Class 7 – Science – Notes – Chapter 5 – Acids, Bases and Salts

 1. Classifying Acids and Bases Based on Taste

The term “Acid” originates from the Latin word “acer”, meaning sharp or sour. Many naturally occurring substances like lemon juice, vinegar, curd, and oranges have a sour flavour, indicating they contain acids.

On the other hand, substances like soap, baking soda, or toothpaste feel slippery to touch and taste bitter. These are bases. Therefore, acids are generally sour-tasting, and bases are bitter and slippery.

 2. Physical Properties of Acids

Acids exhibit the following physical characteristics:

1. Corrosiveness – Strong acids can burn or irritate skin and materials.
2. Electrical Conductivity – Acids dissociate into ions in water and conduct electricity.
3. Neutralization – Acids can react with bases to produce salt and water.
4. Taste – Acids taste sour (only tasted in very dilute and safe forms, never in the lab).

 3. What is an Indicator?

An indicator is a chemical compound that helps identify whether a solution is acidic or basic by undergoing a visible change, often in color.

For example, methyl yellow turns yellow in alkaline solutions but may shift color under acidic conditions. Indicators help determine the pH environment of a solution.

 4. Natural Indicators

Natural indicators are obtained from plants or natural sources and change color depending on the pH of the solution.

• Example: Hydrangea flowers turn blue in acidic soils and pink or red in basic soils.
• Other examples: Turmeric, beetroot juice, red cabbage juice, etc.

Natural indicators are eco-friendly and widely used in experiments and household tests.

5. Synthetic Indicators

Synthetic indicators are man-made substances produced in laboratories. They are designed to change color at specific pH ranges.

• Common examples:
  • Phenolphthalein: Colorless in acid, pink in base.
  • Methyl orange: Red in acid, yellow in base.

These are widely used in chemical titrations to determine acidity or basicity.

 6. Common Acid-Base Indicators (Example)

One widely used indicator is phenolphthalein, which functions as follows:

• In acidic solutions, it remains colorless.
• In basic or alkaline solutions, it turns pink due to the formation of pink-colored anions.

This change occurs because the chemical equilibrium shifts based on the pH level, highlighting whether a solution is acidic or basic.

 7. Acid-Base Indicators and pH Behavior

Indicators are substances that show different colors at different pH values.

• Suppose we have an indicator represented by the weak acid formula: HIn.
• In water:
  HIn + H₂O ⇌ H₃O⁺ + In⁻

Here,

• HIn (acid form) has one color.
• In⁻ (conjugate base) has a different color.

In acidic conditions (low pH), equilibrium stays on the left – acid color appears.
In basic conditions (high pH), equilibrium shifts right – base color is seen.
This behavior makes such indicators ideal for titrations.

 8. Reaction Between Acids and Bases

When an acid reacts with a base, a neutralization reaction occurs, producing salt and water.

• Example reaction:
  HCl + NaOH → NaCl + H₂O

This reaction is foundational in titration experiments, where the concentration of an unknown acid or base is determined using a known concentration of its opposite.

 9. What is a Neutralization Reaction?

A neutralization reaction is a chemical process in which an acid and a base react completely with each other.

• Result: Salt + Water
• Example:
  NaOH + HCl → NaCl + H₂O

This is an exothermic reaction, often releasing heat.

10. Example of Neutralization

During neutralization:

• HCl (acid) reacts with NaOH (base)
• Forms: NaCl (salt) + Water + Heat

Such reactions are common in antacid use, treating acidic soil, and insect stings.

 11. What is a Neutral Solution?

A neutral solution has a pH of exactly 7.

• It is neither acidic (pH < 7) nor basic (pH > 7).
• Example: Pure water is considered neutral.

 12. What Are Salts?

Salts are chemical compounds formed when the hydrogen ion (H⁺) of an acid is replaced by a metal ion or ammonium ion from a base.

• Example:
  HCl + NaOH → NaCl (salt) + H₂O

Salts can be neutral, acidic, or basic depending on their parent acid and base.

13. Neutralization Revisited

Neutralization is the reaction of an acid with a base to form salt and water.

General Reaction:
Acid + Base → Salt + Water

• For instance:
  HCl + NaOH → NaCl + H₂O

This reaction is central in chemistry and used in many practical applications.

14. Uses of Bases

Bases are essential in various industries and daily life. Here are some widely used bases:

1. Sodium Hydroxide (NaOH):
   • Used in making soap, paper, textiles, detergents.
   • Employed in oil refining and medicine production.
2. Calcium Hydroxide (Slaked Lime):
   • Used to treat acidic soil and water.
   • Helps in making bleaching powder.
   • Used as a remedy for acid burns.
3. Ammonium Hydroxide:
   • Used to clean ink stains and grease.
   • Found in cosmetic products and glass cleaners.
4. Potassium Hydroxide (KOH):
   • Used in alkaline batteries as electrolyte.

 15. Uses of Acetic Acid

Acetic acid (commonly known as vinegar) has various uses:

1. Food Industry:
   • Acts as a preservative in pickles and sauces.
2. Fiber Industry:
   • Used to make cellulose acetate, an artificial fiber.
3. Perfume and Solvent Production:
   • Helps produce esters (fragrant compounds) and acetone.

 16. Uses of Citric Acid

Citric acid, a naturally occurring acid in citrus fruits, is widely used:

1. Food and Beverages:
   • Adds flavor and freshness.
   • Functions as a preservative.
2. Cleaning and Cosmetics:
   • Acts as a cleaning agent and found in pharmaceuticals.
3. Metal Industry:
   • Used in fluxes for soldering and galvanizing.

17. Treating Ant Bites (Everyday Neutralization)

• Formic acid, present in ant stings, causes a burning sensation.
• Rubbing the sting area with baking soda (sodium hydrogen carbonate) neutralizes the acid and relieves pain.

18. Everyday Examples of Neutralization

1. Indigestion:
   • Excess HCl in the stomach is neutralized using antacids like milk of magnesia.
2. Soil Treatment:
   • Acidic soil is neutralized using slaked lime (Ca(OH)₂) to promote crop growth.
3. Waste Treatment:
   • Industries neutralize acidic waste before disposing of it to prevent environmental damage.