Demonstration of the specific heat of water

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Greetings dear friends of the Hive scientific community.

As we know water is indispensable for life, perhaps that is why it has many properties that are unique among substances with similar chemical structure.

This time I want to share with you a simple experiment that tests a property known as specific heat, a property that describes the ability of water to absorb heat before increasing its temperature. This particular property is extremely important to maintain the balance on our planet and also allows us to boil water in a dish or cup of cardboard, if that is the case, and that is what we are going to test.

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Testing the specific heat of water. Source: @emiliomoron.

Despite what it may seem, this property does not refer to the boiling or combustion temperature of something, it is a much broader concept that relates to the ability of substances to store heat energy. As you may have noticed when cooking, not all substances heat up at the same rate, when we place water in a pot on the stove, the metal container heats up first but we observe that the water heats up some time later.

Let's see the following demonstration, where we boil water in a cardboard container.

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Boiling water in a cardboard container. Source: @emiliomoron.

As we can see, although the dish is made of cardboard, it withstands the heat of the direct flame, since this container and the water have a different specific heat, being that of the water much higher, which allows it to absorb a lot of heat, preventing the dish from burning.

So, what is specific heat?

Specific heat is a quantity related to the heat capacity of substances, the latter being the amount of heat that a substance can store before changing temperature. Mathematically speaking, it is said to be the amount of heat, Q, necessary to produce in a system a temperature increase of one unit [1].

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So, the amount of heat, Q, required to produce in a system a change in temperature, ∆T, can be expressed as:

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The heat capacity is an extensive property, which means that it is proportional to the amount of matter in the system, so it is usually more convenient to express the corresponding quantity per unit mass, i.e., the amount of heat that must be supplied to a quantity of mass, m, of a substance to raise its temperature by one unit, i.e., the heat that is needed for a given quantity of mass to raise its temperature one degree more, this quantity being the specific heat, c.

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The amount of energy absorbed is expressed in joules (J) or can also be expressed in calories (cal), the most common units of specific heat being J/g.K. or cal/g.°C.

In the experiment, how does the specific heat influence the cardboard dish so that it does not burn?

Some substances heat up quickly, we may have noticed that a piece of metal approaching a heat source heats up almost to the touch while a piece of wood or rubber takes much longer to heat up. Water is one of the substances that heats up slowly, it has a high specific heat capacity so it requires more energy to raise the temperature.

For example, water has a heat capacity of 4181 J/Kg.°C, which is considerably higher than other substances. In the case of the experiment, we can see that to heat water, it must absorb the heat of the flame from the stove, and will boil when it reaches its boiling point of 100 °C. In the process, it removes a large amount of heat from the cardboard, so it cannot start to burn. And even when the water reaches its boiling point, any amount of heat supplied is used to produce the phase change, i.e. to vaporize the water, so it will maintain a constant temperature of 100 °C and the paper has an ignition temperature in excess of 200 °C[2], much higher than the boiling temperature of the water, so this temperature will not be reached until all the water placed in the container has completely evaporated.

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The specific heat of the water prevents the dish from burning. Source: @emiliomoron.

What does it mean that the specific heat is so high?

A high specific heat means that a substance will have greater resistance to increase its temperature, so it can absorb more energy, i.e., we would have to heat water for much longer than a substance with lower specific heat to notice a change in its temperature. Thus, for example, metals such as aluminum and iron have such a low specific heat that when exposed to fire they will heat up almost instantly, but even iron will become red hot before aluminum. In the following table we can compare the specific heat of some substances.

Table of specific heats of some substances
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Source: Wikipedia.com

The high specific heat of water explains why water masses change their temperature more slowly than rocks and soil on the earth's surface, acting as regulators of heat flow between the lithosphere and the atmosphere, which is fundamental for climate regulation on our planet[3].

Just think that when we go to the beach on a sunny day, even if the atmosphere is very hot, the water always has a cooler temperature. We notice it when we walk on the hot sand and then feel the cold water on our feet; this is because the sand has a lower specific heat capacity, and as the sun emits a more or less constant amount of energy, the sand heats up much faster than the water.


