How is a Chemical Reaction Defined?
So what is a chemical reaction? What is a chemical change, and what is a physical change? A chemical reaction occurs when compounds or substances undergo a chemical change to form different compounds or substances. Chemical equations express chemical reactions in terms of symbols and chemical formulas of the compounds involved in reactions. Let’s looks at some chemical change examples, and chemical reaction examples.
Chemical vs. Physical Changes
In a physical change, a compound’s shape may change, but its chemical identity will not. For example, freezing or boiling water is a physical change. Once melted or boiled, the water may be in a different form (solid ice or gaseous water vapor), but it is still water, H2O, and it still has the same chemical composition.
A chemical change happens when a substance’s chemical identity changes. An example of this is rusting. When an iron nail comes into contact with water and is then exposed to the air, it rusts, forming a brown-red substance. This process changes the chemical composition of the original substance.
While some physical changes are easily reversible, such as re-freezing melted ice, reversing a chemical change requires another chemical reaction.
Indications of a Chemical Reaction
Based on its definition, a chemical reaction occurs when a substance undergoes a chemical change. So how can we tell if a chemical change has occurred?
1. Heat or Light Emissions
If a reaction emits heat or light, that is an indication that energy has been released from the reaction. One example of this is a fire or spark, which is evidence of combustion. Because many physical changes also involve heat or light, this is not concrete, definitive evidence of a reaction. But it is a strong argument that one has occurred.
2. Change in Color
A change in color is a possible indication of a chemical reaction.
3. Gas Formation
When substances react and bubbles or a vapor appears, this is an indication that a chemical reaction is occurring. An example of this is when baking soda and vinegar react violently to produce carbon dioxide gas in the famous volcano experiment.
4. Precipitate Formation
A precipitate is a solid product that forms as a result of a chemical reaction between liquid reactants. An example of a precipitate is yellow cadmium sulfide, which is produced when water solutions of ammonium sulfide and cadmium nitrate are combined.
The most definitive way to confirm a chemical reaction is to analyze the chemical compositions of the reactants and products.
Chemical Change Examples
Here’s ten examples of chemical changes:
- Oxidation (rusting)
- Biological decomposition or fermentation
- Cooking an egg
- Chemical decomposition
- Reacting acids and bases together
- Chemical batteries
- Paper Burning
Chemical Reaction Examples
- Electrolysis (separating water into its elements, hydrogen and oxygen, using an electric current)
- Oxidation (the rusting of iron metal when exposed to moisture and oxygen in the air)
- Photosynthesis (the process by which plants convert carbon dioxide and water into glucose and oxygen using energy from sunlight)
- Soft drinks going flat (carbonic acid in soft drinks decomposes into its elements, carbon dioxide and water)
- Table salt formation (combination of sodium and chlorine to form table salt)
- Cellular respiration (the process by which organisms convert oxygen and food molecules into carbon dioxide and energy)
- Driving (the combustion of gasoline and oxygen in engines to produce carbon dioxide, water vapor, and energy)
- Digestion (hydrochloric acid and enzymes break down proteins in organisms’ stomachs)
- Baking soda – vinegar volcano experiment (combining baking soda and vinegar to produce sodium acetate, carbon dioxide, and water)
- Batteries (a galvanic cell converting chemical energy into electrical energy)
- Cooking a steak on the grill (new compounds are formed from the intense heat)
Physical Change Examples
- Ice melting (water changing phases)
- Salt dissolving in water (the compound is simply going into solution)
- Breaking a glass into fragments
- Cutting paper into pieces
- Sugar crystallizing out of solution as the water evaporates
- Water evaporating into the air
- Mixing oil and water
- Melting sulfur (even though the color changes)
- Melting wax