Boiling is the process of changing the state of aggregation of a substance. When we talk about water, we mean a change in the liquid state to vapor. It is important to note that boiling is not evaporation that can occur even at room temperature. Also, do not confuse with boiling, which is the process of heating water to a certain temperature. Now that we have understood the concepts, we can determine at what temperature the water boils.
Process
The process of converting the state of aggregation from liquid to gaseous is complex. And although people do not see this, there are 4 stages:
- In the first stage, small bubbles form at the bottom of the heated vessel. They can also be seen on the sides or on the surface of the water. They are formed due to the expansion of air bubbles, which are always in the cracks of the tank, where the water is heated.
- In the second stage, the volume of the bubbles increases. All of them begin to tear to the surface, since inside them there is saturated steam, which is lighter than water. As the heating temperature rises, the pressure of the bubbles increases, and they are pushed to the surface due to the known force of Archimedes. In this case, you can hear the characteristic sound of boiling, which is formed due to the constant expansion and decrease in the size of the bubbles.
- In the third stage, a large number of bubbles can be seen on the surface. This initially creates turbidity of the water. This process is popularly called "boiling with a white key", and it lasts a short period of time.
- At the fourth stage, the water boils intensely, large bursting bubbles appear on the surface, and splashes can occur. Most often, the spray means that the liquid has warmed up to a maximum temperature. Steam will start to flow out of the water.
It is known that water boils at a temperature of 100 degrees, which is possible only in the fourth stage.
Steam temperature
Steam is one of the states of water. When it enters the air, like other gases, it exerts a certain pressure on it. During vaporization, the temperature of steam and water remains constant until all the liquid has changed its state of aggregation. This phenomenon can be explained by the fact that during boiling all the energy is spent on the conversion of water into steam.
At the very beginning of boiling, moist saturated steam is formed, which after evaporation of all the liquid becomes dry. If its temperature begins to exceed the temperature of water, then such steam is superheated, and in its characteristics it will be closer to gas.
Salt water boiling
It is interesting enough to know at what temperature boils water with a high salt content. It is known that it should be higher due to the content of Na + and Cl- ions in the composition, which occupy the region between water molecules. This chemical composition of water with salt differs from ordinary fresh liquid.
The fact is that in salt water there is a hydration reaction - the process of the addition of water molecules to salt ions. The connection between the molecules of fresh water is weaker than those that are formed during hydration, so the boiling of a liquid with a dissolved salt will take longer. As the temperature rises, molecules in water with salt content move faster, but they become smaller, which is why collisions between them occur less frequently. As a result, steam is formed less, and its pressure is therefore lower than the pressure of fresh water steam. Therefore, for complete vaporization, more energy (temperature) will be required. On average, to boil one liter of water with a content of 60 grams of salt, it is necessary to raise the degree of boiling water by 10% (that is, 10 C).
Pressure dependences of boiling
It is known that in the mountains, regardless of the chemical composition of the water, the boiling point will be lower. This is due to lower atmospheric pressure. A pressure with a value of 101.325 kPa is considered normal. With it, the boiling point of water is 100 degrees Celsius. But if you climb a mountain where the pressure averages 40 kPa, then the water will boil at 75.88 C. But this does not mean that you will have to spend almost half the time to cook food in the mountains. For the heat treatment of products you need a certain temperature.
It is believed that at an altitude of 500 meters above sea level, the water will boil at 98.3 C, and at an altitude of 3000 meters, the boiling temperature will be 90 C.
Note that this law also applies in the opposite direction. If you place the liquid in a closed flask through which steam cannot pass, then with increasing temperature and the formation of steam, the pressure in this flask will increase, and boiling at high pressure will occur at a higher temperature. For example, at a pressure of 490.3 kPa, the boiling point of water will be 151 C.
Boiling distilled water
Distilled is called purified water without any impurities. It is often used for medical or technical purposes. Given that there is no impurity in such water, it is not used for cooking. It is interesting to note that distilled water boils faster than ordinary fresh water, but the boiling point remains the same - 100 degrees. However, the difference in boiling time will be minimal - only a fraction of a second.
In the kettle
Often people are interested in what temperature the water in a teapot boils, since it is these devices that they use to boil liquids. Given that the atmospheric pressure in the apartment is equal to standard, and the water used does not contain salts and other impurities, which should not be there, the boiling point will also be standard - 100 degrees. But if the water contains salt, then the boiling point, as we already know, will be higher.
Conclusion
Now you know at what temperature the water boils, and how atmospheric pressure and the composition of the liquid affect this process. This is nothing complicated, and children receive such information back in school. The main thing is to remember that with a decrease in pressure, the boiling point of the liquid also decreases, and with its increase, it also increases.
On the Internet you can find many different tables, which indicate the dependence of the boiling point of a liquid on atmospheric pressure. They are accessible to everyone and are actively used by schoolchildren, students and even teachers at institutes.