Acids, Bases and Salts
Overview
Acids, Bases and Salts form a foundational chemistry topic in the MP TET Varg-2 Science section. This chapter connects everyday experiences—like the sour taste of lemon, the slippery feel of soap, or the use of antacids—to core chemical concepts. Understanding this topic helps future teachers explain chemical reactions, safety precautions, and real-world applications to upper-primary students.
For the exam, expect questions on identification of acids and bases using indicators, pH scale interpretation, neutralisation reactions, and properties of common salts. The topic frequently appears in both direct knowledge questions and application-based problems involving daily life examples. Mastery requires knowing definitions, distinguishing properties, understanding the pH scale numerically, and recognising common acids, bases and salts used in laboratories and households.
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Key Concepts
- **Acids** are substances that release hydrogen ions (H⁺) when dissolved in water. They taste sour, turn blue litmus red, and react with metals to produce hydrogen gas.
- **Bases** are substances that release hydroxide ions (OH⁻) in water. They taste bitter, feel soapy/slippery, and turn red litmus blue.
- **Alkalis** are bases that dissolve in water. All alkalis are bases, but not all bases are alkalis (e.g., copper hydroxide is a base but not an alkali).
- **Indicators** are substances that show different colours in acidic and basic solutions. Common indicators include litmus, phenolphthalein, and methyl orange.
- **pH Scale** measures the strength of acidity or basicity on a scale of 0 to 14. pH 7 is neutral; below 7 is acidic; above 7 is basic.
- **Neutralisation** is the reaction between an acid and a base to form salt and water: Acid + Base → Salt + Water.
- **Salts** are ionic compounds formed by neutralisation. They consist of a cation from the base and an anion from the acid.
- **Strong vs Weak Acids/Bases**: Strong acids (HCl, H₂SO₄) completely dissociate in water; weak acids (CH₃COOH, H₂CO₃) partially dissociate. Similarly for bases.
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Formulas / Key Facts
| Formula/Fact | Context | |--------------|---------| | pH = -log[H⁺] | Mathematical definition of pH (concept level for upper-primary) | | pH 7 = Neutral | Pure water has pH 7 | | pH < 7 = Acidic | Lower pH means stronger acid | | pH > 7 = Basic | Higher pH means stronger base | | HCl + NaOH → NaCl + H₂O | Classic neutralisation reaction | | 2HCl + Zn → ZnCl₂ + H₂↑ | Acid reacts with metal to release hydrogen gas | | NaHCO₃ + HCl → NaCl + H₂O + CO₂↑ | Baking soda neutralises acid, releases CO₂ | | CaO + H₂O → Ca(OH)₂ | Quick lime reacts with water to form slaked lime |