Core Concepts
In this article, we will discuss the chemical structure, chemical properties, and key applications of water, one of the world’s most important molecules.
Introduction
Water is an inorganic compound made up of oxygen and hydrogen that is critical to living organisms. Represented by the chemical formula H2O, it is one of the most highly studied molecules because of its unique chemical and physical properties. These properties arise from both water’s chemical structure and the crucial bonds that the compound forms within itself and with other molecules.
The Composition of Water
To understand the chemical and physical intricacies of water, let’s first discuss how its makeup was discovered! From 1731 to 1810, experimental and theoretical chemist and physicist Henry Cavendish was actively conducting research on atmospheric air and its components. Within his essay titled “On Factitious Airs,” published around 1784, he, along with John Warltire, discussed inflammable air. Inflammable air, also known as hydrogen gas (H2), was found to make its container dewy upon combustion. When Cavendish replicated the experiment, he found that approximately one-fifth of the air was condensed into a dew. After meticulous observation, he determined the dew to be pure water, definitively proving that water is made up of oxygen and hydrogen molecules! Credit is often given to James Watt for publishing a paper on water’s composition prior to Cavendish, who was reporting his findings to a colleague before publication.
Properties of Water
Now equipped with Cavendish’s discoveries, scientists have been able to deeply study this fascinating compound and its components. To fully explore water, there are three core areas to discuss: its bonding patterns, its physical states, and its solubility.
Bonding
In explaining what makes water special, let’s first begin on an atomic level, starting with its geometry! The geometry is important because it governs how the compound behaves and interacts with other molecules.
As we discuss the structure of water, it is also important to note its shape. The three atoms within it, one oxygen atom flanked by two hydrogen atoms, do not exist in a straight line relative to each other. In a linear structure, the oxygen atom’s lone pairs of electrons would be uncomfortably close to the hydrogen atoms’ electrons. Instead, the lone pairs repel these electrons by pushing the hydrogen atoms farther away, bending the overall molecule’s shape.
Connecting these three atoms together are two polar covalent bonds. A covalent bond refers to when two atoms share electrons to complete each atom’s octet. Atoms want a full octet of electrons to achieve stability, and in oxygen’s case, it requires two additional electrons to achieve a full octet. Two hydrogen atoms can help fulfill this by presenting oxygen with one electron each, making the water molecule very stable!
Another factor that impacts the properties of a water molecule is its polarity, or its uneven distribution of electrons. Oxygen is more electronegative than hydrogen, meaning that it tends to pull electrons toward itself. In a water molecule, this results in the oxygen atom accumulating more electron density than the hydrogen atoms, instead of sharing it equally among all three atoms. The electronegativity difference creates a dipole, the uneven distribution of electrons, within the molecule. Ultimately, this leaves both hydrogen atoms with partial positive charges, while electron-dense oxygen has a partial negative charge. Every water molecule is like this, and we see why it matters when several of them interact with each other.
A weak connection forms when water molecules encounter each other. Opposite charges attract, so the partial positive charge of each hydrogen atom on one molecule attracts the partial negative charge of another molecule’s oxygen atom. This hydrogen bonding allows water molecules to interact easily with each other! Hydrogen bonding is when a hydrogen atom, which is relatively electropositive, weakly interacts with another molecule’s more electronegative atom, like oxygen.
Though individual hydrogen bonds are fairly weak, they become much more powerful when many molecules are interacting with each other! Because of this strength, it can require a significant amount of energy to break the hydrogen bonds, depending on the state the water exists in. This is why water has such a high boiling point, at 100°C or 212°F. All in all, the organization of electrons lends itself to the water molecule’s bent shape and contributes to the strength of its bonds, as shown below!
But why is the state in which the water exists relevant? As it turns out, it actually plays a big role in how the compound’s chemical and physical properties manifest. Let’s take a look!
The States of Matter
What makes water pivotal in the world of molecules is that, at any given moment, it can naturally exist on Earth in all three states of matter. A compound’s state of matter often depends on the temperature of its environment. With Earth’s diverse range of climates, it’s possible for water to take on multiple physical forms across various environments around the globe, allowing us to appreciate it in all its splendor.
Different states of matter behave differently on a molecular level. Therefore, manipulating the water’s temperature is a simple way to change not only its state, but also its properties. At room temperature, it exists as a liquid, the molecules able to move freely around one another. Around 100°C, this increase in heat energy allows the molecules to move at a faster rate, transitioning the liquid into a gas! Contrarily, water’s freezing point is 0°C, at which point its molecules adopting a very rigid crystalline structure that we know as ice.
