Samacheer Class 8 Science - Air

Oxygen possesses several important physical properties that define its behavior and applications. Oxygen is a colorless, odorless, and tasteless gas, making it imperceptible to human senses. It is a poor conductor of both heat and electricity, which limits its use in electrical applications. Oxygen dissolves readily in cold water, a property that is crucial for aquatic life as fish and other organisms depend on dissolved oxygen for respiration. It is denser than air, meaning it will sink below air in the atmosphere. Under specific conditions of high pressure and low temperature, oxygen can be liquefied, transforming from a gaseous state to a liquid state, which allows for easier storage and transport in industrial applications. One of the most significant physical properties of oxygen is that it supports combustion, meaning substances burn readily in the presence of oxygen, releasing energy in the form of heat and light. This property makes oxygen essential for burning processes and is why oxygen-rich environments are more conducive to fire.

Nitrogen has numerous important industrial and practical applications. Liquid nitrogen is used as a refrigerant due to its extremely low temperature, making it valuable in preserving biological samples, food products, and other temperature-sensitive materials. Nitrogen provides an inert atmosphere for conducting certain chemical reactions, preventing unwanted oxidation or contamination during chemical processes. It plays a crucial role in the production of ammonia through the Haber's process, which combines nitrogen with hydrogen under high temperature and pressure in the presence of a catalyst. The ammonia produced is then converted into fertilizers such as ammonium nitrate and ammonium sulfate, which are essential for agriculture and crop production worldwide. Additionally, ammonia is converted into nitric acid, which has numerous industrial applications. Nitrogen is also used for inflating tires of vehicles because it is inert and does not react with rubber or other tire materials, and it maintains pressure more consistently than regular compressed air, improving tire performance and longevity.

Nitrogen reacts with non-metals like hydrogen, oxygen etc., at high temperature to form their corresponding nitrogen compounds.
Non-metal + Nitrogen \(\underrightarrow { \Delta } \) Nitrogen compound
Example:

Global warming refers to the long-term increase in the average temperature of Earth's atmosphere and oceans. It is primarily caused by an enhanced greenhouse effect resulting from increased concentrations of air pollutants, particularly greenhouse gases such as carbon dioxide, methane, and nitrous oxide. These gases trap heat in the atmosphere by absorbing infrared radiation that would otherwise escape to space, causing the temperature of the atmosphere to rise. The increase in these pollutants is largely due to human activities such as burning of fossil fuels, deforestation, and industrial processes. This gradual warming of the planet has significant consequences including melting of polar ice caps and glaciers, rising sea levels, changes in weather patterns, and disruption of ecosystems. Global warming is a critical environmental issue that requires collective action to reduce greenhouse gas emissions and mitigate its effects on the planet.

Dry ice is the solid form of carbon dioxide, obtained when carbon dioxide gas is cooled to extremely low temperatures and compressed. It is a white, crystalline substance that is much colder than regular ice. Dry ice has several important uses in various fields. It is commonly used as a refrigerant for preserving and transporting temperature-sensitive materials such as biological samples, vaccines, and frozen foods, as it maintains extremely low temperatures without leaving any liquid residue. The most visually striking property of dry ice is that it sublimates directly from solid to gas at room temperature, and during this process, the gas is so cold that moisture in the surrounding air condenses on it, creating a dense, white fog. This fog effect is extensively used in stage shows, theatrical productions, and movie effects to create dramatic visual atmospheres and special effects. Dry ice is preferred over regular ice in many applications because it does not melt into a liquid, eliminating the mess associated with water, and it provides much lower temperatures suitable for various industrial and scientific applications.

When a limited amount of CO 2 is passed through lime water, it turns milky due to the formation of insoluble calcium carbonate.
Ca(OH) 2 + CO 2 → CaCO 3 + H 2 O
When an excess amount of CO 2 is passed through lime water, it first turns milky and the milkyness disappears due to the formation of soluble calcium hydrogen carbonate, Ca(HCO 3 ) 2 .

When various substances burn in oxygen, they combine with oxygen to form specific oxide compounds. When carbon burns in oxygen, it produces carbon dioxide (CO₂), a colorless gas that is essential for photosynthesis in plants. When sulfur burns in oxygen, it produces sulfur dioxide (SO₂), a pungent, colorless gas that is a major air pollutant. When phosphorus burns in oxygen, it can produce either phosphorus trioxide (P₂O₃) or phosphorus pentoxide (P₂O₅), depending on the amount of oxygen available and the conditions of combustion. When magnesium burns in oxygen, it produces magnesium oxide (MgO), a white solid compound that is highly exothermic and releases considerable heat and light. When iron burns in oxygen, it produces iron oxide (Fe₂O₃), commonly known as ferric oxide, which is a reddish-brown solid. When sodium burns in oxygen, it produces sodium oxide (Na₂O), a white solid compound. These combustion reactions demonstrate the reactivity of different elements with oxygen and the formation of their respective oxides, which have various properties and applications in industry and daily life.

1. Potassium combine with CO 2 to form potassium carbonate.
4K + 3CO 2 → 2K 2 CO 3 + C
2. When a limited amount of CO 2 is passed through lime water, it turns milky due to the formation of insoluble calcium carbonate.
Ca(OH) 2 + CO 2 → CaCO 3 + H 2 O
(Calcium carbonate)
Sodium hydroxide (base) is neutralized by carbon dioxide (acidic) to form sodium carbonate (salt) and water.

