Water is a unique and essential substance for life on Earth. It exists in all three states of matter and is tasteless, odourless, and has no calories. However, it plays a crucial role in life as it is involved in photosynthesis, respiration, metabolism, and enzyme function.
Despite being a small molecule, water has some unusual properties. It has high melting and boiling points, forms strong bonds with other molecules, and is cohesive and adhesive. This article will explore the properties of water, starting with its structure and how it relates to its physical properties such as cohesion, adhesion, and surface tension. We'll also delve into water's high specific heat capacity, melting and boiling points, and why ice is less dense than water. Water is often referred to as the universal solvent, and we'll explain why. We'll also explore some of the chemical properties of water such as its self-ionisation and amphoteric nature. In summary, we'll take a chemistry-focused view of the properties of water, highlighting its importance and unique characteristics. By understanding water's properties, we can better appreciate the role it plays in our lives and the world around us.
The official name for water is dihydrogen monoxide. Looking more closely at this name gives us an idea of its structure. -hydrogen tells us that it contains hydrogen atoms, and di- indicates that it has two. -oxide refers to oxygen atoms, and mono- tells us that it has just one. Put this all together and we’re left with water: H2O. Here it is, shown below:
Water consists of two hydrogen atoms joined to a central oxygen atom by single covalent bonds. The oxygen atom has two lone pairs of electrons. These squeeze the two covalent bonds tightly together, reducing the bond angle to 104.5° and making water a v-shaped molecule.
For more about the different shapes of molecules and the effect of lone pairs of electrons on bond angles, check out Shapes of Molecules.
Water's structure has a significant impact on its bonding abilities. Hydrogen bonds are a type of intermolecular force that occurs due to the difference in electronegativity between hydrogen and an extremely electronegative atom such as oxygen.
Electronegativity is an atom's ability to attract a bonded pair of electrons, resulting in the bonding electrons being found closer to one atom than the other in a covalent bond. In water, the two hydrogen atoms are bonded to a central oxygen atom by covalent bonds, resulting in hydrogen bonding between adjacent water molecules.
Oxygen is more electronegative than hydrogen, causing oxygen to pull the bonded pair of electrons towards itself and away from hydrogen. This results in the hydrogen becoming electron-deficient and the molecule becoming polar, with oxygen being slightly negatively charged and hydrogen slightly positively charged. We represent these partial charges using the symbol δ. To fully understand the concepts mentioned here, we recommend reading our article on intermolecular forces. Understanding water's bonding abilities is crucial as it plays a vital role in many of its unique properties.
But how does this lead to the formation of hydrogen bonds? Well, hydrogen is a small atom. In fact, it is the smallest atom in the whole of the periodic table! This means that its partial positive charge is densely packed into one tiny space. We say that it has a high charge density. Because it is so positively charged, it is particularly attracted to negatively charged particles, such as other electrons.
What do we know about the oxygen atom in water? It contains two lone pairs of electrons! This means that hydrogen atoms in water molecules are attracted to the lone pairs of electrons in oxygen atoms in other water molecules. The attraction between the densely charged hydrogen atom and oxygen’s lone pair of electrons is known as a hydrogen bond.
In summary, hydrogen bonds occur when a hydrogen atom is covalently bonded to an extremely electronegative atom with a lone pair of electrons. The hydrogen atom becomes electron-deficient and is attracted to the other atom's lone pair of electrons, resulting in a hydrogen bond.
Only specific elements such as oxygen, nitrogen, and fluorine are electronegative enough to form hydrogen bonds. Chlorine is also theoretically electronegative enough, but it doesn't form hydrogen bonds due to its larger size. The negative charge of its lone pairs of electrons is spread out over a larger area, resulting in a lower charge density that isn't enough to attract the partially charged hydrogen atom. However, chlorine does experience permanent dipole-dipole forces. For further information on this topic, we recommend reading our article on intermolecular forces where we cover this topic in more detail. Understanding hydrogen bonding is crucial as it plays a significant role in various chemical and physical properties of water and other substances.
Now that we’ve covered the structure and bonding of water, we can explore how this affects its physical properties. In this next section, we’ll look at the following properties: Cohesion Adhesion Surface tension Specific heat capacity Melting and boiling points Density Ability as a solvent
Cohesion is the ability of particles of a substance to stick to each other. If you splash a small amount of water across a surface, you’ll notice that it forms droplets. This is an example of cohesion. Instead of spreading out uniformly, water molecules stick to each other in clusters. This is due to the hydrogen bonding between neighbouring water molecules.
Adhesion is the ability of particles of a substance to stick to another substance.
When water is poured into a test tube, it tends to climb up the edges of the vessel, forming a meniscus. This phenomenon is an example of adhesion, which is the attraction of water molecules to other substances. Adhesion occurs because water can form hydrogen bonds with other substances, such as the sides of the test tube. The hydrogen bonding between water molecules and the test tube surface creates an attractive force that causes the water to climb up the edges of the vessel.
To measure the volume of the water accurately, it is essential to measure from the bottom of the meniscus. This is because the meniscus causes the water level to appear higher than it actually is, and measuring from the top of the meniscus would result in an inaccurate measurement. In summary, adhesion is a result of the hydrogen bonding between water molecules and other substances. The meniscus formed in a test tube is an example of adhesion, and measuring the volume of water accurately requires measuring from the bottom of the meniscus.
Don’t get cohesion and adhesion mixed up. Cohesion is a substance’s ability to stick to itself, whereas adhesion is a substance’s ability to stick to another substance.
Surface tension describes the way that molecules on the surface of a liquid act like an elastic sheet, and try to take up the least surface area possible.
