Which is the best evidence that a chemical change has occurred?

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Updated February 01, 2020

By Riti Gupta

Reviewed by: Lana Bandoim, B.S.

Elements, atoms and compounds are often too small for you to see individually. So how can you know if a chemical change has occurred? Luckily, you often don't need any sort of microscope; there are plenty of chemical changes that you can see. Let's take a look at some of the ways you can identify the evidence of a chemical change.

A chemical reaction takes place when one or more substances are changed into a new substance. This means that the chemical composition of a substance changes. It is important to remember that matter is neither created nor destroyed in a chemical reaction. Instead, a substance is simply rearranged to form another one. This new substance can be in any state of matter. Here are the five signs of chemical change.

You've probably seen this kind of chemical change before if you have left your shiny blue bike outside over the rainy winter for too long. You saw rust, of course. Rust forms on your bike when the iron in the metal frame reacts with the oxygen and water in the air. It forms a new compound called iron oxide. This change in color, from blue to reddish brown, indicates that a chemical change has taken place.

When energy is either absorbed or released, it is indicative of a chemical change. Fireworks are an example of a chemical change that produces a temperature change and emits light. Fireworks contain fuel and an oxidizing agent. As the oxidizer breaks down the fuel, it releases an enormous amount of energy. Some of this energy is released as heat, so remember to not get too close!

When two salts are mixed, it is possible for a solid substance to form and precipitate out of solution. This means that if you start with two liquids and then see a solid substance in your beaker, it is clear evidence of a chemical change.

Precipitation reactions are very important in making sure water is safe for human consumption. Water purification facilities make use of the fact that they can add a chemical to the water which will react with an impurity in the water and precipitate. Then the now-pure water can be removed from the solid which contains the impurities.

Production of gas is a clear sign that a chemical change has occurred. A quite delicious example of that is baking a cake. When you bite into a moist cake and see the tiny holes in the cake, this is indicative of the rising agent (either baking soda or baking powder) reacting with the acidic components of the cake to create carbon dioxide. This gas is what helps a cake rise in the oven.

Given this information, would water boiling be an example of a chemical change?

You may be tempted to say, "Well, I see bubbles! That means there is a chemical change." It is important to keep in mind that even though the phase of the water has changed, the chemical structure has not, and you have not made a new compound or destroyed a compound, which is important when it comes to evidence of a chemical change. Instead, this kind of change is called a physical change.

Make sure you keep in mind what a chemical change really is when looking at different chemical states.

Is the light bulb going on by now? There are many reactions that produce light and allow you to see that a chemical reaction has taken place. One example that you may have even witnessed firsthand is the reaction in a light stick. When you bend the light stick and break the small glass vial on the inside you initiate a reaction between hydrogen peroxide and phenyl oxalate ester, which results in the emission of light.

Every day, matter undergoes changes. These changes can be classified as either physical changes or chemical changes.

Physical changes can almost always be reversed. For example, when liquid water freezes and forms ice, you still have water. It is just in a different state. If you dissolve sugar in a glass of water, you can evaporate the water to get back to the solid sugar. This is a special kind of physical change called a phase change. A phase change is a change in state or form in which no new substance forms.

Our world is also full of chemical changes that take place every day. They help us stay warm, feed ourselves, drive cars and play games on our smartphones. Everything you see around you is either undergoing or has undergone chemical change. This includes inside of you!

A chemical change, or chemical reaction, is a process by which one or more substances are changed into others through chemical means. This may involve a substance joining together with another substance. It may also involve a substance breaking apart into different substances. A chemical change is a change in which at least one new substance forms.

Conservation of Mass

When a chemical or physical change happens, you can observe different types of things. One thing that will always stay constant is mass. This means that the total mass of what you begin with will equal what you end up with. This is easiest to measure in a closed system in which no matter can escape.

