- Many salts like copper sulphate (CuSO₄·5H₂O) appear dry but actually contain water of crystallisation, which is released on heating and restores the salt’s original colour upon rehydration.
- Water of crystallisation refers to the fixed number of water molecules present in a formula unit of a salt (e.g., 5 in copper sulphate, 10 in washing soda, 2 in gypsum).
- Plaster of Paris (CaSO₄·½H₂O) is formed by heating gypsum and, when mixed with water, reforms into gypsum (CaSO₄·2H₂O); it is used in medical casts, toys, and decorative items.
Definitions [11]
Definition: Acid-Base Indicators
The materials which indicate the presence of an acid or a base in a solution. These are called Acid-Base Indicators or sometimes simple indicators.
Define acids according to Bronsted-Lowry theory.
A substance that donates a proton \[\ce{(H+)}\] to another substance is known as an acid.
Define conjugate acid-base pair.
A pair of an acid and a base differing by a proton is called conjugate acid-base pair.
Definition: Olfactory Indicators
There are some substances whose odour changes in acidic or basic media. These are called olfactory indicators.
Definition: Neutralisation Reaction
The reaction between an acid and a base to give a salt and water is known as a neutralisation reaction.
Definition: Dilution
Mixing an acid or base with water results in decrease in the concentration of ions (H3O+/OH–) per unit volume. Such a process is called dilution and the acid or the base is said to be diluted.
Define pOH.
The pOH of a solution can be defined as the negative logarithm to the base 10, of the molar concentration of OH− ions in solution.
pOH = -log10[OH-]
Define pH.
The pH of a solution is defined as the negative logarithm to the base 10, of the concentration of H+ ions in solution in mol dm–3.
pH is expressed mathematically as
pH = -log10 [H+] or pH = -log10 [H3O+]
Definition: pH scale
pH scale is a scale for measuring the hydrogen ion concentration in a solution.
Definition: Sodium Chloride
The salt formed by the combination of hydrochloric acid and sodium hydroxide solution is called sodium chloride. This is the salt that you use in food.
Definition: Plaster of Paris
On heating gypsum at 373 K, it loses water molecules and becomes calcium sulphate hemihydrate (CaSO4 . \[\frac {1}{2}\] . H2O). This is called Plaster of Paris.
Chemica Equations [5]
Chemical Equation: Baking Soda or Sodium Hydrogencarbonate
\[\begin{aligned}
NaCl+H_2O+CO_2+NH_3 & \to\mathrm{NH}_4\mathrm{Cl~}+\mathrm{~NaHCO}_3 \\
& \text{(Ammonium (Sodium} \\
& \text{chloride) hydrogencarbonate)}
\end{aligned}\]
This reaction takes place when it isheated during cooking: \[2\mathrm{NaHCO}_3\xrightarrow{\mathrm{Heat}}\mathrm{Na}_2\mathrm{CO}_3+\mathrm{H}_2\mathrm{O}+\mathrm{CO}_2\]
(Sodium (Sodium
hydrogencarbonate) carbonate)
Chemical Equation: Sodium Hydroxide
2NaCl(aq) + 2H2O(l) → 2NaOH(aq) + Cl2(g) + H2(g)
Chemical Equation: Bleeching Powder or Calcium Oxychloride
2Ca(OH)2 + 2Cl2 → Ca(ClO)2 + CaCl2 + 2H2O
Chemical Equation: Washing Soda or Sodium Carbonate
Na2CO3 + 10H2O → Na2CO3 . 10H2O
(Sodium
carbonate)
Chemical Equation: Plaster of Paris
\[\mathrm{CaSO_{4}\cdot\frac{1}{2}H_{2}O+1\frac{1}{2}H_{2}O\rightarrow CaSO_{4}\cdot2H_{2}O}\]
(Plaster of Paris) (Gypsum)
Key Points
Key Points: The pH Scale
- The pH scale (0–14) measures the concentration of H⁺ ions in a solution; values < 7 indicate acids, > 7 indicate bases, and 7 is neutral.
- A universal indicator shows different colours at different pH levels, helping to determine the strength of an acid or base.
- Strong acids/bases produce more H⁺ or OH⁻ ions in solution, while weak acids/bases produce fewer ions at the same concentration.
Key Points: Importance of pH in Everyday Life
| S.No. | Area of Life | Role of pH |
|---|---|---|
| 1. | Human Body | Functions within pH range 7.0 – 7.8; imbalance can affect survival. |
| 2. | Agriculture |
Crops grow best in a specific pH:
|
| 3. | Acid Rain | Rainwater with pH < 5.6 contains acids (H₂SO₄, HNO₃, H₂CO₃) from pollutants; these harm soil, marine life, and monuments. |
| 4. | Medicine | Diseases are diagnosed by checking blood and urine pH. |
| 5. | Digestive System | Excess HCl lowers pH, causing pain; antacids like Mg(OH)₂ neutralise it. |
| 6. | Tooth Decay | pH < 5.5 corrodes tooth enamel; toothpaste neutralizes mouth acids. |
| 7. | Bee Sting | Injects acid into the skin; baking soda (a base) neutralises the sting. |
Key Points: Chemicals from Common Salt
- Common salt (NaCl) is a neutral salt and serves as a raw material for many useful chemicals like sodium hydroxide, bleaching powder, baking soda, and washing soda.
- Sodium hydroxide (NaOH) is produced by the electrolysis of brine and is used in the manufacture of soaps, detergents, and paper.
- Bleaching powder is made by reacting chlorine with dry slaked lime and is used for disinfection, textile bleaching, and as an oxidising agent.
- Baking soda (NaHCO₃) is used in cooking, antacids, and fire extinguishers. It is produced using NaCl and has mild alkaline properties.
- Washing soda (Na₂CO₃·10H₂O) is obtained by recrystallisation of sodium carbonate and is used in cleaning, softening hard water, and various industrial processes.
Key Points: Water in Salt Crystals
Concepts [14]
- Acids and Bases in Daily Life
- Acids and Bases in the Laboratory
- Acids and Bases React with Metals
- Reaction of Metal Carbonates with Acids
- Acids and Bases Reaction with each other
- Reaction of Metallic Oxides with Acids
- Reaction of a Non-metallic Oxide with Base
- Common Properties of Acids and Bases
- The pH Scale
- Importance of pH in Everyday Life
- Salts > Family of Salts
- Salts > pH of Salts
- Salts > Chemicals from Common Salt
- Salts > Water in Salt Crystals
