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Acids, Bases, and Salts - Science Notes

Posted by EduEvoke on July 18, 2025
class 10 class 10 notes class10 science
Acids, Bases, and Salts - Class 10 Science Notes

Acids, Bases, and Salts

Here are notes on Acids, Bases, and Salts, as per the Class 10 Science curriculum:

Chapter: Acids, Bases, and Salts

I. Introduction to Acids, Bases, and Salts

  • Acids: Substances that generally taste sour, turn blue litmus paper red, and produce hydrogen ions ($H^+$) or hydronium ions ($H_3O^+$) when dissolved in water.
  • Bases: Substances that generally taste bitter, feel soapy or slippery, turn red litmus paper blue, and produce hydroxide ions ($OH^-$) when dissolved in water. Bases that are soluble in water are called alkalis.
  • Salts: Ionic compounds formed by the neutralization reaction between an acid and a base. They are composed of a positive ion (cation) from a base and a negative ion (anion) from an acid.

II. Indicators

Indicators are substances that show a change in color or odor when added to an acidic or basic solution.

  • Natural Indicators:
    • Litmus: Obtained from lichens.
      • Acidic solution: Blue litmus turns red.
      • Basic solution: Red litmus turns blue.
      • Neutral solution: Litmus remains purple.
    • Turmeric:
      • Acidic solution: Remains yellow.
      • Basic solution: Turns reddish-brown.
  • Synthetic Indicators:
    • Methyl Orange:
      • Acidic solution: Turns red.
      • Basic solution: Turns yellow.
    • Phenolphthalein:
      • Acidic solution: Remains colorless.
      • Basic solution: Turns pink.
  • Olfactory Indicators: Substances whose odor changes in acidic or basic media.
    • Onion and Vanilla Essence: Lose their characteristic smell in a basic medium, but their smell persists in an acidic medium.

III. Properties of Acids

A. Physical Properties:

  • Sour taste (Do not taste strong acids).
  • Corrosive in nature (can cause burns).
  • Turn blue litmus red.
  • Conduct electricity in aqueous solutions.

B. Chemical Properties:

  1. Reaction with Metals:
    • Acids react with most active metals to produce a salt and hydrogen gas.
    • Acid + Metal $\rightarrow$ Salt + Hydrogen gas
    • Example: $2HCl(aq) + Zn(s) \rightarrow ZnCl_2(aq) + H_2(g)$
    • Hydrogen gas can be tested by bringing a burning candle near it; it extinguishes the flame with a "pop" sound.
  2. Reaction with Metal Carbonates and Metal Bicarbonates (Hydrogen Carbonates):
    • Acids react with metal carbonates and metal bicarbonates to produce a salt, carbon dioxide gas, and water.
    • Acid + Metal Carbonate $\rightarrow$ Salt + Carbon Dioxide + Water
    • Acid + Metal Bicarbonate $\rightarrow$ Salt + Carbon Dioxide + Water
    • Examples:
      • $2HCl(aq) + Na_2CO_3(s) \rightarrow 2NaCl(aq) + H_2O(l) + CO_2(g)$
      • $HCl(aq) + NaHCO_3(s) \rightarrow NaCl(aq) + H_2O(l) + CO_2(g)$
    • Carbon dioxide gas can be tested by passing it through lime water (calcium hydroxide solution), which turns milky due to the formation of a white precipitate of calcium carbonate.
      • $Ca(OH)_2(aq) + CO_2(g) \rightarrow CaCO_3(s) + H_2O(l)$
    • If excess $CO_2$ is passed, the milkiness disappears due to the formation of soluble calcium bicarbonate.
      • $CaCO_3(s) + CO_2(g) + H_2O(l) \rightarrow Ca(HCO_3)_2(aq)$
  3. Reaction with Metal Oxides:
    • Metal oxides are generally basic in nature. Acids react with metal oxides to form salt and water.
    • Acid + Metal Oxide $\rightarrow$ Salt + Water
    • Example: $CuO(s) + 2HCl(aq) \rightarrow CuCl_2(aq) + H_2O(l)$
    • The blue-green color observed when copper oxide reacts with hydrochloric acid is due to the formation of copper(II) chloride.
  4. Reaction with Bases (Neutralization Reaction):
    • Acids react with bases to form salt and water. This reaction is called neutralization.
    • Acid + Base $\rightarrow$ Salt + Water
    • Example: $HCl(aq) + NaOH(aq) \rightarrow NaCl(aq) + H_2O(l)$
    • This is an exothermic reaction (heat is released).

