Question

What is meant by 'demineralised' water and how can it be obtained ?

Short answer

Demineralised water has had its mineral ions removed, typically via ion exchange, reverse osmosis, or distillation.

Step-by-step solution

Understanding Demineralised Water

Demineralised water, also known as deionized water, is water that has had most of its mineral ions, such as cations like sodium, calcium, iron, and copper, and anions like chloride and sulfate, removed. This process ensures that it is free from salts and minerals that might interfere with industrial applications or laboratory experiments.

Methods of Obtaining Demineralised Water

There are several methods to obtain demineralised water. The most common methods include: 1. **Ion Exchange:** This is the most common method where water is passed through ion exchange resins that replace unwanted ions with hydrogen and hydroxide ions, thus purifying the water. 2. **Reverse Osmosis:** This involves forcing water through a semi-permeable membrane that removes larger ions and unwanted molecules. 3. **Distillation:** Involves boiling the water and collecting the condensed steam, leaving minerals behind.

Key concepts

Ion Exchange

Ion exchange is a popular and effective method used to obtain demineralised water. This process involves the use of ion exchange resins, which are small, porous beads containing active sites that attract and hold ions. The water flows over these beads, and as it does, the unwanted ions in the water are exchanged for hydrogen (H⁺) and hydroxide (OH^-) ions present in the resins. The result of this ion exchange is purified, demineralised water.

The ion exchange process can involve two main types of resins:

• Cation exchange resins: These resins replace positively charged ions like sodium (Na⁺), calcium (Ca²⁺), and iron (Fe³⁺) with hydrogen ions.
• Anion exchange resins: These replace negatively charged ions such as chloride (Cl^-) and sulfate (SO₄^2-) with hydroxide ions.

After the water passes through both types of resins, it's left with just water molecules, forming H₂O. This makes ion exchange a reliable method for producing demineralised water used in various applications.

Reverse Osmosis

Reverse osmosis is another highly efficient method for obtaining demineralised water. It works by pushing water through a semi-permeable membrane under high pressure. This membrane acts as a filter, allowing only the small water molecules to pass through while blocking larger molecules, such as salt and minerals.

Here’s how reverse osmosis works:

• Water is forced through the membrane, which acts as a barrier to impermeable substances.
• The pressure applied across the membrane keeps the larger ions and impurities from proceeding, while only demineralised water comes out on the other side.
• The impurities are left behind and typically flushed away, making reverse osmosis a clean and effective process.

Reverse osmosis is widely used not only for demineralisation but also in desalination and water purification processes, providing high-quality purified water for industrial and household use.

Distillation

Distillation is a traditional method of obtaining demineralised water that relies on heating and condensation. The process starts by boiling the water, which turns it into steam, leaving the heavier mineral content behind.

The steps in the distillation process include:

• Heating the water to convert it to vapor, which naturally leaves most of the minerals and impurities in the boiling chamber.
• Capturing the steam and cooling it down in a condenser, where it turns back into liquid.
• The collected condensate is demineralised water, separate from the minerals that remain in the original container.

Although distillation is effective in completely removing dissolved solids, it’s an energy-intensive process. It's often used where high purity water is necessary, such as in laboratory settings and some specific industrial processes.