Importance of pH in Everyday Life & Salts | Science Class 10 PDF Download

pH Scale

S.P.L. Sorenson, a Danish chemist, introduced the concept of measuring the concentration of hydrogen ions (H⁺(aq)) in a solution in 1909. The p in pH stands for the German word potenz, meaning "power". The pH scale commonly used ranges from 0 (very acidic) to 14 (very alkaline) at 25 °C.

pH Scale

Universal Indicator

A universal indicator is a mixture of dyes that shows different colours over the pH range 0-14 and therefore indicates whether a solution is acidic, neutral or basic. A solution with pH = 7 is neutral.

Definition and Expression of pH

pH is defined as the negative base-10 logarithm of the hydrogen ion concentration in moles per litre:

pH = -log₁₀[H⁺]

In pure water at 25 °C, [H⁺] = [OH^-] = 1 × 10^-7 mol L^-1. Hence the pH of pure water at 25 °C is 7.

At 25 °C the relationship between pH and pOH is:

pH + pOH = 14

Variation of pH with Change in Concentration of H⁺(aq) and OH^-(aq) Ions

All aqueous solutions contain both H⁺(aq) and OH^-(aq) ions. The product of their concentrations in water is constant at a given temperature. At 25 °C this product, called the ionic product of water (K_w), equals 1 × 10^-14 mol² L^-2.

K_w = [H⁺(aq)][OH^-(aq)]

At 25 °C:

K_w = (1 × 10^-7)(1 × 10^-7) = 1 × 10^-14 mol² L^-2

• If [H⁺(aq)] = [OH^-(aq)] = 1 × 10^-7 mol L^-1, the solution is neutral (pH = 7).
• If [H⁺(aq)] > [OH^-(aq)] (equivalently [H⁺] > 1 × 10^-7 mol L^-1), the solution is acidic (pH < 7).
• If [H⁺(aq)] < [OH^-(aq)] (equivalently [H⁺] < 1 × 10^-7 mol L^-1), the solution is basic or alkaline (pH > 7).

Table: pH Value of Some Common Substances

Importance of pH in Everyday Life

Are Plants and Animals pH Sensitive?

Many living organisms function only within a narrow pH range. The human body maintains blood pH tightly between about 7.0 and 7.8. Aquatic animals such as fish can tolerate only small changes in the pH of their environment. Large shifts in pH caused by pollution or acid rain can harm or kill aquatic life.

Example: Rainwater normally has a pH close to 5.6 because of dissolved carbon dioxide; when industrial pollution increases acidity (acid rain), the pH of river or lake water can fall to levels harmful for aquatic organisms.

Soil pH and Plant Growth

Most plants grow best when the soil pH is close to neutral (around pH 7). If soil is too acidic or too alkaline, nutrient availability changes and plants grow poorly or may fail to grow.

Treatment of acidic or alkaline soil: Acidic soil (pH can be as low as 4) can be made less acidic by adding basic materials such as quicklime (calcium oxide), slaked lime (calcium hydroxide) or chalk (calcium carbonate). These bases react with excess acid in soil and raise the pH. Alkaline soil can be made less alkaline by adding decaying organic matter (manure or compost), which helps reduce alkalinity and improve soil structure.

pH in the Digestive System

The stomach produces gastric juice that contains a large amount of hydrochloric acid. Gastric juice has a very low pH (about 1.4), which helps kill microbes and aids digestion. The stomach is protected from this acid by a layer of mucus and other physiological mechanisms.

When excess acid is produced (for example after overeating or due to spicy food) a person may feel acidity or heartburn. To relieve this, people use antacids - medicines that contain basic substances to neutralise excess stomach acid.

Example: Magnesium hydroxide (milk of magnesia), Mg(OH)₂, is a commonly used antacid.

pH Change as the Cause of Tooth Decay

pH Level of Tooth

The saliva in the mouth is slightly basic and helps neutralise acids. After eating, bacteria in dental plaque ferment food residues (sugars) and produce acids, lowering the local pH in the mouth. Tooth enamel starts to demineralise when the pH at the tooth surface falls below about 5.5. Regular cleaning of teeth and use of fluoride-containing toothpastes help prevent decay by neutralising acids and strengthening enamel.

Self-defence by Animals and Plants through Chemical Means

The sting of the honey bee contains formic acid, which causes irritation and pain. Applying a paste of sodium hydrogen carbonate (baking soda) helps reduce pain by neutralising the acid.

The nettle plant has stinging hairs that inject methanoic acid (formic acid, HCOOH) into the skin, causing burning pain. Rubbing a mild basic paste such as baking soda can help neutralise the acid. A related remedy in nature is the dock plant that often grows beside nettles; its leaves contain mild basic substances that can soothe nettle stings.

Salts

A substance formed by the partial or complete replacement of H⁺(aq) ions of an acid by a metal ion or other electropositive ion is called a salt. Salts are generally ionic compounds that, when dissolved in water, dissociate into cations (other than H⁺) and anions (other than OH^-).