IV. Properties of Bases

A. Physical Properties:

  • Bitter taste (Do not taste strong bases).
  • Slippery or soapy to touch.
  • Corrosive in nature.
  • Turn red litmus blue.
  • Conduct electricity in aqueous solutions.

B. Chemical Properties:

  1. Reaction with Metals:
    • Some active metals react with strong bases to produce a salt and hydrogen gas.
    • Base + Metal $\rightarrow$ Salt + Hydrogen gas
    • Example: $2NaOH(aq) + Zn(s) \rightarrow Na_2ZnO_2(aq) + H_2(g)$ (Sodium zincate)
    • Note: Not all metals react with bases to produce hydrogen.
  2. Reaction with Non-metal Oxides:
    • Non-metal oxides are generally acidic in nature. Bases react with non-metal oxides to form salt and water.
    • Base + Non-metal Oxide $\rightarrow$ Salt + Water
    • Example: $Ca(OH)_2(aq) + CO_2(g) \rightarrow CaCO_3(s) + H_2O(l)$
    • This reaction is similar to the neutralization reaction of an acid and a base.
  3. Reaction with Acids (Neutralization Reaction):
    • Bases react with acids to form salt and water. (Already discussed under acids).

V. Strength of Acids and Bases (pH Scale)

  • Strength of an acid or base depends on the concentration of $H^+$ ions or $OH^-$ ions produced in water.
  • Strong Acid: Completely dissociates in water to produce a high concentration of $H^+$ ions (e.g., HCl, $H_2SO_4$, $HNO_3$).
  • Weak Acid: Partially dissociates in water to produce a low concentration of $H^+$ ions (e.g., $CH_3COOH$ (acetic acid), Carbonic acid ($H_2CO_3$)).
  • Strong Base: Completely dissociates in water to produce a high concentration of $OH^-$ ions (e.g., NaOH, KOH).
  • Weak Base: Partially dissociates in water to produce a low concentration of $OH^-$ ions (e.g., $NH_4OH$ (ammonium hydroxide), $Mg(OH)_2$ (magnesium hydroxide)).

pH Scale:

  • The pH scale is a measure of the acidity or basicity of a solution. The "p" in pH stands for "potenz" (German for power).
  • It ranges from 0 to 14.
  • pH = -log[$H^+$] (where [$H^+$] is the concentration of hydrogen ions).
  • pH < 7: Acidic solution. Lower the pH, stronger the acid.
  • pH = 7: Neutral solution (e.g., pure water).
  • pH > 7: Basic (alkaline) solution. Higher the pH, stronger the base.
  • A change of one unit on the pH scale represents a tenfold change in $H^+$ ion concentration. For example, a solution with pH 5 has ten times less $H^+$ concentration than a solution with pH 4.

Importance of pH in Everyday Life:

  1. pH in our Digestive System: Our stomach produces concentrated HCl (pH $\approx$ 1.2-1.5) to aid digestion. Excess acid causes indigestion and pain, which can be neutralized by antacids (mild bases like magnesium hydroxide).
  2. pH of Soil: Plants require a specific pH range for healthy growth. Acidic soil can be treated with quicklime ($CaO$) or slaked lime ($Ca(OH)_2$), while basic soil can be treated with organic matter.
  3. pH in Tooth Decay: Tooth decay starts when the pH of the mouth falls below 5.5 (due to acid production by bacteria breaking down food particles). Using basic toothpastes helps neutralize these acids.
  4. Self-defense by Animals and Plants:
    • Bee sting contains methanoic acid (formic acid), causing pain and irritation. Applying baking soda (a mild base) provides relief.
    • Nettle leaves inject methanoic acid when touched, causing a burning sensation. Rubbing the affected area with a dock leaf (which contains a base) helps.

VI. Salts

  • Salts are formed by the reaction of an acid and a base.
  • Types of Salts (based on the strength of parent acid and base):
    1. Neutral Salts: Formed from a strong acid and a strong base (e.g., NaCl, $KNO_3$). Their aqueous solutions have a pH of approximately 7.
    2. Acidic Salts: Formed from a strong acid and a weak base (e.g., $NH_4Cl$). Their aqueous solutions are acidic (pH < 7).
    3. Basic Salts: Formed from a weak acid and a strong base (e.g., $Na_2CO_3$, $CH_3COONa$). Their aqueous solutions are basic (pH > 7).
  • Water of Crystallization: Some salts contain a fixed number of water molecules as an integral part of their crystal structure. This water is called water of crystallization.
    • Examples:
      • Copper Sulphate Pentahydrate ($CuSO_4 \cdot 5H_2O$) - Blue color
      • Gypsum (Calcium Sulphate Dihydrate) ($CaSO_4 \cdot 2H_2O$)
      • Washing Soda (Sodium Carbonate Decahydrate) ($Na_2CO_3 \cdot 10H_2O$)
    • When these hydrated salts are heated, they lose their water of crystallization and become anhydrous (e.g., blue copper sulphate turns white on heating).