Acid rain is precipitation that contains high levels of sulfuric and nitric acids, formed when sulfur dioxide and nitrogen oxides from burning fossil fuels and industrial processes react with water vapor in the atmosphere. Acid rain has numerous harmful effects on the environment and human health. It irritates the eyes and skin of human beings, causing discomfort and potential health issues. It inhibits germination and growth of seedlings, reducing agricultural productivity and affecting forest regeneration. It changes the fertility of the soil by altering its pH and nutrient composition, which destroys plants and reduces their ability to grow. Acid rain also causes significant damage to aquatic ecosystems by making water bodies more acidic, which harms fish and other aquatic organisms. Additionally, it causes corrosion of many buildings, bridges, monuments, and other structures made of limestone, marble, and metal, leading to their deterioration and requiring expensive repairs. To prevent acid rain and its effects, several measures can be implemented. Minimizing the usage of fossil fuels such as petrol and diesel reduces the emission of harmful gases. Using CNG (Compressed Natural Gas) as a cleaner alternative fuel produces fewer pollutants. Utilizing non-conventional sources of energy such as solar, wind, and hydroelectric power eliminates emissions entirely. Proper disposal of industrial wastes ensures that harmful chemicals do not enter the atmosphere. These preventive measures collectively help reduce acid rain formation and protect the environment.

In a soda bottle, carbon dioxide gas is dissolved in the liquid under high pressure, which keeps the gas in solution. When the bottle is opened, the pressure inside the bottle is released and exposed to atmospheric pressure, which is much lower. As a result, the dissolved carbon dioxide gas expands and comes out of solution, increasing the pressure inside the bottle. During summer, the temperature is high, which further increases the kinetic energy of the gas molecules, causing them to move faster and exert greater pressure on the bottle walls. The combination of gas expansion due to pressure release and increased molecular motion due to higher temperature can cause the pressure inside the bottle to increase significantly. If this pressure becomes too great, the bottle may burst or explode due to the inability of the container to withstand the internal pressure. This is why soda bottles are more likely to burst when opened during hot summer months compared to cooler seasons.

Fish have a specialized respiratory system adapted for extracting oxygen from water. The gills of fish are richly supplied with blood capillaries, which are tiny blood vessels that can readily absorb the oxygen dissolved in water through a process of diffusion. When water passes over the gills, oxygen diffuses from the water into the blood, allowing fish to obtain the oxygen they need for respiration and survival. However, when fish are taken out of water, they are no longer in contact with the aqueous medium that contains dissolved oxygen. Although the atmosphere contains oxygen gas, fish cannot absorb and utilize oxygen directly from the air using their gills. The structure and function of gills are specifically designed to extract dissolved oxygen from water, not gaseous oxygen from air. Additionally, when fish are out of water, their gills collapse and dry out, which further prevents them from functioning properly. As a result, the supply of oxygen to the fish is cut off, and they cannot breathe. Without access to oxygen, fish will suffocate and die within a short period of time, typically within a few minutes, depending on the species and environmental conditions.

Astronauts cannot breathe in space because there is no atmosphere beyond Earth's atmosphere, and therefore no oxygen available for respiration. When astronauts venture into space, they must carry their own oxygen supply with them in their spacesuits and spacecraft to survive. Modern spacecraft and spacesuits are equipped with systems that can generate oxygen through various methods. One method involves using energy from solar arrays to split water molecules into hydrogen and oxygen through a process called electrolysis. In this process, electrical energy is used to break the chemical bonds in water, separating it into its constituent elements. The oxygen produced is then stored and used for breathing, while the hydrogen can be used for other purposes such as fuel. This method is advantageous because water can be carried aboard spacecraft and recycled, making it a sustainable source of oxygen for long-duration space missions. Additionally, some spacecraft use chemical oxygen generators that produce oxygen through chemical reactions, and others use stored liquid oxygen. These systems ensure that astronauts have a continuous supply of breathable oxygen while they are in the vacuum of space.

Nitrogen is increasingly being used as a substitute for compressed air in vehicle tires, and this preference is based on several practical advantages. The primary reason people prefer nitrogen over compressed air in tires is that nitrogen gas escapes from the tire much more slowly than compressed air does. This is because nitrogen molecules are larger than the oxygen and nitrogen molecules that make up compressed air, and they diffuse through the tire rubber at a slower rate. As a result, tires filled with nitrogen maintain their pressure for a longer period of time compared to tires filled with compressed air. This means that nitrogen-filled tires require less frequent pressure adjustments and monitoring. Additionally, maintaining proper tire pressure is crucial for vehicle safety, fuel efficiency, and tire longevity. By using nitrogen, drivers can ensure that their tires maintain optimal pressure for extended periods, reducing the risk of under-inflation, which can lead to poor fuel economy, increased tire wear, and potential safety hazards. Nitrogen also does not support combustion and does not contain moisture, which can prevent oxidation and corrosion of the tire interior, further extending tire life. These advantages make nitrogen a superior choice for tire inflation in modern vehicles.