Chemical vs Physical Changes

Theory Exercises

Physical vs Chemical Changes

Physical Changes

A physical change is a change in the matter that doesn't alter its identity. The substance remains the same, but its form or state may be different.

Characteristics of Physical Changes:

The identity of the substance is preserved
The chemical composition remains unchanged
Generally reversible (can be undone)
No new substances are produced
Often involves a change in state or appearance

Examples of Physical Changes:

Melting: Ice → Water (heating)
Freezing: Water → Ice (cooling)
Evaporation: Water → Water vapor (heating)
Condensation: Water vapor → Water (cooling)
Dissolving: Salt dissolving in water
Breaking or cutting: Paper tearing, glass breaking
Crushing: Crushing rock into powder
Boiling: Water boiling at 100°C

Ejemplo: When ice melts, it changes from solid to liquid, but it's still water (H₂O). No new substance is formed, and if you freeze the water again, you get ice back.

Properties Affected in Physical Changes:

Property	Example
State	Solid → Liquid → Gas
Size and Shape	Cutting paper changes its shape
Color	Not typically affected (though it may appear different due to physical state)
Density	Changes when state changes
Appearance	Can change while identity remains the same

Chemical Changes

A chemical change is a change in matter that produces one or more new substances with different properties and composition. A chemical reaction has occurred.

Characteristics of Chemical Changes:

New substances are formed
The chemical composition changes
Generally irreversible (difficult or impossible to undo)
Often produces energy (heat, light, sound)
Involves breaking and forming chemical bonds
Usually shows observable signs (color change, gas evolution, heat, light)

Examples of Chemical Changes:

Combustion: Burning wood or fuel
Oxidation: Rusting of iron
Decomposition: Rotting of food
Cooking: Cooking an egg
Neutralization: Acid + Base → Salt + Water
Photosynthesis: Water + Carbon dioxide → Glucose + Oxygen
Fermentation: Sugar → Alcohol + Carbon dioxide

Ejemplo: When wood burns, it reacts with oxygen to form ash, carbon dioxide, and water. The original wood is gone, and completely new substances have been formed. This cannot be reversed easily.

Observable Signs of Chemical Changes:

Color change (e.g., copper wire turns black when heated)
Gas evolution (e.g., vinegar + baking soda produces bubbles)
Heat/Light production (e.g., combustion)
Precipitate formation (e.g., mixing solutions forms a solid)
Temperature change (e.g., ice melting feels cold, but burning feels hot)
Smell change (e.g., decomposing matter)
Sound (e.g., explosion)

Key Differences

Aspect	Physical Change	Chemical Change
New Substances	No	Yes
Identity	Preserved	Changed
Chemical Composition	Unchanged	Changed
Reversibility	Often reversible	Usually irreversible
Energy	May absorb or release	Usually produces energy
Observable Signs	May be subtle	Usually obvious
Example	Melting ice	Burning paper

Differentiating Between the Two

Ask yourself these questions:

Is a new substance formed?

- YES → Chemical change - NO → Physical change

Can it be easily reversed?

- YES → Likely physical change - NO → Likely chemical change

Are there obvious signs (color change, gas, heat, light)?

- YES → Likely chemical change - NO → Likely physical change

Does the chemical composition change?