Common Salts and Their Uses:

  1. Common Salt (Sodium Chloride, NaCl):
    • Obtained from seawater (evaporation) and rock salt deposits.
    • Essential component of our food.
    • Used as a raw material for many important chemicals:
      • Sodium Hydroxide (NaOH)
      • Baking Soda ($NaHCO_3$)
      • Washing Soda ($Na_2CO_3 \cdot 10H_2O$)
      • Bleaching Powder ($CaOCl_2$)
  2. Sodium Hydroxide (NaOH) - Caustic Soda:
    • Produced by the electrolysis of brine (aqueous solution of NaCl). This process is called the Chlor-alkali process because it produces chlorine (chlor) and sodium hydroxide (alkali).
    • At Anode: $2Cl^-(aq) \rightarrow Cl_2(g) + 2e^-$
    • At Cathode: $2H_2O(l) + 2e^- \rightarrow H_2(g) + 2OH^-(aq)$
    • Overall: $2NaCl(aq) + 2H_2O(l) \xrightarrow{electricity} 2NaOH(aq) + Cl_2(g) + H_2(g)$
    • Uses: In making soaps, detergents, paper, artificial fibers, refining petroleum, and as a laboratory reagent.
  3. Bleaching Powder (Calcium Oxychloride, $CaOCl_2$):
    • Produced by the action of chlorine gas on dry slaked lime ($Ca(OH)_2$).
    • $Ca(OH)_2(s) + Cl_2(g) \rightarrow CaOCl_2(s) + H_2O(l)$
    • Uses:
      • For bleaching cotton and linen in the textile industry.
      • For bleaching wood pulp in paper factories.
      • As an oxidizing agent in many chemical industries.
      • For disinfecting drinking water (to make it free from germs).
  4. Baking Soda (Sodium Hydrogen Carbonate or Sodium Bicarbonate, $NaHCO_3$):
    • Prepared using NaCl as one of the raw materials (Solvay process):
    • $NaCl + H_2O + CO_2 + NH_3 \rightarrow NH_4Cl + NaHCO_3$
    • Uses:
      • As an ingredient in antacids (neutralizes excess acid in the stomach).
      • In baking powder (a mixture of baking soda and a mild edible acid like tartaric acid). When heated or mixed with water, it produces $CO_2$ gas, causing cakes/bread to rise and become soft and spongy.
      • In soda-acid fire extinguishers.
  5. Washing Soda (Sodium Carbonate Decahydrate, $Na_2CO_3 \cdot 10H_2O$):
    • Obtained by heating baking soda to get sodium carbonate, which is then recrystallized with water.
    • $2NaHCO_3(s) \xrightarrow{heat} Na_2CO_3(s) + H_2O(l) + CO_2(g)$
    • $Na_2CO_3(s) + 10H_2O(l) \rightarrow Na_2CO_3 \cdot 10H_2O(s)$
    • Uses:
      • In glass, soap, and paper industries.
      • For removing permanent hardness of water.
      • As a cleaning agent for domestic purposes.
      • In the manufacture of borax.
  6. Plaster of Paris (Calcium Sulphate Hemihydrate, $CaSO_4 \cdot \frac{1}{2}H_2O$):
    • Prepared by heating gypsum ($CaSO_4 \cdot 2H_2O$) at 100°C (373 K).
    • $CaSO_4 \cdot 2H_2O(s) \xrightarrow{373 K} CaSO_4 \cdot \frac{1}{2}H_2O(s) + 1\frac{1}{2}H_2O(l)$
    • It is a white powder that, when mixed with water, sets into a hard solid mass due to the formation of gypsum.
    • $CaSO_4 \cdot \frac{1}{2}H_2O(s) + 1\frac{1}{2}H_2O(l) \rightarrow CaSO_4 \cdot 2H_2O(s)$
    • Uses:
      • For setting fractured bones in hospitals.
      • For making toys, decorative materials, and smooth surfaces.
      • For fire-proofing materials.

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