This is where the particles on the surface of a liquid are strongly attracted to the other particles in the liquid. These outer particles are pulled into the bulk of the liquid, making the liquid take the shape with the least surface area possible. Due to this attraction, the surface of the liquid is able to withstand external forces, such as the weight of an insect. Water has a particularly high surface tension due to hydrogen bonding between its molecules. This is another example of the cohesive nature of water.
Specific heat capacity is the energy needed to raise the temperature of one gram of a substance by one degree Kelvin or one degree Celsius.
Remember that a change of one degree Kelvin is the same as a change of one degree Celsius.
Changing the temperature of a substance involves breaking some of the bonds within it. Hydrogen bonds between water molecules are very strong and so require a lot of energy to break. This means that water has a high specific heat capacity.
Water’s high specific heat capacity means it offers many advantages to living organisms as water resists extreme temperature fluctuations. It helps them maintain a constant internal temperature, optimising enzyme activity.
Water has high melting and boiling points due to the strong hydrogen bonds between its molecules, which require a lot of energy to overcome. This becomes apparent when you compare water to similar-sized molecules that don’t experience hydrogen bonds. For example, methane (CH4) has a molecular mass of 16 and a boiling point of -161.5 ℃, whereas water has a similar molecular mass of 18, but a much higher boiling point of exactly 100.0 ℃!
You might know that most solids are denser than their respective liquids. However, water is a bit unusual - it is the other way round. Solid ice is a lot less dense than liquid water, which is why icebergs float at the top of the sea instead of sinking to the ocean floor. To understand why, we need to look more closely at water’s structure in the two states.
As a liquid, water molecules are constantly moving about. This means that the hydrogen bonds between the molecules are constantly being broken and reformed again. Some of the water molecules are very close together whilst others are further apart.
As a solid, water molecules are fixed into position. Each water molecule is bonded to four adjacent water molecules by hydrogen bonds, holding it in a lattice structure. The four hydrogen bonds mean that the water molecules are held a fixed distance from each other. In fact, in this solid state, they are held further apart than in their liquid form. This makes solid ice less dense than liquid water.
The final physical property that we’ll look at today is water’s ability as a solvent.
A solvent is a substance that dissolves a second substance, called a solute, forming a solution.
Water is often referred to as the universal solvent. This is because it can dissolve a wide range of different substances. In fact, almost all polar substances dissolve in water. This is because water molecules are also polar. Substances dissolve when the attraction between them and a solvent is stronger than the attraction between solvent molecule and solvent molecule, and solute molecule and solute molecule.
In the case of water, the negative oxygen atom is attracted to any positively charged solute molecules, and the positive hydrogen atoms are attracted to any negatively charged solute molecules. This attraction is stronger than the forces holding the solute together, so the solute dissolves.
All the ideas we explored above were examples of physical properties. These are properties that can be observed and measured without changing the chemical composition of the substance. For example, the water molecules in steam have the exact same chemical identity as the water molecules in ice - the only difference is their state of matter. However, chemical properties are properties that we see when a substance undergoes a chemical reaction. We’re going to focus on two of water’s chemical properties in particular.
Ability to self-ionise Amphoteric nature
As a liquid, water exists in an equilibrium. Most of its molecules are found as neutral H2O molecules, but some ionise into hydronium ions, H3O+, and hydroxide ions, OH-. The molecules are constantly switching backwards and forwards between these two states, as shown by the equation below:
2H2O ⇋ H3O+ + OH-
This is known as self-ionisation. Water does this all by itself - it doesn’t need another substance to react with.
Water is an amphoteric substance that can act as both an acid and a base. When water self-ionizes, it forms hydronium ions, H3O+ and hydroxide ions, OH-. Hydronium ions can act as an acid by losing a proton to form H2O and H+, while hydroxide ions can act as a base by accepting a proton to form H2O.
Water reacts with other bases by acting as an acid, donating a proton, and with other acids by acting as a base, accepting a proton. This property of water makes it a versatile substance that can participate in a wide range of chemical reactions.
In addition to its amphoteric nature, water has several other unique properties that result from its hydrogen bonding and self-ionization. These properties include cohesion, adhesion, high surface tension, high specific heat capacity, high melting and boiling points, and the ability to dissolve a wide range of substances, making it known as the universal solvent.
Finally, water's solid form, ice, is less dense than liquid water, which is why ice floats on water. Understanding the properties of water is essential in various areas of science, such as biology, chemistry, and environmental science.
What are the properties of water?
Water is tasteless, odorless, and colourless. It is cohesive and adhesive and has high surface tension. It also has a high specific heat capacity and high melting and boiling points. It is a good solvent and is also unusual in that solid ice is less dense than liquid water. Water also self-ionises and is amphoteric.
What are the physicochemical properties of water?
Physicochemical is another word for physical and chemical. Water’s physicochemical properties include its cohesive and adhesive nature, its high specific heat capacity, surface tension and melting and boiling points, its ability as a solvent, and its amphoteric nature. Water also self-ionises and is less dense as a solid than as a liquid.
What are the physical properties of water?
Water is tasteless, odorless, and slightly blue in colour. It is cohesive and adhesive and has high surface tension. It also has a high specific heat capacity and high melting and boiling points. It is a good solvent and is also unusual in that solid ice is less dense than liquid water.
What are amphoteric properties?
Substances with amphoteric properties are substances that behave as both an acid and a base. One such example is water.
What is responsible for the cohesive property of water?
Water is cohesive, meaning it sticks to itself. This is due to the strong hydrogen bonds between molecules.
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