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The boiling point of a liquid is defined as the temperature at which its vapor pressure is the same as the atmospheric pressure above said liquid. If we artificially increase the vapor pressure of the liquid, we can lower the boiling temperature. On the other hand, if we decrease the vapor pressure of the liquid, we will increase it.
Therefore, as we add more energy to boil the salt water, there will be less vapor pressure than if it had no salt. In this way, the boiling point will be reached when the water has reached a higher temperature. Although salt interferes with the network of hydrogen bonds that are in water molecules, the effect it produces is not very great if we use it in reasonable concentrations.
For example, to really increase the boiling point of water, we would need to add 230 grams of salt to one liter of water. In this way, the boiling point would increase 2°C. But this would result in a very salty and unhealthy taste, and would have an even higher concentration of salt than seawater.
What happens if you add the salt after the water has boiled?
Some people prefer to add salt to the water after it has boiled. Although at that moment the boiling point will no longer be accelerated, because the water will already be boiling, corrosion of metal pots can be avoided. This is because the chloride and sodium ions in the salt water will have less time to react with the metal.
experiments
To verify this phenomenon, some simple experiments can be carried out at home:
- Put pure water in one container and water with 20% salt in another, and let them boil. The fire must be the same for both containers and constant. With the help of a stopwatch, observe the time it takes for the water in each to reach the boiling point.
- Put 200 ml of water in a container and heat it until it boils. With a thermometer, measure the temperature. Then add 10 grams of salt, which is about a level tablespoon, and stir. Once dissolved, heat until the solution boils. Upon reaching the boiling point, again take the temperature. Continue adding salt and taking the temperature until you notice that there is undissolved salt in the bottom of the container. Finally, dump all the recorded data into a table and compare them.
- Put pure water in one container and salt water to taste in another. Heat until boiling and introduce any pasta of your choice in each one. Control the cooking time with a stopwatch until the pasta reaches the desired point. Compare the data obtained.
Bibliography
- Bea Sánchez, JL Formulation and preparation of mixtures . (2020). Spain. Synthesis. Timberlake, KC General Organic and Biological Chemistry. (2013). Spain. Peasron.
- Timberlake, KC General Organic and Biological Chemistry. (2013). Spain. Peasron.
- Sardeira, S.; Ceretti, H.; Reciulschi, E. Solutions. Solution properties. CDN. Available at: https://cdn.educ.ar/dinamico/UnidadHtml__get__822de3ea-9e90-45e1-9304-fe3234be2df1/15075-edi/data/5a8e0668-c858-11e0-830f-e7f760fda940/index.htm
- Did you know that water does not always boil at 100ºC? 2019. Retema.es. Retrieved from https://www.retema.es/noticia/sabias-que-el-agua-no-siempre-hierve-a-100c-qHRid .
