Water-ICSE-Class 9|Biswajit Das

1. Water as a Solvent 💧

Water is a universal solvent due to its polar nature. The water molecule (H2​O) has a bent structure with a partial negative charge on the oxygen atom and a partial positive charge on the hydrogen atoms. This polarity allows water to dissolve a wide range of substances by surrounding the solute particles and breaking them apart.

• Dissolving Ionic Compounds: Water molecules orient themselves to surround the positive and negative ions of a solute, pulling them away from the crystal lattice.
• Dissolving Covalent Compounds: Water can dissolve some covalent compounds, particularly those that are also polar (like sugar and ethanol), by forming hydrogen bonds with them.

2. Types of Water 🚰

Based on its hardness, water is classified into two main types:

• Soft Water: Water that readily forms a lather with soap. It is free from dissolved salts of calcium and magnesium. Examples include rainwater and distilled water.
• Hard Water: Water that does not form a lather with soap easily. This is due to the presence of dissolved bicarbonates, chlorides, and sulfates of calcium and magnesium. The salts react with soap to form a white precipitate (scum).2C17​H35​COONa (soap) + CaSO4​→(C17​H35​COO)2​Ca (scum) + Na2​SO4​

Hardness in water can be of two types:

• Temporary Hardness: Caused by the presence of dissolved calcium and magnesium bicarbonates. It can be removed by boiling.Ca(HCO3​)2​Δ​CaCO3​↓+H2​O+CO2​
• Permanent Hardness: Caused by the dissolved chlorides and sulfates of calcium and magnesium. It cannot be removed by boiling.

3. Methods for Removing Hardness 🛠️

Hardness can be removed from water through several methods:

• Boiling: This method only removes temporary hardness. The bicarbonates decompose to form insoluble carbonates, which settle down.
• Adding Washing Soda: Sodium carbonate (Na2​CO3​) can remove both temporary and permanent hardness. It reacts with the calcium and magnesium salts to form insoluble carbonates, which are then filtered out.CaSO4​+Na2​CO3​→CaCO3​↓+Na2​SO4​
• Ion Exchange Process (Permutit Process): This is a modern method for removing both types of hardness. Hard water is passed through a bed of a sodium aluminum silicate compound called permutit or zeolite. The calcium and magnesium ions in the water are exchanged for sodium ions from the permutit.Na2​Z+Ca2+→CaZ+2Na+ (where Z is the silicate radical)The exhausted permutit can be regenerated by treating it with a concentrated sodium chloride solution.CaZ+2NaCl→Na2​Z+CaCl2​

4. Efflorescence, Deliquescence, and Hygroscopy 💧

These are properties of certain chemical substances related to their interaction with atmospheric water vapor.

• Efflorescence: The property of some crystalline substances to lose their water of crystallization when exposed to dry air, resulting in a powder. Example: Washing soda (Na2​CO3​⋅10H2​O) loses water and becomes a white powder (Na2​CO3​⋅H2​O).
• Deliquescence: The property of some soluble crystalline substances to absorb moisture from the air and dissolve in it to form a saturated solution. Example: Caustic soda (NaOH) and calcium chloride (CaCl2​).
• Hygroscopy: The property of some substances to absorb moisture from the air without changing their physical state. They remain a solid or a liquid. Examples: Concentrated sulfuric acid (H2​SO4​) and quicklime (CaO).

Key Difference: A hygroscopic substance only absorbs moisture, while a deliquescent substance absorbs moisture and then dissolves in it.

5. Preparation of Water

Water can be prepared in the laboratory by passing dry hydrogen gas over heated black copper(II) oxide. The hydrogen reduces the copper oxide to form reddish-brown copper and water.

CuO+H2​Δ​Cu+H2​O

Another common method is the combustion of hydrogen gas in air or oxygen, which produces water vapor.

2H2​+O2​Δ

6. Reactions of Water

Water is a stable compound, but it reacts with a variety of substances under different conditions.

1. With Metals

• Highly Reactive Metals: Metals like sodium and potassium react vigorously with cold water to form the corresponding hydroxide and hydrogen gas.2Na+2H2​O→2NaOH+H2​
• Less Reactive Metals: Metals like iron react with steam (water vapor) at high temperatures to form a metallic oxide and hydrogen gas.3Fe+4H2​O→Fe3​O4​+4H2​

2. With Non-metals

• Red-hot Carbon: When steam is passed over red-hot carbon, a mixture of carbon monoxide and hydrogen (known as water gas) is produced.C+H2​OΔ​CO+H2​

3. With Metallic Oxides

• Basic Oxides: Water reacts with basic oxides (also called basic anhydrides) to form their corresponding bases. For example, calcium oxide (quicklime) reacts with water to form calcium hydroxide (slaked lime). This is an exothermic reaction.CaO+H2​O→Ca(OH)2​

4. With Non-metallic Oxides

• Acidic Oxides: Water reacts with acidic oxides (also called acidic anhydrides) to form their corresponding acids.SO2​+H2​O→H2​SO3​ (sulfurous acid)SO3​+H2​O→H2​SO4​ (sulfuric acid)CO2​+H2​O⇌H2​CO3​ (carbonic acid)

5. With Anhydrous Salts

Water can combine with anhydrous salts to form crystalline hydrated salts. This property is used to test for the presence of water. Anhydrous copper sulfate, which is white, reacts with water to form blue hydrated copper sulfate.

• Anhydrous Copper Sulfate:CuSO4​+5H2​O→CuSO4​⋅5H2​O(White) (Blue)
• Anhydrous Cobalt Chloride:CoCl2​+6H2​O→CoCl2​⋅6H2​O(Blue) (Pink)