NCERT Summary: Acids, Base & Salts

Acids and Bases

What are Acids?

• Acids are substances that taste sour and change the colour of certain indicators.
• Common natural examples include curd, lemon juice, orange juice and vinegar.
• Many acids produce hydrogen ions (H+) when dissolved in water; this is the chemical basis for their acidic behaviour.

What are Bases?

• Bases are substances that often taste bitter and feel soapy to touch.
• Common examples include baking soda (sodium hydrogen carbonate), antacids, and caustic lime (calcium oxide).
• Bases usually produce hydroxide ions (OH-) when dissolved in water; this is the chemical basis for their basic (alkaline) behaviour.

Properties Commonly Used to Identify Acids and Bases

• Acids taste sour; bases taste bitter and may feel slippery or soapy.
• Acids and bases can be corrosive; strong acids/bases can harm skin and materials, so handle with care.
• Both acids and bases conduct electricity in aqueous solution because of the presence of ions.
• The strength of an acid or base depends on how fully it ionises in water; concentration depends on how much of the substance is dissolved.

Indicators

• Chemicals that show whether a solution is acidic or basic by changing colour are called indicators.
• Natural indicators include litmus, turmeric and petals of the china rose (gudhal).
• Litmus is one of the most commonly used indicators: acids turn blue litmus to red, and bases turn red litmus to blue.
• China rose (hibiscus) petals show dark pink in acidic solutions and green in basic solutions.
• Turmeric remains yellow in acidic solutions and turns red (or reddish) in basic (alkaline) solutions.
• There are also synthetic indicators such as phenolphthalein and methyl orange, used in experiments and titrations.

Neutral Substances

• Substances that are neither acidic nor basic are called neutral.
• Neutral substances do not change the colour of indicators (for example, pure water usually does not change litmus colour).

Neutralization

• A neutralization reaction occurs when an acid reacts with a base and both lose their characteristic properties.
• The general result of a neutralization reaction is the formation of a salt and water, and usually heat is released (the reaction is exothermic).
• General word equation: Acid + Base → Salt + Water + Heat
• Example chemical equations: HCl + NaOH → NaCl + H₂O
• Another example: H₂SO₄ + 2 NaOH → Na₂SO₄ + 2 H₂O
• The salt formed by neutralization can be acidic, basic or neutral depending on the strength of the acid and base that react.

Everyday Uses and Applications of Neutralization

• Ant sting: Ants inject formic acid. The sting can be neutralised by rubbing moist baking soda (sodium hydrogen carbonate, NaHCO₃) or applying calamine lotion which contains zinc carbonate (ZnCO₃).
• Indigestion: Excess stomach acid can cause discomfort. Antacids such as Milk of Magnesia (magnesium hydroxide, Mg(OH)₂) neutralise stomach acid and relieve indigestion.
• Soil treatment: Overuse of chemical fertilisers may make soil acidic. Acidic soil is made less acidic by adding bases such as quicklime (calcium oxide, CaO) or slaked lime (calcium hydroxide, Ca(OH)₂). If soil is too alkaline, the condition can be corrected by adding organic matter which helps lower the pH.
• Factory wastes: Industrial effluents that contain acids are often neutralised by adding basic substances before disposal to prevent environmental damage.

pH Scale - A Simple Measure of Acidity and Basicity

• The pH scale measures how acidic or basic a solution is and usually ranges from 0 to 14.
• On this scale, solutions with pH < 7 are acidic, a solution with pH = 7 is neutral, and solutions with pH > 7 are basic (alkaline).
• pH is a useful way to describe the strength of acids and bases in everyday contexts such as soil testing, pool water, and agriculture.

Safety and Precautions

• Strong acids and bases can harm skin and eyes. Always handle them with care, use protective gloves and goggles when required, and wash hands after experiments.
• Never taste or directly smell chemicals to identify them.
• If an acid or base splashes into the eyes, rinse immediately with plenty of water and seek medical help.

Summary

• Acids are sour, produce H⁺ ions in water and turn blue litmus red.
• Bases are bitter and slippery, produce OH⁻ ions in water and turn red litmus blue.
• Indicators such as litmus, turmeric and china rose petals help identify acidic or basic nature by colour change.
• Neutralization between an acid and a base yields a salt and water and often releases heat; this reaction has many practical uses from treating ant stings to correcting soil pH.