Basic chemistry

What is pH scale?

pH and pH scale

PH and pH scale

ph scale
ph scale

The pH or hydrogen potential is a measure of the acidity or basicity of a solution.

The pH scale is used to determine the degree of acidity of a substance. It represents the concentration of ions (H+)in a solution.The more ions a substance contains (H+), the more acidic it is . The more it contains ions (OH−), the more it is basic .

The pH scale is divided into steps from 0 to 14, with 0 being the highest degree of acidity while 14 is the lowest degree of acidity.

The lower the pH value (the closer it gets to 0), the more acidic the substance is.

Substances can also be classified according to their strength. An acid with a pH approaching 0, like hydrochloric acid, is a strong acid, while an acid with a pH approaching 7, like coffee, will be considered a weak acid. For bases, a substance whose pH is close to 14, such as sodium hydroxide, will be classified as a strong base, while a base with a pH approaching 7 will be considered as a weak base, as the sea ​​water.

PH, a logarithmic scale

The pH is a logarithmic scale, that is, when a solution becomes ten times more acidic, its pH decreases by one. If a solution becomes 100 times more acidic, its pH will decrease by two.

Lemon juice, which has a pH of 2, is 100 times more acidic than tomato juice, whose pH is 4.

Soap, whose pH is 10, is 10 times more basic than sodium bicarbonate, which has a pH of 9.

To determine the pH value from the molar concentration, use the exponent (not including the negative sign) of the concentration in scientific notation.

Variation in acidity or basicity H+ Scientific notation concentration (mol / L) pH
Strong acid
Weak acid
Low base
Strong base
1 1×100 0
0.1 1×10-1 1
0.01 1×10-2 2
0,001 1×10-3 3
0.0001 1×10-4 4
0.00001 1×10-5 5
0.000001 1×10-6 6
0.0000001 1×10-7 7
0.00000001 1×10-8 8
0.000000001 1×10-9 9
0.0000000001 1×10-10 10
0.00000000001 1×10-11 11
.000000000001 1×10-12 12
.0000000000001 1×10-13 13
.00000000000001 1×10-14 14

The following table gives examples of pH for some solutions of our daily life.

Acidic solutions
Basic solutions
substance pH substance pH
Hydrochloric acid 0.0 Saliva 7.2
Acid battery <1.0 Blood 7.4
Gastric acid 2.0 Egg white 7.8
vinegar 2.9 Sea water 8.0
Orange or apple juice 3.5 Milk of magnesia 10.5
beer 4.5 Ammonia 11.5
tea 5.5 Bleach 12.0
Acid rain <5.6 lime 12.5
milk 6.5 Pipe unblocker 14.0


For more details on the pH and the pOH, it is possible to consult the following sheet: The relation between the pH and the concentration of the ions hydronium and hydroxide

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