Water

Clean, dry air is not a single substance but a mixture of several gases. The vast majority, about 99%, is made up of two elements: nitrogen (approximately 78%) and oxygen (approximately 21%). The remaining 1% consists of other gases, including argon (about 0.9%), carbon dioxide (about 0.04%), and small amounts of other noble gases like neon and helium. Oxygen is the most critical component for life as we know it. It is essential for aerobic respiration, the chemical process in our cells that releases energy from the food we eat (like glucose) to power all life processes.

A common experiment to determine the percentage of oxygen in the air involves reacting a known volume of air with a substance that removes the oxygen. One method uses damp iron filings in an inverted measuring cylinder or test tube placed in a beaker of water. The iron reacts with the oxygen in the trapped air, forming rust. As the oxygen is used up, water is drawn up into the measuring cylinder to fill the space. The initial volume of air is recorded, and the final volume of the remaining gas is measured after the reaction stops. The decrease in volume corresponds to the volume of oxygen used. The percentage of oxygen can then be calculated as (change in volume / initial volume of air) × 100.

Air pollutants are harmful substances released into the atmosphere. Most are by-products of burning fossil fuels (coal, oil, and natural gas). Key pollutants include:

1. Carbon Monoxide (CO): A toxic gas produced during the incomplete combustion of carbon-containing fuels, such as in car engines with an insufficient oxygen supply. It reduces the ability of blood to transport oxygen.
2. Sulfur Dioxide (SO₂): Produced when fossil fuels containing sulfur impurities (especially coal) are burned in power stations and factories. It dissolves in rainwater to form acid rain, which damages buildings, forests, and aquatic life. It also causes respiratory problems.
3. Oxides of Nitrogen (NOx): Formed at the high temperatures inside car engines and power station furnaces, where nitrogen and oxygen from the air react. Like SO₂, they contribute to acid rain and cause respiratory issues.

Rusting is the specific name for the corrosion of iron and its alloys, like steel. It is a chemical process that converts iron into hydrated iron(III) oxide, which is the familiar reddish-brown, flaky substance we call rust. For rusting to occur, two substances must be present: oxygen (usually from the air) and water. Rusting is an oxidation reaction where iron loses electrons. The process can be accelerated by the presence of salt (which is why cars rust faster in winter when roads are salted) or by acidic conditions.

Since rusting requires oxygen and water, prevention methods focus on keeping one or both of these away from the iron surface. This is achieved in several ways:

1. Barrier Methods: Creating a physical barrier between the iron and the environment. Examples include painting (for car bodies, gates), oiling or greasing (for engine parts, bike chains), and coating with plastic (for wire fencing, dish racks).
2. Sacrificial Protection: This involves using a more reactive metal to protect the iron. Zinc is more reactive than iron, so if a piece of iron is coated in zinc (a process called galvanising), the zinc will react and corrode first, 'sacrificing' itself to protect the iron. This method is used for roofing sheets and ship hulls.
3. Alloying: Iron can be mixed with other elements like chromium and nickel to create stainless steel, an alloy that is highly resistant to rusting.

Water from rivers and reservoirs is not safe to drink directly. It must be treated to remove impurities and harmful microbes. The process typically involves several stages:

1. Screening: Water passes through large screens to remove large debris like leaves, branches, and plastic bags.
2. Coagulation and Flocculation: A chemical coagulant, such as aluminium sulfate, is added. This causes tiny suspended particles like clay and silt to stick together, forming larger, heavier clumps called 'floc'.
3. Sedimentation: The water is kept in large, still tanks. The heavy floc particles sink to the bottom as sediment, which is later removed.
4. Filtration: The water is then passed through layers of sand and gravel. These filter beds remove any remaining fine suspended particles.
5. Chlorination: A small, controlled amount of chlorine gas or a chlorine compound is added to the water. This is a crucial step that kills bacteria and other harmful microorganisms, making the water safe to drink (potable).

In chemistry, it's important to be able to test for the presence of water and to determine if it is pure. These are two separate tests.

Test for the presence of water: This is a chemical test to confirm that water molecules (H₂O) are present. There are two common tests:

1. Anhydrous copper(II) sulfate: This is a white powder. In the presence of water, it turns blue as it becomes hydrated.
2. Anhydrous cobalt(II) chloride paper: This is blue paper. In the presence of water, it turns pink.

Test for purity: This is a physical test. A pure substance has sharp, fixed melting and boiling points. For water at standard atmospheric pressure:

• The boiling point is exactly 100°C.
• The freezing point (or melting point) is exactly 0°C.

If a sample of water contains dissolved impurities (like salt), its boiling point will be raised (above 100°C) and its freezing point will be lowered (below 0°C).