Chemical reactions are processes in which substances change into other substances.

You know a chemical reaction takes place if one or more of these occur:
 

• Color changes - Different combinations of molecules reflect light differently. A color change indicates a change in molecules.
   
• Heat content changes - In all chemical reactions, the heat content of the reactants and the heat content of the products is never the same. Sometimes the difference is great and can be easily detected. At other times, the difference is slight and more difficult to detect.
   
• Gas produced - Whenever a gaseous product forms in a liquid solution, bubbles can be seen. A colorless gas produced in a reaction of solids is much harder to detect.
   
• Precipitate forms - Precipitates are insoluble products formed by a reaction taking place in a liquid solution. This insoluble product will eventually settle to the bottom, but might immediately appear by turning the clear solution cloudy.

• A compound breaks into parts.
   
• compound → element + element
   
• 2H₂O → 2H₂ + O₂

Some decomposition complications with heat:

• Some acids, when heated, decompose into an acidic oxide and H₂O. H₂SO₃ → SO₂ + H₂O  
• Metallic hydroxides, when heated, decompose into a metallic oxide and H₂O. Ca(OH)₂ → CaO + H₂O  
• Metallic carbonates, when heated, decompose into a metallic oxide and CO₂. Li₂CO₃ → Li₂O + CO₂  
• Metallic chlorates, when heated, decompose into metallic chlorides and O₂. 2KClO₃ → 2KCl + 3O₂  

• Elements are joined together.
   
• element + element → compound
   
• 2H₂ + O₂ → 2H₂O
   
• Compounds are joined together
   
• compound + compound → compound
   
• 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

• A single element replaces an element in a compound.
   
• element + compound → element + compound
   
• Zn + 2HCl → H₂ + ZnCl₂

• An element from each of two compounds switch places.
   
• compound + compound → compound + compound
   
• H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O

• A hydrocarbon (a compound containing only carbon and hydrogen) combines with oxygen.
   
• The products of combustion are always carbon dioxide and water.
   
• hydrocarbon + oxygen → carbon dioxide + water
   
• CH₄ + 2O₂ → CO₂ + 2H₂O
   
• When metallic substances combine with oxygen, the result is an oxidation-reduction reaction.

Neutralization Reactions

• Special types of double displacement reactions that involve the reaction between an acid and base to form a salt and water.
   
• acid + base → salt + water
   
• Heat is usually given off in neutralization reactions.
   
• A suspension of solid magnesium hydroxide in water is widely used as an antacid to neutralize excess stomach acid:
    Mg(OH)_{2 (s)} + 2HCl _(aq) → MgCl_{2 (aq)} + 2H₂O _(l)

Oxidation-Reduction Reactions

• Any reaction in which elements experience a change in oxidation number.
   
• one atom gains e^&minus and another atom looses e^&minus
    S + O₂ → SO₂  
• In the reaction above, sulfur and oxygen both have an oxidation number of zero before the reaction. After the reaction, sulfur is +4 and oxygen is −2.

Precipitation Reactions

• Aqueous reactions that involve the formation of a precipitate (solid).
   
• soluble compound + soluble compound → insoluble compound
    2KI _(aq) + Pb(NO₃)_{2 (aq)} → 2KNO_{3 (aq)} + PbI_{2 (s)}  
• The physical state symbol (aq) says the reaction is taking place in a water solution. The physical state symbol (s) says the lead (II) iodide is a solid - therefore insoluble in the solution.

Types of Reactions

 
Predicting Reaction Products

 
Predicting Reaction Products

 
Chemical Reactions