Using English to give Evidence

Evidences of Chemical Reactions

     All chemical bonds posses potential energy.  In a chemical reaction this energy is changed when old bonds are broken and new bonds are formed. Chemical changes are different from physical changes.  When a physical change occurs there is no breaking and forming of bonds.  There are certain things that will help us identify if a chemical reaction has taken place.  We call these evidences of chemical reactions.  

1. Light is given off: Potential energy in valance electrons is released in the form of light.

2.Gas is given off: Indicates an increase in kinetic energy of the atom (compound) as it is changed to a gas.

3. Temperature change: Exothermic is an increase in the speed of the molecules (heat). Exothermic is the taking in of energy.

4. Color change: the increase or decrease of potential energy stored in new bonds.

5. Precipitate formed: increase or decrease of potential energy in new bonds formed.

6. Electrical potential: The release of chemical bond energies to cause the movement of  valence electrons. 

Some chemical changes are easy to observe. The products formed by the rearrangement of atoms look different than the reactants. Other changes are not easy to see but can be detected in other ways. Color Change Substances often change color during a chemical reaction. For example, when gray iron rusts, the product that forms is brown, as shown in the photograph below. Formation of a Precipitate Many chemical reactions form products that exist in a different physical state from the reactants. A solid product called a may form when chemicals in two liquids react, as shown in the photograph below. Seashells are often formed this way when a sea creature releases a liquid that reacts with seawater. Formation of a Gas Chemical reactions may produce a gas, like that formed when antacid pills are mixed with stomach acid. The photograph below shows an example in which carbon dioxide gas is produced by a chemical reaction. Temperature Change Most chemical reactions involve a temperature change. Sometimes you can recognize the change when you see a flame. Notice the burning of the metal magnesium in the photograph. Other temperature changes are not immediately obvious. If you touch concrete before it hardens, you may notice that it feels warm. This warmth is due to a chemical reaction.

All chemical reactions, whether simple or complex, involve changes in substances. One or more starting substances, the reactants, are changed into one or more new substances, the products.

Reactants à Products

In a chemical reaction the ways in which atoms are joined together are changed. Bonds are broken and new bonds are formed as reactants are converted into products. The atoms are not created or destroyed. They are just rearranged.

Chemical reactions can be described in different ways. For example, we could say: "Iron reacts with oxygen to produce iron (III) oxide (rust)." Alternatively, we could identify the reactants and product in this reaction by writing a word equation.

Iron + oxygen à iron (III) oxide

In a word equation, the reactants are written on the left, and the products are written on the right. They are connected by an arrow ( à ) that is read as "yields" or "reacts to produce." Word equations communicate the reaction but can get cumbersome. To be more efficient chemists use chemical formulas for writing equations. For example the rusting of iron would be represented as follows:

Fe + O₂ à Fe₂O₃

Equations that show just the formulas of the reactants and products are called skeleton equations. A skeleton equation is a chemical equation that does not indicate the relative amounts of the reactants and products. The physical state of a substance in a reaction can be indicated in the equation by using the following symbols: (s) for solid, (l) for liquid, (g) for gas, and (aq) for a solution in water. These usually follow the substance in the equation and can also be written as subscripts. To write a skeleton equation, you must write the correct formulas of the reactants and products with the reactants on the left side of the yield sign and the products on the right.

Just writing the skeleton equation of reaction does not always correctly communicate what is happening in the reaction. To represent chemical reactions correctly, equations must be balanced so that they show the amount of reactants and products in the reaction. In every balanced equation each side of the equation has the same number of atoms of each element. This is necessary to be consistent with the law of conservation of mass. Remember, in a chemical reaction, atoms are not created nor destroyed; they are simply rearranged.