Complete Chapter Notes in Simple Language
Indicators are substances that help us identify whether a solution is acidic or basic. They generally show a change in colour when added to an acidic or basic solution.
Some substances change their smell in acidic or basic media. Such substances are called olfactory indicators.
Hydrochloric acid (HCl), sulphuric acid (H2SO4), nitric acid (HNO3) and acetic acid (CH3COOH).
Sodium hydroxide (NaOH), calcium hydroxide [Ca(OH)2], potassium hydroxide (KOH), magnesium hydroxide [Mg(OH)2] and ammonium hydroxide (NH4OH).
When a suitable metal reacts with an acid, hydrogen gas is evolved and a salt is formed.
The gas evolved can be collected in soap bubbles. A burning candle is brought near the gas-filled bubble to test the gas.
The salt formed here is sodium zincate.
Carbon dioxide is passed through lime water. It turns milky because calcium carbonate is formed.
On passing excess carbon dioxide, soluble calcium hydrogencarbonate is formed.
When an acid reacts with a base and their effects are cancelled, salt and water are formed. This is called a neutralisation reaction.
Metallic oxides are therefore described as basic oxides.
Carbon dioxide reacts with calcium hydroxide to produce salt and water. Therefore, non-metallic oxides are considered acidic in nature.
| Reaction | Products |
|---|---|
| Metal oxide + Acid | Salt + Water |
| Non-metal oxide + Base | Salt + Water |
Acids show their acidic properties because they produce hydrogen ions in aqueous solution.
Hydrogen ions do not exist alone in water. They combine with water molecules and form hydronium ions (H3O+).
Acid solutions contain ions. These ions carry electric current through the solution.
All bases do not dissolve in water. A base that dissolves in water is called an alkali.
HCl produces hydrogen/hydronium ions in the presence of water. Therefore, dry HCl gas does not change the colour of dry litmus.
When an acid or base is mixed with water, the concentration of H3O+ or OH− ions per unit volume decreases. This process is called dilution.
The pH scale is used to express the acidic or basic nature of a solution. The chapter describes a general scale from 0 to 14.
| pH | Nature |
|---|---|
| Less than 7 | Acidic |
| 7 | Neutral |
| More than 7 | Basic / Alkaline |
Higher hydronium ion concentration means a lower pH value.
A universal indicator is a mixture of several indicators. It gives different colours for different hydrogen-ion concentrations.
For the same concentration, acids that produce more H+ ions are called strong acids, while acids producing fewer H+ ions are called weak acids.
The chapter states that our body works within a pH range of about 7.0 to 7.8. Living organisms can survive only within a relatively narrow pH range.
Rain water having a pH below 5.6 is called acid rain. It can lower the pH of river water and make survival of aquatic organisms difficult.
Plants require a suitable soil pH for healthy growth. Soil pH can be checked by preparing a water extract of the soil and testing it with universal indicator paper.
Basic materials such as quick lime, slaked lime and chalk can be used to neutralise excessive acidity in soil.
The stomach produces hydrochloric acid, which helps in digestion. Too much acid can cause pain and irritation.
Antacids are mild bases that neutralise excess stomach acid. Magnesium hydroxide (milk of magnesia) is one example mentioned in the chapter.
Tooth decay starts when the pH of the mouth falls below about 5.5.
Bacteria in the mouth produce acids by breaking down sugar and food particles remaining after eating.
Bee sting leaves an acid that causes pain and irritation. A mild base such as baking soda can provide relief.
Nettle leaves inject methanoic acid, causing burning pain.
Salts having the same positive or negative radical are said to belong to the same family.
| Family | Examples |
|---|---|
| Sodium salts | NaCl, Na2SO4 |
| Chloride salts | NaCl, KCl |
| Natural Source | Acid |
|---|---|
| Vinegar | Acetic acid |
| Sour milk / Curd | Lactic acid |
| Orange | Citric acid |
| Lemon | Citric acid |
| Tamarind | Tartaric acid |
| Tomato | Oxalic acid |
| Ant sting | Methanoic acid |
| Nettle sting | Methanoic acid |
| Acid + Base | Nature of Salt | pH |
|---|---|---|
| Strong acid + Strong base | Neutral | 7 |
| Strong acid + Weak base | Acidic | Less than 7 |
| Strong base + Weak acid | Basic | More than 7 |
Sodium chloride (NaCl) is the common salt used in food. It can be obtained from seawater. It is also found naturally as rock salt.
Rock salt contains impurities and often appears brown. It is found in large deposits and is mined like coal.
Common salt is an important raw material for manufacturing:
When electricity is passed through an aqueous solution of sodium chloride called brine, sodium hydroxide, chlorine and hydrogen are produced.
Bleaching powder is produced by the action of chlorine on dry slaked lime, Ca(OH)2.
The chapter represents bleaching powder as Ca(ClO)2, while noting that its actual composition is complex.
Baking soda is sodium hydrogencarbonate: NaHCO3.
Washing soda is sodium carbonate decahydrate:
Sodium carbonate can be obtained by heating baking soda. Recrystallisation of sodium carbonate gives washing soda.
Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt.
On heating, copper sulphate crystals lose water of crystallisation and become white. Adding water restores the blue colour.
Gypsum contains two molecules of water of crystallisation.
When gypsum is heated at 373 K, it loses water and forms calcium sulphate hemihydrate, called Plaster of Paris.
POP changes back into gypsum and forms a hard solid mass when mixed with water.
Litmus, turmeric, phenolphthalein, methyl orange and olfactory indicators help identify acidic/basic media.
Produce H+/H3O+ ions in aqueous solution.
Produce OH− ions in aqueous solution.
Acid + Base → Salt + Water.
Below 7 = acidic, 7 = neutral, above 7 = basic.
Important raw material for NaOH, baking soda, washing soda and bleaching powder.
NaHCO3
Na2CO3·10H2O
CaSO4·2H2O
CaSO4·½H2O
Zn + H2SO4 → ZnSO4 + H2
NaHCO3 + HCl → NaCl + H2O + CO2
NaOH + HCl → NaCl + H2O
Ca(OH)2 + CO2 → CaCO3 + H2O
2NaCl + 2H2O → 2NaOH + Cl2 + H2
2NaHCO3 → Na2CO3 + H2O + CO2
CaSO4·½H2O + 1½H2O → CaSO4·2H2O
Class 10 Science — Chapter 2
Acids, Bases and Salts
Complete Revision Notes