Earth, fire, air, and water have long been considered prime elements and essence of all life. Now we know those “natural forces” are not really elements, but something else entirely. Fire, for example, is a chemical reaction between oxygen in the air and fuel such as wood or coal. Earth and air are actually combinations of many different elements.
Water is a molecule, composed of 2 hydrogen atoms and one oxygen atom. But, ancient philosophers got one thing right - water is essential for life. There is a reason why all ancient civilizations were founded next to the rivers. Life, as we know it, depends on water.
Water in the human body
Our body consists of 65% of water. That’s amazing! But what’s more interesting is that certain organs like the brain and the heart (73%), as well as the lungs (83%), depend on water consumption even more.
Our body is about 65% water; the brain, heart and lungs hold even more.
All humans and living beings are made from a lot of water
Water is our essential need, we need to drink on average 2L per day just to survive. And in the case of dehydration, our brain shifts to water saving mode. As a consequence, cognitive functions quickly decline to prevent unnecessary “wasting”. In our body, water has multiple roles. Maybe the most obvious is that it regulates our temperature by sweating. But even more importantly, it provides fuel for every cell in our body. It’s also used to metabolize and transport food as well as to flush waste. It forms saliva needed for digestion and lubricates joints. Water is really essential for our everyday functioning.
Chemical structure of water
Structurally, the molecule of water looks like a letter V. The formula for the water (H2O) is probably well known to everyone. Two hydrogen atoms bond with one oxygen atom by what is called a covalent bond. That means that electrons are shared between the oxygen and hydrogen atoms, not transferred.
Water is made from one oxygen and two hydrogen atoms. The oxygen end is slightly negative and the hydrogen ends slightly positive, which makes water a polar molecule.
Oxygen is the more electronegative of the two, more “greedy” for electrons, so that part of the molecule has a slightly negative charge. The other part (the hydrogen part) has a slightly positive charge. This type of molecule, with differently charged ends, is called a polar molecule. That polarity makes water molecules highly attracted to each other. The positively charged end of one molecule is attracted to the negatively charged end of another water molecule. We say that multiple water molecules are connected by hydrogen bonds. That allows for some interesting properties we will discuss next.
Adhesion and cohesion
Cohesion and adhesion define “stickiness” of a substance. Cohesion is an attraction between similar things, while adhesion is an attraction between different things. Water has high cohesion, it is actually highest of all nonmetallic liquids. That means that molecules of water like to stay close together (remember hydrogen bonds?). You can see this property in action if you observe a drop of water on the wax paper. A drop of water consists of many water molecules bonded by a hydrogen bond and it retains its shape.
Cohesion holds water drops together; adhesion makes them cling to other surfaces.
Adhesion and cohesion at work. See how water behaves differently
But there are cases when the attraction to other types of molecules overpowers cohesion.
Did you ever observe water on the glass? It becomes a wet mess, unlike elegant drops we saw before. Why? Well, glass molecules are even more polar than the water, so they become more attractive. This is adhesion. Adhesion and cohesion work together to achieve some really amazing things. One of them is capillary action, movement of water upwards, against the gravity. This is the principle on which the plants transport water around and it also helps with blood circulation in our body.
The universal solvent
A solvent is a thing which can dissolve other things. And water can dissolve more things than any other liquid. That’s why it’s often called the "universal solvent". Those things which we put in liquids are called solutes.
Since water is polar, it can dissolve other polar molecules and ionic compounds. Examples include table salt, food colors, and sugar. Since they dissolve in the water we call them hydrophilic molecules. Those that do not mix with water, such as oils, we call hydrophobic, water-fearing molecules. They do not dissolve in water since they are nonpolar. Nonpolar molecules interfere with the hydrogen bonding of water molecules. We explored more this topic in our Lava lamp experiment so try it out if you’re interested.
Density and heat capacity
What happens to water when it freezes into ice?
Make your prediction, then tap an answer to check!
Did you know that water is the only substance on the Earth occurring in all 3 forms: solid, liquid and gas? And another peculiarity: The density of the water is higher in a liquid form than in solid (ice). That’s another property unique to the water. But how is that happening? Around 0 degrees Celsius, hydrogen bonds start forming crystalline structures. In crystalline structures, atoms are more spread apart so the density is lower. That’s why the ice floats on the water!
Ice is less dense than liquid water, which is why it floats.
The density of the water is higher in a liquid form than in solid
Water has a very high heat capacity. Actually, it has the highest heat capacity of all liquids! That means it can absorb a lot of heat before its temperature rises. The opposite is also true: water cools down slowly. If you’ve ever been around the sea, you know the climate is much more pleasant. Water serves as a buffer that prevents huge differences in temperature.
Since the human body consists of much water, this property also helps us in regulating body temperature. Think of the various temperature conditions we find ourselves in, from freezing winters to scorching summers. We can manage it without many issues because our body uses water as a temperature buffer. If you want to see water’s heat capacity in action, check out 5 amazing balloon experiments where we explored it in details.
Key takeaways
- Water is a molecule of two hydrogen atoms and one oxygen atom (H₂O), joined by covalent bonds.
- Because oxygen pulls electrons harder, water is a polar molecule - slightly negative at the oxygen end, slightly positive at the hydrogen ends.
- That polarity creates hydrogen bonds, giving water high cohesion and adhesion (capillary action) and making it the universal solvent.
- Water is the only substance on Earth found naturally as a solid, liquid, and gas, and uniquely it is less dense as ice, so ice floats.
- Water has the highest heat capacity of any liquid, so it warms and cools slowly and helps regulate temperature - in the climate and in our bodies.
Frequently Asked Questions
Why is water called the universal solvent?
Because water can dissolve more substances than any other liquid. Its polar molecules pull apart other polar molecules and ionic compounds - like salt, sugar, and food colouring - surrounding them and mixing them in. It can't dissolve non-polar substances such as oils, which is why they stay separate.
Why does ice float on water?
When water freezes, hydrogen bonds arrange the molecules into a crystal structure that holds them further apart than in liquid water. This makes ice less dense than the water around it, so it floats. Most other substances become denser and sink when they solidify.
What makes water a polar molecule?
Oxygen is more electronegative than hydrogen, so it pulls the shared electrons closer. That leaves the oxygen end of the molecule slightly negative and the hydrogen ends slightly positive. Having oppositely charged ends is exactly what makes a molecule "polar".
What is the difference between cohesion and adhesion?
Cohesion is the attraction between molecules of the same substance - water sticking to water, which is why droplets keep their shape. Adhesion is the attraction between different substances - water sticking to glass. Together they produce capillary action, which helps plants move water upward.
How much of the human body is water?
About 65% of the body is water, but some organs contain even more - the brain and heart are around 73% water and the lungs about 83%. We need to drink roughly 2 litres a day to stay hydrated and keep our cells working.
Why does water take so long to heat up and cool down?
Water has the highest heat capacity of any liquid, meaning it can absorb a lot of energy before its temperature rises - and release it slowly as it cools. This is why the seaside has a milder climate and why our water-rich bodies handle temperature changes so well.
And that is all from us but just the beginning for amazing water. If you enjoyed this article, we recommend that you experiment a little. Why don’t you try to make your own plastic? Or go explore osmosis with gummy bears or Diffusion with hot and cold water. Many, many exciting scientific adventures await you.
