Boiling water is an event we see or take part in every day—be it to make coffee or cook or simply make water potable. We don’t think too much about boiling water when we do it.
But what is this process of boiling? Let’s recall what we learned in school about it. Boiling is defined as a process of heating a liquid up to a temperature where the liquid converts into vapor.
The temperature at which a liquid turns into vapor is called its boiling point. It is the temperature where the atmospheric pressure is equal to the vapor pressure of the liquid.
In the process of boiling, not just a few molecules at the surface, but all the molecules in the liquid take place, making it a bulk phenomenon.
The normal boiling point of water is 211.94 degrees Fahrenheit and the standard boiling point of water is 211.29 degrees Fahrenheit. (Normal temperature = boiling temperature at 1 atm; Standard temperature = boiling temperature at 1 bar)
In this article, we will look at boiling water and understand if it is a physical change or a chemical change. But before we classify boiling water under either of the categories, we need to understand what a physical change and chemical change is.
- What Change Does Boiling Water Involve?
Change is a constant part of life. It is happening around us at all times. Chemists often deal with different changes more actively in their work. The changes that a matter can undergo tell a lot about the nature of the matter.
Chemists have classified changes that they observe into two categories—physical change and chemical change.
What Is a Physical Change?
Chemists classify changes in which no bonds are formed or broken as physical change. This implies that the compounds in the change remain the same even after the change and no new material is formed.
The properties of the materials involved in a physical change stay the same even after the physical change. The properties here include color, boiling point, etc.
A physical change involves the movement of the molecules and not a change in the nature of the molecules. Some typical physical changes include:
- Change of State: Change from a solid state to a liquid state, or liquid to a gaseous state, and vice versa.
- Separation of Mixture: When a mixture separates without changing the properties of the matters in the mixture, it is a physical change.
- Making Solutions: Solutions are a special kind of mixture where the chemical properties of the matter (mostly liquids) do not change.
- Physical Deformation: Changes through cutting, denting, or stretching of a matter are physical changes because they do not alter the chemical properties of the matter.
Melting is an example of a physical change. In the case of melting, the state of the matter changes from a solid form to a liquid one. Melting of ice is a physical change.
The change of ice into liquid on exposure to higher temperatures is only a physical change as the water molecules do not change properties during the change.
In the liquid state, each water molecule is made up of two hydrogen atoms and one oxygen atom. In the solid state too, the composition of the water molecule remains the same.
When you cut up a piece of cloth into many small pieces, the matter in your hands becomes smaller than before. But it does not mean that the cloth ceases to be a cloth. The properties of a cloth remain the same, even if the shape and size are different from before the change.
Physical changes can be either reversible or irreversible.
- Melting is a reversible change as melted ice (water) can be frozen back into ice.
- Cutting of grass would be an irreversible physical change. After you cut grass, it can grow back into how it used to look, but that would be new matter replacing the old space. The cut-off part can’t be attached back to reverse the process of cutting.
What Is a Chemical Change?
In contrast with physical change, a chemical change involves breaking as well as forming bonds of molecules—sometimes simultaneously.
A material that undergoes a chemical change turns into a new material with a different set of properties. Chemical changes are also much more complicated to reverse compared to reversible physical changes.
One example of a chemical change is the burning of a candle. It involves the change of wax and the wick through fire to produce carbon dioxide and light. This process involves not only a change in the appearance of the material but also the resulting molecules from the change.
Although chemical changes are characterized by the change in molecular structure, we don’t actually see the structure of the molecules changing. There are other observations that tell us if a chemical change has taken place.
Some chemical changes involve a change in energy while other chemical changes involve the formation of new substances with different properties.
To identify chemical changes, some of the observations that help are:
- Changes in Temperature: In many chemical changes, the result is an increase or a decrease in temperature.
- Emission of Light: In many chemical changes, there is an emission of light as a result.
- Change in Color Unexpectedly: Many chemical changes can be recognized by a change in color of the substances involved. These changes in color are not results of the original color of the initial substances mixing together, but are the results of new substances being formed with different colors.
- Formation of Bubbles: When a chemical reaction with a liquid results in the formation of bubbles without boiling it, it indicates the formation of a new gaseous substance.
- Change in Smell or Taste: When the taste or smell of a substance changes, it shows that a chemical reaction has taken place. An example would be the rotting of food.
- Formation of a Solid after Mixing of Two Liquids: In some chemical changes, a solid substance is formed as a result of two solids interacting. This is a chemical reaction called precipitation.
What Change Does Boiling Water Involve?
Considering all that we learned about chemical changes and physical changes, it would be easy for us to understand what change boiling water involves if we observe the process.
The first remark that can be made after observing water boiling is that it changes its state from liquid to gas if you boil it long enough. After reaching the boiling point, the water starts to evaporate and change its state.
The second remark that can be made is that when the water vapor from boiling water is collected on a cool lid, it immediately turns back into its liquid form. This proves that this process of changing the state of the water is reversible.
Boiling of water does not result in a change of color, taste, or smell. It does not emit any light. The change in temperature is also due to external heat being provided to it or heat being taken away from it.
Considering all these observations, we can confidently claim that boiling of water is a physical change.