Chlorine: The Diatomic Green Gas
What is Chlorine?
Chlorine is a chemical element with the symbol Cl and atomic number 17. In its pure form, it is not a single atom but exists as a gas made of molecules, each containing two chlorine atoms bonded together. This is why it's called a diatomic molecule and written as $Cl_2$. It belongs to a family of elements known as the halogens (Group 17 on the periodic table), which are all very reactive. The name "chlorine" comes from the Greek word chloros, meaning greenish-yellow, which perfectly describes its color.
Properties of Chlorine Gas
Chlorine gas has a set of distinct physical and chemical properties that make it both useful and dangerous. It is heavier than air, which means it will sink to the ground if released, and it has a strong, pungent, and irritating odor that is detectable even at very low concentrations.
| Property | Description |
|---|---|
| Color | Greenish-yellow |
| State at Room Temperature | Gas |
| Molecular Formula | $Cl_2$ |
| Odor | Pungent and irritating (like bleach) |
| Solubility in Water | Moderately soluble; it reacts with water to form hypochlorous acid ($HOCl$) |
| Toxicity | Highly toxic; it was used as a chemical weapon in World War I |
| Reactivity | Very reactive, especially with metals and organic compounds |
How is Chlorine Gas Produced?
Chlorine is too reactive to be found in nature as a pure element. It is almost always found bonded with other elements, most commonly as sodium chloride (NaCl) in salt beds or dissolved in seawater. To obtain pure chlorine gas, we need to force these compounds to break apart using electricity, a process known as electrolysis.
The most common method is the Chlor-Alkali Process[1], which uses an electric current to split a brine solution (concentrated salt water). The main chemical reaction can be simplified as:
$2NaCl + 2H_2O \xrightarrow[Electricity]{} Cl_2 + H_2 + 2NaOH$
This means that from salt water, we get three valuable products: Chlorine gas ($Cl_2$), Hydrogen gas ($H_2$), and Sodium Hydroxide ($NaOH$), which is a strong base used in soap and cleaning products. This makes the process very efficient and economically important.
The Many Uses of Chlorine and Its Compounds
While the green gas itself is dangerous, its compounds are incredibly useful and have become integral to modern life. About two-thirds of all manufactured chemicals involve chlorine or its compounds at some stage of their production.
| Use | How it Works & Examples |
|---|---|
| Water Purification | A small amount of $Cl_2$ is added to drinking water and swimming pools. It kills bacteria, viruses, and other disease-causing microbes (pathogens), preventing the spread of illnesses like cholera and typhoid. |
| Bleaching | Chlorine compounds like sodium hypochlorite ($NaOCl$) are the active ingredients in household bleach. They break down colored stains and are used to whiten paper and cloth. |
| Plastics (PVC) | A major use of chlorine is in making Polyvinyl Chloride (PVC), a durable plastic used in pipes, window frames, credit cards, and medical devices. |
| Medicines & Solvents | Many pharmaceuticals, including some antibiotics and anesthetics, contain chlorine. Chlorine is also used to make solvents for dry cleaning and degreasing. |
| Agriculture | Chlorine is a component of many pesticides and herbicides that protect crops from pests and weeds. |
A Tale of Two Cities: Chlorine in Water Purification
Imagine two cities. City A adds a carefully measured amount of chlorine to its water supply. City B does not. In City A, the water is clear and safe to drink, with a drastically reduced risk of waterborne diseases. In City B, the water from the reservoir may look clean, but it could harbor invisible bacteria like E. coli or viruses that cause severe illness. This practical application of chlorine has been one of the most significant public health advances in history, saving countless lives by preventing outbreaks of cholera, dysentery, and typhoid.
The process works because chlorine is a powerful oxidizing agent. When added to water, it reacts to form hypochlorous acid ($HOCl$), which penetrates the cell walls of microbes and destroys them from the inside, effectively disinfecting the water.
Common Mistakes and Important Questions
Q: Is the chlorine used in pools the same as the toxic gas?
Yes and no. The substance used is typically a chlorine compound, like calcium hypochlorite (solid) or sodium hypochlorite (liquid), which release hypochlorous acid ($HOCl$) into the water—the same disinfecting agent formed when pure $Cl_2$ gas is used. However, handling the pure gas is extremely dangerous and is only done by trained professionals at large water treatment facilities. Pool owners use much safer, stabilized compounds.
Q: I've heard that chlorine in water can be bad for you. Is that true?
At the low concentrations used for water disinfection, chlorine is considered safe for human consumption and is vital for preventing disease. However, chlorine can react with naturally occurring organic matter in water to form small amounts of byproducts like trihalomethanes (THMs). While long-term exposure to high levels of THMs may pose a health risk, the public health benefits of chlorination in preventing immediate, deadly waterborne diseases far outweigh the potential risks. Water authorities carefully monitor and control these levels.
Q: Why does chlorine bleach things?
Bleaching is an oxidation process. The chlorine compounds break the chemical bonds in the colored molecules (chromophores) that make a substance appear colored. By breaking these bonds, the molecule no longer absorbs visible light in the same way, and the color disappears, leaving the material white or lighter in color.
Footnote
[1] Chlor-Alkali Process: An industrial process for the electrolysis of sodium chloride (NaCl) solutions. It is the primary method for producing chlorine gas, hydrogen gas, and sodium hydroxide (caustic soda).