Acids, Bases and Salts · Lesson 12 of 12
Chapter Summary and Practice
“Acids, bases, salts and pH reunite for one final chemical family gathering.”
• Connect indicators, acid-base reactions, ions, pH and salts into one coherent understanding. • Revise the major chemical equations and reaction patterns from the chapter. • Balance important chemical equations systematically. • Apply pH and neutralisation ideas to everyday situations. • Distinguish common salts, hydrated salts and important chemicals derived from common salt.
The chapter begins with simple indicator colour changes and develops into a deeper picture of acids and bases as substances that form characteristic ions in water. Those ions explain electrical conductivity, neutralisation and pH. The same ideas then lead naturally to salts, industrial chemicals and hydrated crystals.
At A Glance
Indicators
Indicators provide observable evidence of acidity or basicity. Litmus and turmeric are natural indicators. Phenolphthalein and methyl orange are synthetic indicators. Onion, vanilla and clove oil can act as olfactory indicators.
Important Acid And Base Reactions
| Reactants | Products |
|---|---|
| Acid + Metal | Salt + Hydrogen |
| Acid + Metal carbonate | Salt + Carbon dioxide + Water |
| Acid + Metal hydrogencarbonate | Salt + Carbon dioxide + Water |
| Acid + Base | Salt + Water |
| Metal oxide + Acid | Salt + Water |
| Non-metallic oxide + Base | Salt + Water |
Ions In Water
Acids produce H⁺ or H₃O⁺ ions in water. Bases produce OH⁻ ions. These ions explain conductivity and the characteristic behaviour of acidic and basic solutions. Water is essential for ion formation in reactions such as hydrogen chloride forming hydronium ions.
pH
pH expresses the acidic or basic nature of a solution. Lower pH corresponds to greater hydronium ion concentration. Acidic solutions lie below neutral and basic solutions above neutral. Strong acids or bases produce more characteristic ions than weak ones at the same concentration.
Salts
Salts may belong to families based on common positive or negative ions. Their aqueous solutions can be neutral, acidic or basic depending on the strengths of the acid and base from which they are formed.
Chemicals From Common Salt
Electrolysis of brine gives sodium hydroxide, chlorine and hydrogen. Chlorine is used to make bleaching powder. Sodium hydrogencarbonate is baking soda. Sodium carbonate decahydrate is washing soda.
Water Of Crystallisation
Hydrated salts contain fixed numbers of water molecules in their crystal structures. Copper sulphate pentahydrate, washing soda and gypsum are examples. Heating gypsum produces Plaster of Paris, which sets by taking up water and reforming gypsum.
Important Equations
Balancing Chemical Equations Step By Step
Write correct formulae first. Count every element on both sides. Choose an element or an unchanged polyatomic group that is unequal. Adjust coefficients only. Recount after each change. Continue until every element balances. Finish by checking that the coefficients are the smallest whole-number ratio.
Problem
Balance Mg + HCl → MgCl₂ + H₂.
- 1.Magnesium is already one on each side.
- 2.MgCl₂ contains two chlorine atoms.
- 3.Place two before HCl.
- 4.The two hydrogen atoms on the left now form one H₂ molecule.
- 5.The balanced equation is Mg + 2HCl → MgCl₂ + H₂.
Problem
Balance CaCO₃ + HCl → CaCl₂ + H₂O + CO₂.
- 1.Calcium and carbon are already balanced.
- 2.CaCl₂ needs two chlorine atoms, so place two before HCl.
- 3.Two hydrogen atoms from 2HCl form one H₂O molecule.
- 4.Count oxygen: three on the left and three on the right.
- 5.The balanced equation is CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂.
Problem
Balance Al + H₂SO₄ → Al₂(SO₄)₃ + H₂.
- 1.Keep every chemical formula unchanged.
- 2.The product contains two aluminium atoms, so place two before Al.
