Physical and Chemical Changes (G3 Lower Secondary Science)

Physical change

Process where atoms and chemical bonds are not rearranged. No new substances are formed.

Examples: Expansion, contraction, state changes

Freezing

Physical change where liquid → solid

Melting

Physical change where solid → liquid

Condensation

Physical change where gas → liquid

Boiling

Physical change where liquid → gas throughout the whole liquid at the boiling point

Evaporation

Physical change where liquid → gas at the surface of the liquid at any temperature

Chemical change

Process where atoms and chemical bonds are rearranged. Atoms are not created or destroyed (conservation of mass), but new substances are formed.

Also known as chemical reaction

Reactant

What takes part in the chemical reaction, written before → in the chemical equation

Product

What is produced in the chemical reaction, written after → in the chemical equation

Conservation of mass

In a chemical reaction, the number of atoms of each element does not change. Atoms are neither created nor destroyed.

Thermal decomposition

Chemical reaction that occurs under heating, where a substance is broken down
E.g., Calcium carbonate → Calcium oxide + Carbon dioxide

Photosynthesis

Chemical reaction that occurs under light
Opposite of cellular respiration
Carbon dioxide + Water (+ Light) → Glucose + Oxygen

Oxidation

Chemical reaction that occurs with exposure to oxygen

Cellular respiration

Type of oxidation (a chemical reaction)
Opposite of photosynthesis
Glucose + Oxygen → Carbon dioxide + Water (+ Energy)

Rusting

Type of oxidation (a chemical reaction)
Iron + Oxygen + Water → Rust

Combustion

Burning, which is oxidation (a chemical reaction)
E.g., Fuel + Oxygen → Carbon dioxide + Water

Electroplating

A type of chemical reaction that occurs under an electric current

Acidic solutions

pH < 7
Taste sour
Can be corrosive
Named “___ acid” (e.g., hydrochloric acid)

Neutral solutions

pH = 7
Pure water

Alkali solutions

pH > 7
Taste bitter
Can be corrosive
Examples include “___ hydroxide” (e.g., sodium hydroxide)

Litmus (colors)

Turns red in acidic solutions
Turns blue in alkali solutions

Universal indicator (colors)

Acidic: Red, orange, yellow
Neutral: Green
Alkali: Blue, purple
(Happens to parallel the colors of the rainbow)

Neutralization

Reaction between acid and base (to give salt and water)

Acid + Base →

Salt + Water

Acid + Carbonate →

Salt + Carbon dioxide + Water

Acid + Metal →

Salt + Hydrogen

Salt

Ionic compound produced by reaction of acid with alkali/carbonate/metal
Cation (+) - From alkali/carbonate/metal
Anion (-) - From acid

Hydrochloric acid + Sodium hydroxide →

Sodium chloride + Water

(Salt + Water; this is an acid-base reaction)

Sulfuric acid + Magnesium hydroxide →

Magnesium sulfate + Water

(Salt + Water; this is an acid-base reaction)

Hydrochloric acid + Calcium carbonate →

Calcium chloride + Carbon dioxide + Water

(Salt + Carbon dioxide + Water; this is an acid-carbonate reaction)

Sulfuric acid + Sodium carbonate →

Sodium sulfate + Carbon dioxide + Water

(Salt + Carbon dioxide + Water; this is an acid-carbonate reaction)

Hydrochloric acid + Magnesium →

Magnesium chloride + Hydrogen

(Salt + Hydrogen; this is an acid-metal reaction)

Nitric acid + Calcium →

Calcium nitrate + Hydrogen

(Salt + Hydrogen; this is an acid-metal reaction)