Examples (illustrative):

(Partial replacement: only one hydrogen atom is replaced.)

(Complete replacement: all ionisable hydrogen atoms are replaced.)

When soluble salts dissolve in water they produce ions. For example:

• NaCl → Na⁺(aq) + Cl^-(aq)
• CuSO₄ → Cu²⁺(aq) + SO₄^2-(aq)

Naming of Salts

• Salts obtained from sulfuric acid are called sulfates. Example: Na₂SO₄, CuSO₄.
• Salts obtained from nitric acid are called nitrates. Example: KNO₃, NaNO₃.
• Salts obtained from hydrochloric acid are called chlorides. Example: NaCl, CaCl₂, KCl.
• Salts obtained from phosphoric acid are called phosphates. Example: Ca₃(PO₄)₂, Na₃PO₄.
• Salts obtained from carbonic acid are called carbonates. Example: Na₂CO₃, K₂CO₃, CaCO₃.
• Salts obtained from acetic acid are called acetates. Example: CH₃COONa, (CH₃COO)₂Ca.

Classification of Salts

Salts may be classified as follows:

• Normal salts: Formed by complete neutralisation of an acid by a base (strong acid + strong base → neutral salt). Example: NaCl from HCl + NaOH.
• Acidic salts: Formed by partial replacement of ionisable hydrogen atoms of a polybasic acid by metal or ammonium ions. Example: NaHSO₄ (acidic salt of H₂SO₄).
• Basic salts: Formed by partial replacement of hydroxyl groups of a polyhydric base by acid radicals. Example: basic lead carbonate (a basic salt).

Some normal salts with their parent acids

Family of Salts

Salts that share the same cation or the same anion are said to belong to the same family. For example, NaCl, KCl and LiCl all belong to the chloride family (common anion Cl^-).

pH of Salts

The pH of an aqueous solution of a salt depends on the strengths of the parent acid and base:

• Salt of a strong acid + strong base → approximately neutral (pH ≈ 7).
• Salt of a weak acid + strong base → basic solution (pH > 7).
• Salt of a strong acid + weak base → acidic solution (pH < 7).
• Salt of a weak acid + weak base → pH depends on relative strengths; it must be determined experimentally or by equilibrium calculations.

General Methods of Preparation of Soluble Salts

• By direct combination of elements: Heat two elements together to form the corresponding salt (where applicable).
  
• By the action of dilute mineral acids on active metals:

  Examples:

  Zn + H₂SO₄ (dil) → ZnSO₄ + H₂(g)

  2Al + 6HCl (dil) → 2AlCl₃ + 3H₂(g)

  2Na + 2HCl (dil) → 2NaCl + H₂(g)

• By decomposition:

  (a) Decomposition of metal hydrogen carbonates by acid:

  NaHCO₃(s) + HCl (dil) → NaCl + H₂O + CO₂(g)

  (b) Decomposition of metal carbonates by acid:

  CuCO₃(s) + 2HCl (dil) → CuCl₂ + H₂O + CO₂(g)

  
• By neutralisation: Acid + base → salt + water.
  
• By the action of alkalis on metals (or metal salts):
  

General Methods of Preparation of Insoluble Salts

• By direct combination of elements: For example, heating a metal with sulphur can produce metal sulphides that are often insoluble.
  
• By double decomposition (precipitation): Mixing two soluble salts in solution can produce an insoluble salt (a precipitate).

Sodium Chloride (Common Salt / Table Salt)

Sodium chloride (NaCl) is formed by the neutralisation of hydrochloric acid with sodium hydroxide:

HCl + NaOH → NaCl + H₂O

Common salt is an ionic compound of sodium and chlorine: (Na⁺Cl^-)_n.

The main natural source of common salt is seawater. Sea water contains about 3.5% dissolved salts by mass; sodium chloride makes up roughly 2.7-2.9% of seawater. Salt is also obtained from saline lakes and as rock salt, formed when ancient seas or lakes evaporated. Sambhar Lake in Rajasthan is a known source of salt in India.

Chemicals from Common Salt

Common salt is an important raw material for many industrial chemicals used in daily life, such as:

• Sodium hydroxide (NaOH), also called caustic soda.
• Sodium hydrogen carbonate (baking soda, NaHCO₃).
• Sodium carbonate (washing soda, Na₂CO₃).
• Bleaching powder (contains hypochlorite species).

Chlor-alkali Process for Obtaining Sodium Hydroxide

Electrolysis of concentrated sodium chloride solution (brine) yields chlorine gas at the anode, hydrogen gas at the cathode and sodium hydroxide in solution near the cathode. The overall balanced reaction in the industrial chlor-alkali process is:

2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂

Chlor-alkali Process

The products-chlorine, hydrogen and sodium hydroxide-are all commercially important. The name chlor-alkali comes from these products: "chlor" for chlorine and "alkali" for sodium hydroxide.