pH Scale

Last Updated : 28 Apr, 2026

Acids and bases are chemical substances with opposite properties.

  • Acids are substances that release hydrogen ions (H⁺) in a solution.
  • Bases are substances that release hydroxide ions (OH⁻) in a solution.

pH is a measure of how acidic or basic a substance is. It tells us the concentration of hydrogen ions (H⁺) in a solution.

ph_scale

The strength of acids and bases can be measured using the pH Scale. This scale helps to determine how acidic or basic a solution is by measuring the concentration of hydrogen ions present in it.

  • A pH value of 7 represents a neutral solution.
  • A higher concentration of hydrogen ions makes the solution more acidic, while a lower concentration makes it more basic.
  • As the pH value decreases from 7 to 0, the acidity of the solution increases.
  • As the pH value increases from 7 to 14, the alkalinity of the solution increases.
  • Litmus paper is used to test whether a substance is acidic or basic, and it changes color accordingly.
  • The pH scale is based on standard solutions whose values are internationally accepted.
  • It is widely used in chemistry and daily life.
  • The food and drinks we consume can influence the pH level in our body.

pH Indicator

pH indicators are substances used to determine whether a solution is acidic or basic. They show a change in colour (or smell) when added to a solution, depending on its pH value. These indicators help in identifying the nature of a solution but do not provide the exact pH value.

Types of pH Indicators

pH indicators are mainly classified into three types:

1. Natural Indicators

Natural indicators are obtained from natural sources such as plants.

  • Litmus (obtained from lichens): Acid turns red, Base turns blue.
  • Turmeric: Base turns reddish-brown.
  • China rose (hibiscus): Acid turns pink, Base turns green.

2. Synthetic Indicators

Synthetic indicators are man-made chemicals that give a clearer and sharper colour change. These are widely used in laboratories for accurate identification.

  • Phenolphthalein: Acid → colourless, Base → pink
  • Methyl orange: Acid → red, Base → yellow

3. Olfactory Indicators

Olfactory indicators are substances whose smell changes in acidic or basic solutions. These are less commonly used but help in identifying the nature of substances based on odour.

  • Onion: smell disappears in base
  • Vanilla essence: smell disappears in base

Limitations of pH Scale

Although the pH Scale is very useful for measuring the acidity and basicity of solutions, it also has some limitations. These limitations explain the situations where the pH scale may not give complete or accurate information about a substance.

  • The pH scale is generally limited to values between 0 and 14. Very strong acids and bases can have pH values below 0 or above 14, which are not commonly represented at the Class 10 level.
  • The pH scale works properly only for aqueous (water-based) solutions. It cannot be used directly for non-aqueous solutions.
  • pH does not directly show the strength of an acid or base; it shows only the concentration of hydrogen ions (H⁺) in a solution. Strength depends on the degree of ionisation.
  • Very dilute solutions may give pH values close to 7, even if they are acidic or basic, which can sometimes cause confusion.
  • The pH scale does not give information about the nature of the salt present in a solution.
  • Measurement using indicators may not be fully accurate. For precise pH values, a pH meter is required.
  • Temperature affects pH values, but the basic pH scale (0–14) is usually considered at 25°C. Changes in temperature can slightly change pH.

Importance

The pH Scale is very important in many areas of daily life and science. It helps us understand the acidity or basicity of different substances and plays a significant role in processes related to health, agriculture, environment, and industry.

1. Digestive System 

  • In the human body, all the physiological reactions take place at the pH of 7 - 7.8.
  • Hydrochloric acid is secreted as food enters the stomach(turns the pH of the stomach between 1 and 3) because of overeating or other reasons excess of HCl (hydrochloric acid) is released.
  • The excess of HCl in the stomach causes indigestion which produces pain and irritation (pH level of the stomach decreases).
  • To cure indigestion, we can take bases called antacids (anti-acid). Antacids are a group of mild bases.
  • Being basic in nature, they react with excess acid in the stomach and neutralise it. (which have no toxic effects on our body)
  • Milk of Magnesia (Magnesium hydroxide)

Mg(OH)2(s) + 2HCl(aq) → 2H2O(l) + MgCl2(aq)

  • Baking Soda (Sodium hydrogen carbonate)

NaHCO3 + HCl ➝ NaCl + CO2 + H2O

2. Tooth Decay

  • When we eat food, the bacteria present in our mouths break down the sugar to form acids.
  • The acid lowers the pH in the mouth making it acidic.
  • When the pH of the mouth falls below 5.5, our tooth starts decaying.
  • Tooth Decay happens because the acid becomes strong enough to attack the enamel (of our teeth) and corrode it.
  • The best way to prevent tooth decay is to clean the mouth thoroughly after eating food.

3. Plants and Animals

Most of the plants grow best when the pH of the soil is close to 7. If the soil is too acidic (low pH), then it is treated with:

  • Quicklime (Calcium Oxide)
  • Slaked lime (Calcium Hydroxide)
  • If the soil is too alkaline then it can be reduced by adding Manure or Compost (decaying organic matter) which contains acidic materials.
  • The aquatic animals can survive in water bodies within a narrow range of pH change.
  • Calcium Carbonate is often added to acidic lake water to neutralise the acid that comes from acid rain.

4. Self Defence by Animals and Plants through Chemical Warfare

  • When a honey bee stings a person, it injects Formic Acid into the skin.
  • By rubbing a mild baking soda neutralises the acidic liquid injected by a bee sting.
  • When a wasp stings, it injects an alkaline liquid by rubbing mild acid vinegar on the stung area gives relief.
  • When a person touches the leaves of a nettle plant, the stinging hair of nettle leaves injects Methanoic Acid into the skin.
  • It can be neutralized by rubbing Baking Soda or Dock Plant which grows beside the nettle plant and is basic in nature.

HCOOH + NaHCO3 ⟶ HCOONa + CO2 + H2O

5. Lemon juice to clean copper vessels

  • A dull green coloured film forms on a copper vessel when it is exposed to damp air over an extended period of time.
  • Copper hydroxide (Cu(OH)2) and copper carbonate (CuCO3) are combined to form this greenish layer.
  • The reaction is,

2Cu + H2O + CO2 + O2 ⟶ Cu(OH)2 .CuCO3

  • Because these goods are basic in nature, acidic substances such as lemon or tamarind juice are required to clean them.
  • These sour compounds are chosen for cleaning since they are good at cleaning copper vessels.
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