Each state of matter has different uses and applications. In nature, solid ice helps regulate the ocean temperatures beneath it and influences sea levels. Gaseous water vapor is a key player in the water cycle, and can be harnessed for energy purposes via a steam turbine. And we certainly wouldn’t be here without liquid water, which comprises about 60% of your body and around 70% of the earth’s surface.
But that’s not where water’s wonders end. Next, we’ll investigate its solubility, another facet of its interactions with other molecules that showcases its ultra-unique behavior and properties.
Solubility
In its liquid form, water holds significant utility to scientists as a solvent. In fact, it is a hailed as a universal solvent because it is capable of dissolving more substances than most other solvents that we know of!
The reason this occurs can be explained by the rule of solubility, which we can simplify to “like dissolves like.” Recall that water is polar, thanks to the dipole that arises from its internal electronegativity difference. According to this rule, polar solvents can dissolve polar solutes, and nonpolar molecules are able to dissolve non-polar solutes. As a result, water can readily dissolve other polar compounds, like most acids, alcohols, sugars, and salts, with ease. When water acts as the solvent, we consider this an aqueous solution.
The flip side of this rule is that polar solvents can’t dissolve nonpolar solutes, and nonpolar solvents can’t dissolve polar solutes. Thus, nonpolar solutes like oils and fats are insoluble in water. It also means that if you were to drop an ice cube into room-temperature benzene or hexane, the ice cube wouldn’t dissolve once it melts into water.
Nonetheless, water can dissolve more substances than you might expect! This is just one example of how it’s a useful compound practically everywhere: in the lab, in your body, and even in your kitchen! Next, we’ll dive a bit deeper into water’s many other applications.
Applications of Water
Let’s zoom out a bit to discuss how, apart from being a universal solvent, water is used every day in both large and small ways. Not only does it serve as a solvent, but it also facilitates a variety of key chemical reactions.
Water in Reactions
Water has the interesting capacity to act as an acid or a base. Known as an amphiprotic compound, water may either donate or accept protons, depending on the circumstances of the reaction. This is not the only compound with this special talent, but it is the most common one out there!
When water donates a proton, it is acting as an acid, according to the Brønsted-Lowry definition. By giving a proton to another partner in the reaction, the water molecule becomes a hydroxide ion (OH–). By contrast, a water molecule that accepts a proton is functioning as a base. The addition of the extra proton converts it into a hydronium ion (H3O+).
In organic chemistry, water plays a central role in a few common reactions. We’ll start with hydrolysis, which literally refers to the splitting of a water molecule! Hydrolysis is a key reaction pertaining to esters, as a hydrolyzed ester results in a carboxylic acid and an alcohol. This reaction often includes the use of a hydrogen ion (H+) as a catalyst.
Two other reactions, hydration and dehydration, utilize water to transform alcohols and alkenes! These reactions are two sides of the same coin, with opposite outcomes. When starting with an alkene reactant, the catalyst attacks the water molecule. This allows a hydrogen atom and an –OH group to add on to the alkene, whose double bond breaks. When starting with an alcohol reactant, the –OH group attacks the hydrogen ion catalyst, leading to the reformation of the alkene double bond!
Other reactions feature water as a major participant, but keep in mind that these reactions are not just theoretical. They play critical roles in our bodily systems, which makes them essential to keeping us alive! Without water, some of the fundamental metabolic processes that keep us running would not be able to proceed: the citric acid cycle, fatty acid catabolism, glucose metabolism, and more. Plus, plants depend on it, too! Plants need water for survival, and the process of photosynthesis to create energy involves it as a reactant.
Water in Industry
On an even larger scale, this monumentous molecule has industrial applications as well. Hydropower (or hydroelectric power) is a form of renewable energy that uses naturally flowing water to generate electricity. In data centers, metal work, nuclear power, and more, it is the coolant to keep systems from overheating. It’s also inherent to the food industry, where it cleans, dilutes, sanitizes, cooks, and blends throughout the manufacturing process. Textiles require a significant amount of water for dyeing, bleaching, and printing. Virtually all industries that involve manufacturing use water at some point in that process!
Conclusion
Necessary for life itself, water is deeply integrated into every aspect of life as we know it today. Although we’ve learned a lot about it over the centuries, we still study its wide applications across fields. Despite its structural simplicity, water’s properties continually improve its uses, enabling us to harness it for increasingly complex activities.