- 3.The product contains three sulphate groups, so place three before H₂SO₄.
- 4.Three H₂SO₄ molecules contain six hydrogen atoms.
- 5.Place three before H₂ to give six hydrogen atoms on the product side.
- 6.Check aluminium, sulphur, oxygen and hydrogen.
- 7.The balanced equation is 2Al + 3H₂SO₄ → Al₂(SO₄)₃ + 3H₂.
Revise, Reflect, Refine
| Common Mistake | Better Approach |
|---|---|
| Changing subscripts while balancing | Change coefficients only. |
| Calling every hydrogen-containing compound an acid | Check whether it produces H⁺ or H₃O⁺ in water. |
| Thinking every base is an alkali | Only water-soluble bases are alkalis. |
| Confusing strong with concentrated | Strength concerns ion formation; concentration concerns amount per volume. |
| Adding water to concentrated acid | Add acid slowly to water with stirring. |
| Assuming every salt is neutral | Check the strengths of its parent acid and base. |
Practice
Practice Problems
- Explain how litmus distinguishes an acid from a base.
- Explain why glucose is not classified as an acid even though it contains hydrogen.
- Write and balance the reaction between zinc and dilute sulphuric acid.
- Write and balance the reaction between calcium carbonate and hydrochloric acid.
- Explain the lime water test for carbon dioxide.
- Explain why excess carbon dioxide makes the milkiness of lime water disappear.
- Write the ionic equation for neutralisation.
- Explain why dry hydrogen chloride does not show acidic behaviour with dry litmus.
- Explain why acid should be added to water during dilution.
- Compare a strong acid with a weak acid at the same concentration.
- Explain how pH affects tooth decay.
- Predict the nature of a salt formed from a strong acid and weak base.
- Name the three products of the chlor-alkali process and state where the gases form.
- Explain why baking powder makes a cake rise.
- Explain the meaning of water of crystallisation.
- Compare gypsum and Plaster of Paris.
The Journey Beyond
The ideas developed here connect visible observations with microscopic ions. Indicator colours reveal changes in chemical environment. Ion formation explains conductivity and neutralisation. pH connects ion concentration to biological and environmental effects. Salts then show how acid-base chemistry leads to useful household and industrial materials.
Quiz
Which equation represents the essential ionic change during neutralisation?
Which reaction produces salt, carbon dioxide and water?
Which statement correctly distinguishes a strong acid from a concentrated acid?
Which compound is known as Plaster of Paris?
- Write and balance the reaction between magnesium and hydrochloric acid.
- Write and balance the reaction between calcium carbonate and hydrochloric acid.
- Explain the lime water test for carbon dioxide and what happens when excess carbon dioxide is passed.
- Explain why dry hydrogen chloride gas does not turn dry blue litmus red.
- Write the ionic equation for neutralisation and explain what it means.
- Explain the difference between a strong acid and a concentrated acid.
- Explain how pH is connected with tooth decay and antacid action.
- Predict the nature of a salt formed from a weak acid and strong base.
- Write the chlor-alkali equation and identify the products at the electrodes.
- Write the reaction for heating baking soda.
- Explain water of crystallisation using copper sulphate as an example.
- Compare gypsum and Plaster of Paris and explain how Plaster of Paris sets.
Key Takeaways
• Indicators reveal whether a solution is acidic or basic through observable changes. • Acids produce H⁺ or H₃O⁺ ions in water, while bases produce OH⁻ ions. • Neutralisation combines hydrogen ions and hydroxide ions to form water. • The pH scale describes the acidic or basic nature of aqueous solutions. • Salt solutions may be acidic, basic or neutral depending on their parent acid and base. • Common salt is an important raw material for sodium hydroxide, chlorine and several useful compounds. • Water of crystallisation is structurally associated with hydrated salts. • Plaster of Paris hardens by reacting with water and reforming gypsum.
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Are The Crystals Of Salts Really Dry?
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