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


Acidic and basic solutions

pH of aqueous solutions

1) What is pH ?

Acidity is a rather familiar concept because many foods and drinks are acidic in varying degrees: vinegar, lemon juice, tomatoes, citrus fruits etc..
Unlike acidic solutions there are basic solutions. It is difficult for us to understand what is a basic solution because food and beverages are almost never basic. There are basic solutions among cleaning products like bleach or detergents.

The pH is a number that indicates whether a solution is acidic or basic and with what degree.
The pH is a chemical variables that have no unity.



2) How to measure pH ?

The pH can be measured using pH paper:
It consists of strips soaked with chemicals that take different colors depending on pH.
A drop of solution must be placed on a strip. Comparing its color to a color index, we can deduce the pH value.

Comment: a pH paper gives only an approximate value of the pH to the nearest unit.

The pH of a solution can be measured using a device called a pH meter.
It consists of a probe (itself consists of electrodes). Once this probe is immersed in a solution, the pH meter shows the pH value.
The pH meter provides a quick and accurate measurement (usually to the nearest tenth).

3) pH of acidic and basic solutions

- If the pH is between 0 and 7 then the solution is acidic.
The lower the pH is, the more acidic the solution is

- If the pH is between 7 and 14 then the solution is basic.
The higher the pH is, the more basic the solution is

- If the pH is 7 then the solution is neither acid nor basic, it is said to be neutral