Thanks to the specific heat of the water, the ocean can absorb a lot of solar radiation and hardly heat up. Source: pixabay.com.

This aspect is also important for aquatic life, for example, because the ocean can receive a large amount of solar radiation, and the water will barely heat up to any appreciable degree throughout the day, thus maintaining a thermal difference that will not affect the marine ecosystem.

On the other side, once the water has risen in temperature it will have to release a large amount of energy to cool down, so now the heat flow will be reversed, heating in the process the air masses in the surrounding environment, thus keeping the nights in the coastal areas a little warmer.

And what is the reason for the high specific heat of water?

If we look at the table above, ammonia, in addition to water, has a high specific heat. These unique values are due to the fact that these substances can form hydrogen bridges. These intermolecular forces are the explanation for many of the anomalous properties that we observe in water, such as the decrease in density upon solidification.

As we know, molecules must absorb energy to break bonds and produce a phase change, in the case of water the molecules are strongly bound to each other, the maximum energy in a hydrogen bridge between two water molecules is about 23 kJ/mol, without considering electrostatic interactions with other molecules. Therefore, it is necessary to supply a lot of energy to make the water molecules separate, and as long as they remain together, they cannot vibrate, and to increase their temperature the water molecules must increase their vibration, in the meantime, they will continue to accumulate energy.


Illustration of hydrogen bridge bonds in water, the blue lines represent the bonds. Source: Wikipedia.com.

In this sense, all the energy supplied to heat the water is divided between the breaking of the bonds, which are constantly forming and breaking, making them difficult to break. The reason for this difficulty lies in the structure of the water molecule, as it is made up of two hydrogen atoms and an oxygen atom that has two pairs of free electrons, it is able to form four bonds between molecules. The total number of bonds fluctuates with time and depends on temperature, but from simulations it has been estimated that on average each water molecule participates in 3.5 bonds per hydrogen bond[4].

In conclusion

Specific heat is a property of substances that depends on their molecular organization, so it is different for each one, and is related to the ability of substances to absorb or release heat per unit mass in order to vary their temperature.

The importance of the high specific heat of water is fundamental for life on the planet, since this extraordinary property allows large bodies of water to be a means of transporting energy and to contribute to the regulation of temperature on our planet.


Well friends, I hope you liked the information about this important property. See you next time!.


References

  1. Wikipedia.com. Capacidad calorífica.
  2. Wikipedia.com. Temperatura de ignición.
  3. Rodríguez, J.M. y Marín, R. (1999). Fisicoquímicas de aguas. Ed. Díaz de Santos, España.
  4. Wikipedia.com. Fuerza por puente de hidrógeno.


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8 comments
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Awesome demonstration. I have never thought of the specific heat of water this way, even though I use to think I know what it means. This totally simplifies it. If I ever need to explain specific heat of water to a layman, this should suffice.

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Hi friend @gentleshaid, I'm glad you liked the demonstration and that you find the information useful to make this property known, I was happy to share it. Greetings and thanks for your appreciation of my post!

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Thanks for your contribution to the STEMsocial community. Feel free to join us on discord to get to know the rest of us!

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Thank you very much for your support friends!

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impresionante experimento. Gracias, voy a intentarlo en casa y a tratar de explicárselo a mis hijos

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Saludos @nahumsamuel, muchas gracias! es un experimento muy didáctico, seguro a los niños les impresionara, ten cuidado de que la llama cubra solo el área del envase que tiene agua.

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Me gustó mucho el experimento, considero que a veces no le damos el justo valor al agua por considerar que siempre la tendremos, poder constantar sus elementos partiendo de su composición quimica nos permite valorar también los aportes a todo nuestro ecosistema

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Así es @emimoron, a veces no valoramos todas las propiedades del agua y su importancia como reguladora de muchos factores ambientales, esnecesario conocer al respecto para saber cuan especial es este líquido. Saludos!

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