What is it called when a chemical compound breaks to form two or more than two new substances?

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Classifying chemical reactions

Chemical reactions are changes in which new substances are formed. Magnesium burning in oxygen to form magnesium oxide is a chemical change. On the other hand, ice melting to form liquid water is a physical change because ice and water are the same chemical substances.

What happens during chemical reactions?

Chemical changes can be identified by looking for changes and the release or absorption of heat, i.e. the reaction vessel gets hot or cold. For example, when magnesium, a silvery metal, burns, a brilliant white light is seen, heat is released and a white powder is formed.

Evidence of chemical reactions
You cannot see the atoms and molecules breaking bonds and forming new bonds, but you can see the results of these changes. A cake rising in the oven, gas bubbles forming in an antacid drink or sausages burning on a BBQ are all evidence of chemical change.

There are four key indicators to watch for in deciding if a chemical reaction has occurred:

Colour change - has a new substance formed with a different colour? In this example blue litmus paper turns red when placed in acid.
Gas formation or consumption - Are gas bubbles being released from a solution? Antacid tablets in water release carbon dioxide gas.
Formation of a precipitate - Has a solid been formed from a mixture of solutions?
The release or absorption of energy - All chemical reactions involve a change in energy. Most release energy to their surroundings, heating the vessel and air around the reaction, but a few absorb energy cooling their surrounds.

Types of chemical reactions
Although there are millions of different chemical reactions, many occurring as complex sequences of events, there is some pattern to the chemistry involved. Most simple chemical reactions can be divided into four types based on the number of reactants and how the reactant molecules and atoms behave.

The four types of chemicals reactions are:

1. Decomposition reactions -This reaction occurs when a single substance breaks down, or decomposes, into two or more simpler substances. One example is electrolysis of water when an electric current is being passed through water forming hydrogen and oxygen gas.

2. Synthesis reactions - These are also known as combination or addition reactions. Synthesis occurs when two or more simple substances combine to form one more complex substance. Synthesis is the opposite to addition reactions. Rockets are propelled by combining hydrogen and oxygen to form water!

3. Single replacement reactions - In these reactions, an atom or group of atoms replaces another atom or group of atoms in a compound to form a new compound. Copper plating a coin or knife blade is an example of single replacement.

4. Double replacement reactions - More complex than single replacement reactions, these reactions involve two pairs of ions in solution swapping partners to form two new compounds. Acid/base reactions are double replacement reactions.

This article will cover the main classifications of chemical reactions: synthesis reaction, decomposition reaction, single replacement reaction, and double replacement reaction. We also discuss what is a combustion reaction, precipitation reaction, and acid base reaction.

Vocabulary

• synthesis reaction- a reaction that occurs when two atoms interact to form one atom
• decomposition reaction– a reaction that occurs when a compound breaks down into two or more atoms
• combustion reaction – when a substance reacts with oxygen, forming light and heat in the form of fire
• single replacement reaction– a reaction that occurs when a new compound is formed when one element is substituted for another element in a compound, creating a new element and a new compound as products
• double replacement reaction – a reaction in which the cationic or the anionic species switch places, creating two new products
• neutralization (acid base reaction)- a double replacement reaction in which an acid reacts with a base to form water and salt
• precipitation reaction– a double replacement reaction in which forms a solid from two miscible liquids

The Four Basic Types of Chemical Reactions

Synthesis Reaction

A synthesis reaction occurs when two reactants interact to form one product. The product created is different from both of the reactants. The general equation represents this type of reaction:

In most cases, synthesis reactions release energy. Reactions that release energy are considered exothermic. A typical example of a synthesis reaction is the formation of table salt. Sodium and chlorine ions interact to form sodium chloride.

Decomposition Reaction

A decomposition reaction occurs when the reactant breaks down into simpler products. Here is the general equation that represents this type of reaction:

Unlike synthesis reactions, decomposition reactions require energy to break the bonds present in the reactant. Reactions that require an input of energy are endothermic. A common example of a decomposition reaction is the decomposition of hydrogen peroxide. The decomposition of hydrogen peroxide results in water and oxygen gas. This is shown in the following equation:

Single replacement Reaction

Single replacement reactions occur when a single element replaces an element in another compound. This produces a new compound and a new element. This type of reaction is represented by the general equation

In this equation, C replaces B in the reaction, and B is now a single element. A common example of a single replacement reaction is the reaction of Tin chloride and zinc. In the reaction, zinc replaces tin to form zinc chloride and tin as a single element. The chemical equation of this reaction is:

Video of a Single replacement reaction

Double replacement Reaction

A double replacement reaction exchanges ionic species in two compounds to form two completely new compounds. Double replacement reactions swap cations or the anions, but not both. The general equation that represents this type of reaction:

An example of a double-replacement reaction is the reaction between Lead nitrate and Potassium iodide. The lead cation and potassium cation switch places.

Precipitation Reactions

Precipitation and neutralization are both double replacement reactions. These reactions both result in two completely new compounds through double replacement. A precipitation reaction occurs when two soluble compounds mix to form an insoluble solid. The solid that separates from the solution is called the precipitant. A classic example of a precipitation reaction is Silver nitrate’s reaction with Potassium chloride, which forms silver chloride, a white solid.

Video of a Precipitation Reaction

Acid Base Reactions

Acid base reactions, or neutralization reactions are double replacement reactions that occur between acids and bases. Typically, acid-base neutralizations produce water and a salt. A common example of neutralization is between hydrochloric acid, a strong acid, sodium hydroxide, a strong base. Learn more about acid-base neutralization reactions.

Combustion Reactions

Combustion reactions are those that involve the burning of compounds. A reactant, usually a hydrocarbon, reacts with oxygen gas (O2), to produce carbon dioxide gas (CO2) and water vapor (H2O). Combustion reactions also produce energy in the form of heat and/or light.

Example: the combustion of fuel propels the movement of cars. C8H18 (octane), or gasoline, reacts with oxygen gas in the air to produce carbon dioxide gas and water vapor, but most importantly, energy.

2C8H18 + 25O2 → 16CO2 + 18H2O

Further Reading

• Balancing Chemical Equations
• Law of Conservation of Mass