Classifying chemical reactions
What happens during chemical reactions? Evidence of chemical reactions
Types of 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?
Click for larger imageDuring chemical reactions the bonds between atoms or ions are broken, the atoms rearranged and new substances formed. A chemical reaction always forms one or more new substances.

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:

  1. Color change - has a new substance formed with a different color? In this example blue litmus paper turns red when placed in acid.
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  1. Gas formation or consumption - Are gas bubbles being released from a solution? Antacid tablets in water release carbon dioxide gas.
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  1. Formation of a precipitate - Has a solid been formed from a mixture of solutions?
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  1. 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.
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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.

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