What is a Chemical Change?

by ReAgent Chemicals

A chemical change is a reshuffling of chemicals that happens when a chemical reaction takes place. It can happen in the matter of a few nanoseconds, or it can take decades to occur. And while some chemical changes are dramatic and obvious, others are barely noticeable. Types of chemical change can be classified into common categories. You can also use different types of indicators and experiments to determine chemical changes.

What is the Difference Between Physical and Chemical Change?

A clear distinction between physical changes and chemical changes is the rearrangement of elemental constituents. The bonds between elements or functional groups in a compound may either break or form new ones with other elements or functional groups.

For example, ice melting into water is a physical change because it only involves water changing phase. The change doesn’t affect the fact that the substance is still water, with one hydrogen and two oxygen atoms per molecule. Even though the molecules switched positions and some molecular bonds were broken, the chemical bonds per molecule were maintained. 

Categories of physical changes include:

  • Sublimation
  • Evaporation
  • Dissolution
  • Liquefaction

On the other hand, if a direct electrical current (DC) was passed through the water using electrodes, oxygen gas and hydrogen gas would be liberated. This would no longer represent a physical change, but a chemical change, because the bond between the elemental constituents is broken. In this example, a chemical change occurred through the process of electrolysis, as shown in the illustration below:

Diagram showing electrolysis of water

There are five main categories of chemical change:

  1. Decomposition reactions

The electrolysis of water is an example of a chemical change that involves the breaking of covalent bonds by using electrical energy. This is called a decomposition reaction. The positively charged hydrogen ions are attracted to the anode, while the negatively charged oxygen ions are attracted to the cathode. The process splits the two elements of water into their gas forms.

Of course, another common example of a decomposition reaction is the rotting of organic matter. For example, a rotting carcass has complex organic chemicals, like proteins, that decompose into ammonia, water, carbon dioxide, hydrogen sulphide, methane, and trace minerals. Therefore, the generalised equation for a decomposition reactions can be written as:


To wrap up, decomposition reactions are the breakdown of compounds into their simpler constituents, usually their elemental constituents.

  1. Combination reactions

When two elements react and form a compound, it’s called a combination reaction. During this type of reaction, new chemical bonds are formed, and energy is typically released in the form of heat. Combination reactions are also known as synthesis reactions because they form new substances. 

The generalised chemical equation for combination reactions can be written as:


An example of this reaction is the reaction between metal sodium and chlorine gas, which forms sodium chloride or table salt. This reaction is highly exothermic and violent, and has the chemical equation:


To correctly write the balanced chemical equations for combination reactions, you must remember the elements that occur in nature as diatomic molecules, which are hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, and iodine.

  1. Single replacement reactions

When a more reactive element replaces a similar but less reactive element in a compound, it’s called a single replacement reaction. The position of an element in the periodic table is predictive, whether it’s more reactive or less reactive than other elements of its group. Therefore, the generalised chemical equation for a single replacement reaction can be written as:


An example of this is the reaction of magnesium metal with an aqueous solution of copper (II) nitrate. Since magnesium is relatively more reactive than copper, it substitutes the copper in the solution. The resulting products are aqueous magnesium nitrate and solid copper metal, as shown in the balanced chemical equation below:


Similar types of reactions occur between metals and water or metals and acid. However, the replaced element is a nonmetal, such as in the reaction between aluminium and hydrochloric acid.

  1. Double-replacement reactions

Double-replacement reactions occur between two ionic compounds. During these reactions, the positive and negative ions exchange partners, forming two new compounds. Therefore, the general form of this type of reaction can be written as:


Here, A and C represent positively-charged cations, while B and D represent negatively-charged anions. Such reactions commonly occur in aqueous solutions. One of the products of the reaction is typically a solid precipitate, though it can also be a gas or a molecular compound.

Precipitates are insoluble byproducts of a double replacement reaction. These are formed when the cations and anions combine to create insoluble ionic compounds. An example of this is the reaction between the aqueous solutions of potassium iodide and lead (II) nitrate. The balanced chemical equation for this reaction is:


  1. Combustion reactions

Any chemical reaction with oxygen gas that releases energy in the form of heat and light is classified as a combustion reaction

One or more products are formed by combustion reactions. While they’re similar to combination and replacement reactions, the key difference is the presence of oxygen. Any burning of fuel, such as a candle flame or wood fire, is a combustion reaction. The simplest type of combustion is the reaction between hydrogen gas and oxygen gas:


In many cases, such as burning fossil fuels inside an automotive engine, combustion requires a trigger or ignition, like an electrical spark, to start the reaction.

Spontaneous combustion reaction between potassium permanganate and glycerol
This is another example of a combustion reaction between potassium permanganate and glycerol

What Are the Signs of Chemical Change?

Some of the signs of a chemical change can be misinterpreted as physical changes, and vice versa.

For instance, the release of heat energy is a sign of a combustion reaction, but it can also occur as the result of friction when two solid and rough objects slide against each other, such as in the case of flint rocks used to trigger a fire.

Generally speaking, however, signs of chemical change include the following:

  • Energy change: This can either be exothermic or endothermic
  • Change in colour: Many reactions, such as the reaction of copper metal with sulphuric acid, involve change of colour
  • Production of light: This can be a subtle luminescence or a fireball due to explosion
  • Evolution of gas: Gas from the compounds can be released, such as in the case of water electrolysis
  • Production of odour: The evolution of gases can also result in a pungent smell
  • Formation of precipitates: This usually occurs in double-replacement reactions

Are Chemical Changes Reversible?

Technically, all chemical changes are theoretically reversible, but it would be impractical or impossible to reverse many reactions since it would require a high amount of energy and precision.

For instance, when you boil an egg, the protein in the egg albumen and yolk undergoes chemical denaturation. What this means is that the chemical properties and molecular structure of the egg’s protein become disorganised. This is theoretically reversible, but next to impossible to do given our current technology. It would be akin to putting together the shattered pieces of a glass mirror without leaving even a hairline crack.

Therefore, for practical purposes, many chemical changes aren’t reversible. On the other hand, some chemical reactions are readily reversible when certain saturation points are reached. One good example of this is the excess reaction of hydrogen ions with bicarbonate ions, forming carbonic acid:

HCO3+ H+ ⇌ H2CO

Chemical Changes in Everyday Life

Chemical changes are all around us. Think about the metabolic reactions in your body, such as respiration. Basically, respiration is a slowed down process of combustion, which ‘burns’ the sources of energy, just like glucose in the blood gets utilised by cells.

Other examples of chemical changes in everyday life are in cooking, where the application of heat in items like meat denature the protein in the food.

Additionally, when you’re driving your car, a rapid chemical reaction occurs inside the engine as the fossil fuel is burned, reducing it to carbon dioxide, carbon monoxide, soot, and water vapour. Similarly, when you use scented candles or incense, chemical change also occurs.

These are all relatively fast reactions compared to the slow everyday oxidation of metals, like the brass doorknobs in your house, which could take decades before the effects of oxidation are even noticeable.


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