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.


Sates of matter

Water: freezing and melting ( or solidification and fusion )

The essential about freezing and melting temperature

Freezing of pure water

We can cause the freezing of the pure water by immersing it in a refrigerant mixture having a temperature below 0 ° C. Temperature measurements then show that during solidification, pure water keeps a constant temperature of 0 ° C.
The solidification of pure water always occurs at a constant temperature of 0 ° C

Freezing of salt water

During the freezing of a salt water it can be observed that its temperature: regularly decreases below 0°C until it is completely frozen.
More generally, during the freezing of a mixture, its temperature keeps on decreasing below the freezing temperature of the pure substance.
This conclusion is also true for the vaporization, melting, liquefaction, sublimation or deposition of a mixture.
Furthermore, measuring the freezing point of a substance is a method used to check its purity ( this method is called cryoscopy )
      ice and salt water
Photo: Liquid salt water and ice can coexist at negative temperatures because salt water freeze below 0°C

Photo courtesy of
National Oceanic and Atmospheric Administration/Department of Commerce

Melting of pure water

Measurements of temperature in melting ice, would show that pure water keeps a temperature of 0°C.
The melting of pure water always occurs at a constant temperature of 0°C.
The melting temperature of pure water is therefore the same as its freezing temperature and it is also true for all other pure substances: melting and freezing occur at the same temperature.

Melting of another substance: cyclohexane

Cyclohexane is a pure substance whose fusion can be compared to that of pure water.
Its melting occurs at a constant temperature, like pure water, but at 6 ° C while the pure water has a melting temperature of 0 ° C
A pure substance changes state at a constant temperature that is characteristic of this substance.
Particularly a melting or freezing at the constant temperature of 0°C is a characteristic that allows to identify pure water.

Learn more

- Salt and the freezing point of water: This article explains why freezing point of salt water is below 0°C and describes molecules of water in this solution.

- Freezing and melting: notes about temperature, energy exchanges and molecules during freezing and melting of various substances.

- The effect of dissolved Salt and sugar on the melting rate of ice
Projet de sciences par un élève qui a fait un travail très sérieux afin de comparer les effets du sucre et du sel sur la température de congélation de l'eau.

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


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