For instance, let’s take a common combustion reaction such as burning a wood log. To prove the conservation of mass, you could measure the mass of the log before you burned it. After it has been burned, you could measure the mass of the ashes. But the mass of the ashes would be much lower than the mass of the log. So where did the rest of the mass go? It went into the air! Carbon dioxide gas and water vapour are products of combustion reactions. If you collected and measured the gases produced as the log burned, then you would find the missing mass. Antoine Lavoisier did exactly this to prove his point! We now call this idea the Law of the Conservation of Mass.

Other scientists had experimented and proven this law before Lavoisier, but Lavoisier usually gets the credit for it because he published it in what is considered the first chemistry book. This book encouraged a shift from alchemy to modern chemistry. What made his work different was that he used math to describe what happened when mixing elements. His work spotlighted the need to measure and count. We can see this math in action when we balance chemical equations.

How to Tell When a Chemical Reaction May Have Occurred

We can observe many different things when a chemical reaction takes place. We might observe a change in temperature, the emission of light, a change in colour, a release of gas, or a change in the amount of reactants or products. All of these observations are useful when determining whether a chemical reaction has happened.

Heat or Light

Many reactions involve a transfer of energy. In some cases the energy may be felt as heat energy or seen as light energy. Often, both light and heat can be observed at the same time!

When a reaction gives off heat, we call this an exothermic reaction. This means that heat is transferred from the reactants to the external environment. When the opposite occurs and a chemical reaction absorbs heat from its environment, it feels cold. We call this an endothermic reaction. The emission of light from a very hot substance is called incandescence.

Many chemical reactions give off light and/or heat. A familiar example of a chemical reaction that gives off both heat and light is the combustion of propane in a gas stove or BBQs.

Sometimes light is given off without the temperature being high. We call this luminescence. A good example of this happens when you bend a glowstick. The bending allows two chemicals inside to react and give off light. When the light is as a result of an inorganic chemical reaction, we call it chemiluminescence. When a living organism uses chemicals in its body to emit light, we call it bioluminescence.

Colour Change

A change in colour is another sign that a chemical reaction has occurred. A good example of this is when something rusts. Pure iron is naturally silver in colour. We don’t usually observe it this way on Earth because it reacts easily with the oxygen in the air.

The chemical equation of this reaction is:

4Fe + 3O2 + 6H2O → 4Fe(OH)3

As you may already know, rust is a reddish colour. It is responsible for the beautiful colours we see in the Grand Canyon, and the colour of the soil on Mars.

Another impressive colour change reaction is that of potassium permanganate and sugar (glucose). In this reaction, the colour changes as more reactants are turned into products! But no need for lab chemicals to see this for yourself. Try this activity to see this at home!

Release of Gas

Sometimes a chemical reaction produces a gas as one of its products. When this happens, we can observe bubbles or fumes.

An example of a chemical reaction that produces a gas is the reaction between baking soda and lemon juice (citric acid). If you mix baking soda with lemon juice (or vinegar), both substances quickly react to form bubbles. The bubbles are made of carbon dioxide gas (CO2(g)). Carbon dioxide bubbles are also responsible for the fizz you see in soft drinks. The burning of paper or wood (a type of combustion reaction) also produces carbon dioxide gas as well as water vapour.

Misconception Alert!
Be careful to not confuse a phase change with a gas formation. Water vapor formed when heating water is not evidence of a chemical change!

In some chemical reactions, the gas produced can be highly toxic. This can happen at home if certain cleaning products are mixed together. Breathing the gas created from mixing some cleaning products can even result in death. This is why you should never use two different cleaning products together - ever!

Precipitate Formation 

Sometimes when two solutions undergo a chemical reaction, one of the new products formed from the reaction is not soluble. This means that it is not able to dissolve in the new solution. Instead, we can observe a solid that is distinct and separate from the solution. We call this solid a precipitate. You have probably heard the word “precipitation”, but maybe not when talking about chemistry. We also use the word precipitation to refer to the types of water that fall from the sky. The solid in a precipitation reaction also often falls, or sinks, to the bottom of the solution. Precipitation reactions are all double replacement reactions.

The video below shows many examples of a formation of a precipitate, including ones in which colourless solutions form colourful solid products. Who knew that chemistry could be so beautiful!