e.g., soft caramel candy) 118 to 120 °C (244 to 248 °F) 87%
hard ball (e.g., nougat) 121 to 130 °C (250 to 266 °F) 90%
soft crack (e.g., salt water taffy) 132 to 143 °C (270 to 289 °F) 95%
hard crack (e.g., toffee) 146 to 154 °C (295 to 309 °F) 99%
clear liquid 160 °C (320 °F) 100%
brown liquid (e.g., liquid caramel) 170 °C (338 °F) 100%
burnt sugar 177 °C (351 °F) 100%

Source: Wikipedia

 For more information about candy making, see the Exploratorium page about the Science of Cooking: Candy-making Stages.

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