Chlorine is typically used either as a solution or as a part of a compound like sodium hypochlorite (NaClO). As a strong oxidising agent, it’s very useful as a disinfectant even at very dilute concentrations. For instance, it’s commonly used as a swimming pool disinfectant at a concentration range of between 1.0 and 3.0 parts per million (ppm).
Chlorine is a highly corrosive gas that reacts readily with many substances. It has an atomic number of 17 and a standard atomic weight of 35.45. It belongs to the group of elements known as halogens, which are all highly reactive and harmful, or even lethal to biological organisms because of their ability to strip substances of their outer electrons.
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Everyday Uses of Chlorine
Free or elemental chlorine rarely occurs in nature, except for the very small amounts that can be found in volcanic gases. Even in this case, this element eventually reacts with other substances to form compounds, which is why it’s usually found in small concentrations in everyday substances like cleaning agents, healthcare products, and pool disinfectants.
Chlorine is mainly found as a constituent in other compounds or solutions. For example, it’s commonly found in aqueous solutions of sodium hypochlorite, or as the active ingredient in household cleaning agents. While the main industrial and commercial use of this substance is as a disinfectant, it has plenty of other uses, such as in bleaching agents and even renewable energy production. Here are seven everyday uses of chlorine:
- Swimming pool disinfectant: While chlorine by itself isn’t used to disinfect swimming pools, a solution of a compound that contains it, such as sodium hypochlorite, is commonly used. In the case of sodium hypochlorite, this is an inorganic salt that has a hypochlorite group as the counterion of sodium. As an aqueous solution, the sodium and hypochlorite split into ions, making the hypochlorite highly oxidative so that it can strip electrons from the cell membranes of bacteria and other microorganisms in the swimming pool water.
- Drinking water treatment: The chlorination of water intended for drinking, as well as domestic or commercial use, is an essential step in making it potable. This is because chlorination ensures that bacteria, viruses, and protists are eliminated from the drinking water.
- Household disinfectant: Many household cleaning agents have chlorine-based compounds as the active ingredients. Some products are concentrated and need to be diluted with water. For example, you can use a 5% proportion of a chlorine-based cleaning agent to clean surfaces like tables, countertops, and floors.
- Food protection: This substance is also used to protect crops from pests. It also ensures food safety by disinfecting food contact surfaces, such as countertops. Many foodborne pathogens like salmonella and E. coli can lurk on kitchen surfaces that are not disinfected.
- Healthcare products: Aside from hospital disinfection, chlorine is important in the manufacture of many types of medications, such as anti-arthritis drugs and anti-cholesterol medications. In fact, it’s present in about 25% of final drug formulations. Meanwhile, chlorinated solvents are used as separating or purification agents. Chlorine chemistry is also essential in the manufacture of other medical products like contact lenses and respiratory inhalers.
- Renewable energy production: Harnessing solar energy is largely made possible by silicon chips and silicon wafers on solar panels. These silicon-based products are manufactured from grains of sand that are purified using chlorine or chlorine-based chemistry. Similarly, harnessing wind power would be very difficult without the chlorine-based epoxy resins that are made into lightweight turbine blades.
- Building and construction: Building insulation is usually made from plastic foams using chlorine chemistry. They reduce energy consumption by preventing heat inside the house from escaping during cold weather. Conversely, the heat from the outside is prevented from baking the inside of the house during hot days. Chlorine compounds are also used in manufacturing durable paints.
Why is Chlorine Added to the Swimming Pool?
Although pools have continually flowing water running through pipe systems and drains, dirt accumulates in them quickly. From hair and shedded skin cells to fallen leaves, various types of contaminants accumulate in pools and risk the health of pool users.
The most dangerous types of contaminants in swimming pools are pathogenic microorganisms, such as bacteria and fungi. Although viruses are technically not alive, some may still contaminate pools and potentially infect swimmers. With this in mind, chlorine is added to swimming pools to kill the pathogenic microbes in the water. Generally, the chlorine level in a swimming pool should be maintained between one and three parts per million.
