Learning physics and chemistry easily and freely - Science for elementary school, middle school and high school

Free online chemistry lesson for elementary school, middle school and high school.


Air and atmosphere


1) What is an atmosphere ?

Definition of an atmosphere: this is a layer of gases surrounding a celestial body.

Examples of celestial bodies that have an atmosphere: Earth, Mars, Venus, Jupiter, the Sun, Titans.
Examples of celestial bodies with no atmosphere: the Moon, Mercury, Pluto.

Comment: The presence of an atmosphere around a celestial body doesn't systematically means that it can sustain life.

2) Characteristics and layers of Earth's atmosphere.

The Earth's atmosphere is the layer of air surrounding the Earth.
Its thickness is estimated at about 500 km.

Temperature variations in Earth's atmosphere are not regular. In some areas it decreases and it increases in others. This leads to distinguish several layers:
- The troposphere from 0 to (on average) 12 km in altitude. The temperature decreases with altitude. The troposphere contains about 90% of the air.
- The stratosphere extends roughly from 12 to 50 km altitude. The temperature is steadily increasing.
- The mesosphere extends roughly from 50 to 80 km altitude. The temperature is steadily increasing.
- Thermosphere extends  roughly from 80 to 500 km

Comment: sometimes an additional layer called the exosphere is included in atmosphere. Exosphere is a transition area  to interplanetary space that it is still protected from particles emitted from the Sun, by the Earth's magnetic field.

Science class

Mechanics lessons

Water on Earth
Changes of state in the nature: the water cycle
Water in human body
Test for water
Properties of water in different states

States of matter

States of matter and its changes
Boiling water
Water: freezing and melting
Changes of state: mass and volume
Molecules in different states of matter

Mixtures and solutions

Heterogeneous mixtures
Homogeneous mixtures
Vaporization of water
Dissolving a solid in water
Conservation of mass on dissolving
Miscibility in water

Mass and volume

Volume and its units
Measuring volume with a graduated cylinder
Mass and its units
Measuring the mass of a liquid
Mass of a liter of water
Volumetric mass density

Air and atmosphere

Earth's atmosphere protect us
Threats to the Earth's atmosphere
Composition of air
Air and life

Atoms and molecules

Molecules in mixtures and pure substances
Molecules and states of matter
Composition of molecules
Constituents of the atoms
The electrical neutrality of the atom


Basics of combustion
The combustion of carbon
The combustion of butane

Atoms and chemical reactions

Chemical reactions
Chemical equations
Law of conservation of mass
Chemical synthesis


Most common metals
How to distinguish metals ?

Corrosion of metals
Electrical conductivity of solid materials
Electrons and free electrons
Electric current in metals


The conductivity of aqueous solutions
Aqueous solutions and ions
The direction of movement of ions
Formation of ions
Tests for ions

Acidic and basic solutions

pH of aqueous solutions
Ions in acidic and basic solutions
Dilution of acids and bases
Composition of hydrochloric acid
Chemical reaction between iron and hydrochloric acid

Electrochemical cells and chemical energy

Chemical reaction beteween a copper sulphate solution and zinc
Copper sulfate and zinc battery
Basics of electrochemical cell


©2021 Physics and chemistry