Acids, Bases and Salts
CBSE Class 10 · Science · Notes, formulas and practice questions
Acids, bases and salts explains how substances are grouped as acidic or basic, how acids react with metals, carbonates and bases, what the pH scale tells us, and how important salts such as sodium hydroxide, baking soda, washing soda, bleaching powder and plaster of Paris are made and used.
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Most of this chapter is about recognising acidic and basic substances and explaining their behaviour. Acids taste sour, turn blue litmus red and produce hydrogen ions in water; bases taste bitter, feel soapy, turn red litmus blue and produce hydroxide ions. Indicators such as litmus, turmeric and phenolphthalein change colour in acidic or basic conditions, so they let us test substances safely. The first part of the chapter gives everyday examples of each class and introduces the idea of strong and weak acids and bases.
Reactions make the ideas concrete. An acid reacts with a metal to give a salt and hydrogen gas, which can be tested by its pop sound with a burning splint. Acids react with carbonates and hydrogencarbonates to release carbon dioxide, which turns lime water milky. When an acid and a base react with each other, they neutralise to form salt and water; this is called neutralisation and it releases heat. These reactions are used constantly in later chapters, so learning balanced equations for common examples is very useful.
The pH scale runs from 0 to 14 and measures how acidic or alkaline a solution is, with pH 7 neutral. A pH below 7 is acidic and above 7 is alkaline; the further from 7, the stronger the acidic or basic character. Strong acids and bases dissociate completely in water, while weak ones dissociate only slightly. The scale is not just a laboratory idea: soil pH affects crop growth, tooth decay increases when the mouth pH falls below 5.5, and bee stings or acidity in the stomach are treated with mild substances of the opposite type.
The final part deals with salts of everyday importance. Sodium hydroxide is made by electrolysing sodium chloride solution. Chlorine gas reacts with dry slaked lime to give bleaching powder, and baking soda and washing soda are sodium salts used in cooking and cleaning. Plaster of Paris is produced by heating gypsum and sets when mixed with water. Water of crystallisation, the fixed water inside crystal structures, is the key to these changes and explains why crystals can lose water on heating and change their properties.
Key terms
- Acid
- A substance that produces hydrogen ions (H⁺) when dissolved in water. Acids have a sour taste and turn blue litmus red. Hydrochloric acid, sulphuric acid and acetic acid are common examples.
- Base
- A substance that produces hydroxide ions (OH⁻) when dissolved in water. Bases have a bitter taste, feel soapy and turn red litmus blue. Water-soluble bases are called alkalis; sodium hydroxide and calcium hydroxide are examples.
- Indicator
- A substance that changes colour in acidic or basic conditions. Litmus, turmeric, phenolphthalein and methyl orange are common indicators.
- Neutralisation
- The reaction between an acid and a base that forms a salt and water. Hydrogen ions from the acid combine with hydroxide ions from the base to make water, and the reaction gives out heat.
- pH scale
- A scale from 0 to 14 that shows how acidic or alkaline a solution is. pH 7 is neutral, pH below 7 is acidic and pH above 7 is alkaline.
- Strong and weak acids and bases
- A strong acid or base ionises completely in water, so its solution conducts electricity well and reacts quickly. A weak acid or base ionises only partially. Concentration is not the same as strength.
- Water of crystallisation
- Water molecules present in a fixed number in the crystal lattice of a salt. For example, washing soda has the formula Na₂CO₃·10H₂O because ten water molecules are attached to each formula unit. Heating removes this water and changes the crystals.
- Sodium hydroxide
- An alkali with the formula NaOH, also called caustic soda, made by the electrolysis of concentrated sodium chloride solution. It is used in the manufacture of soap, paper and detergents.
- Bleaching powder
- A white powder with the formula CaOCl₂, prepared by passing chlorine gas over dry slaked lime. It is used to bleach cotton and linen and to disinfect drinking water.
Formula sheet
| What | Formula | Notes |
|---|---|---|
| Acid with a metal | Zn + 2HCl → ZnCl₂ + H₂ | A dilute acid reacting with a metal gives a salt and hydrogen gas. The gas burns with a pop sound, which is the standard test for it. |
| Acid with a metal carbonate | Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂ | The products are a salt, water and carbon dioxide. The gas turns lime water milky. |
| Acid with a metal hydrogencarbonate | NaHCO₃ + HCl → NaCl + H₂O + CO₂ | Same three products as with a carbonate. Sodium hydrogencarbonate is baking soda. |
| Neutralisation | Acid + Base → Salt + Water | Hydrogen ions from the acid combine with hydroxide ions from the base to make water, and the reaction gives out heat. |
| Chlor-alkali process | 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂ | Electrolysis of concentrated sodium chloride solution, called brine. It is how sodium hydroxide is manufactured; chlorine comes off at the anode and hydrogen at the cathode. |
Practice questions with answers
1. What happens when dilute hydrochloric acid is added to zinc granules? Write the balanced equation and name the gas.
Zinc reacts with dilute hydrochloric acid to give zinc chloride and hydrogen gas: Zn + 2HCl → ZnCl₂ + H₂. The gas burns with a characteristic pop sound, confirming that it is hydrogen.
2. A solution changes red litmus to blue and has a pH of 9. State whether it is an acid or a base and give one example.
Both observations show that the solution is a base. Bases turn red litmus blue and have a pH above 7. Sodium hydroxide or calcium hydroxide are examples.
3. What is observed when a few drops of phenolphthalein are added to dilute hydrochloric acid and then sodium hydroxide solution is added slowly?
Phenolphthalein remains colourless in acid. As sodium hydroxide is added, the acid is neutralised and the mixture then becomes basic, so the solution turns pink. Further addition keeps it pink because phenolphthalein is pink in a base.
4. Write the equation for the reaction between hydrochloric acid and sodium carbonate. How can the gas produced be tested?
Sodium carbonate reacts with hydrochloric acid as follows: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂. The gas is carbon dioxide, and it is tested by passing it through lime water, which turns milky.
5. Define water of crystallisation. Give the formula of two hydrated salts mentioned in this chapter.
Water of crystallisation is the fixed number of water molecules present in the crystal structure of a salt. For example, washing soda is Na₂CO₃·10H₂O and gypsum is CaSO₄·2H₂O.
6. Why does a bee sting cause pain, and how can it be relieved?
A bee sting injects formic acid into the skin, which causes pain and irritation. Applying a mild base such as baking soda neutralises the acid and gives relief.
7. What is the chemical name and formula of baking soda? Write its reaction with hydrochloric acid.
Baking soda is sodium hydrogencarbonate, NaHCO₃. With hydrochloric acid it produces sodium chloride, water and carbon dioxide: NaHCO₃ + HCl → NaCl + H₂O + CO₂.
8. Write the chemical formula and one important use of washing soda.
Washing soda is sodium carbonate decahydrate, Na₂CO₃·10H₂O. It is used as a cleansing agent and for softening hard water.
9. How is plaster of Paris prepared from gypsum, and why should it be stored in an airtight container?
Gypsum, CaSO₄·2H₂O, is heated to about 373 K, and it loses 1½ water molecules per formula unit to form plaster of Paris, CaSO₄·½H₂O. It should be stored airtight because it absorbs moisture from the air and slowly changes back to gypsum.
10. Describe the preparation of sodium hydroxide by the chlor-alkali process. Name the two gases obtained.
Concentrated sodium chloride solution, called brine, is electrolysed to produce sodium hydroxide, chlorine gas and hydrogen gas: 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂. Chlorine is liberated at the anode and hydrogen at the cathode.
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