Is Chlorine Added to Tap Water?
The tap water that we use everyday is from three sources: rivers, groundwater, and reservoirs seawater. The most common sources in most countries are from surface water and groundwater. Other sources are either very difficult to access or difficult to process, such as seawater.
Regardless of the source, chlorination is a key part of the water treatment process. It’s crucial to ensuring that potential pathogenic contaminants like bacteria and parasitic organisms are killed or at least limited in number.
Scientific Applications for Chlorine
Gaseous elemental chlorine can be prepared in a laboratory through several methods, such as the reaction between an acid that contains chlorine and a metal oxide. This element can also be prepared in the laboratory through the electrolysis process. Consider the following chemical reactions:
- Double displacement reaction
The reaction of manganese dioxide with hydrochloric acid is a double-displacement reaction that releases chlorine gas. One mole of manganese dioxide reacts with four moles of hydrochloric acid to produce one mole of manganese (II) chloride, two moles of water, and one mole of chlorine.
- Electrolysis reaction
The electrolysis process is used both in the laboratory and in the factory to produce chlorine. Refer to the illustration below to understand the chlor-alkali membrane electrolysis process:
How is Chlorine Used in Chemistry?
Chlorine is primarily used as a disinfectant because of its high reactivity as an oxidising agent. It can easily strip away the outer electrons of almost any substance, including the outer phospholipid membranes of bacteria and other microorganisms. This substance is so corrosive that it was weaponised during World War I due to its ability to destroy the mucus membranes of the lungs.
Chlorine belongs to the group of elements known as the halogens, which include five additional elements:
- Fluorine (F)
- Bromine (Br)
- Iodine (I)
- Astatine (At)
- Tennessine (Ts)
If you refer to column 17 of the periodic table of elements, chlorine is second from the top. The respective position of the halogens from top to bottom also indicates their reactivity. Although the five halogen elements are chemically similar, their relative reactivity increases from bottom to top. For instance, fluorine can oxidise chlorine:
Using Liquid Chlorine in a Lab
Chlorine is a greenish yellow gas at room temperature, but it becomes a liquid at −34 °C. It’s the most compressible element, making it relatively easy to transport in pressurised liquid form. About one-half kilogram of liquid chlorine can suddenly expand to 142 litres of gas when released.
Liquid chlorine can also refer to an aqueous solution of sodium hypochlorite. In either case, the liquid form of this substance can be used in several ways in a lab, such as in oxidation reaction experiments or in the preparation of polymers.
Chlorine in Manufacturing
Globally, about 58 million metric tonnes of chlorine are produced each year through the chlor-alkali membrane electrolysis process. The balanced chemical equation for the reaction is the following:
The Uses of Chlorine in Plastic Manufacturing
Chlorine is one of the sustainable ingredients in the manufacture of plastics. For instance, polyvinyl chloride, or PVC, is unlike other types of plastics, which are manufactured wholly from fossil fuel. By comparison, PVC is 57% chlorine that’s obtained from sodium chloride or common salt.
PVC is commonly obtained from the reaction of ethylene with oxygen and hydrogen chloride by using copper as a catalyst.
Why is Chlorine Used in Sewage Treatment?
Adding chlorine to wastewater is a part of several steps in sewage treatment. Chlorination is necessary to kill pathogens and parasites in the sewage, remove bad odour, and balance the chemistry of the water. In the UK, the wastewater treatment process can be summarised into five stages:
- Movement: This simply refers to ensuring that the wastewater reaches the sewage works. It involves drains and sewers
- Preliminary screening: The removal of large non-biodegradable solid waste materials
- Primary settlement: The process of separating suspended solids from the water
- Secondary sewage treatment: A two-stage biological treatment. The microscopic biodegradable particles in the water are decomposed by bacteria
Tertiary treatment: This stage includes chlorination. The water is already clean but it is further treated to kill any pathogen. Minerals and other chemicals are also removed.
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