Why is water considered a pure substance

15.4.1784

It is a sensation for science: water can be divided into two parts. The French banker and hobby chemist Antoine Laurent de Lavoisier (born August 26, 1743) proved this in an astonishing experiment.

A year before his experiment, Lavoisier had heard from England that water could be split into two gases. The British nobleman Henry Cavendish (born October 17, 1731) discovered hydrogen during his experiments. Lavoisier simulated Cavendish's experiments and again split water into hydrogen and oxygen. After he had recovered water from the two gases, he explained to the astonished public: “The combustion of the two types of air and their conversion to water, part by weight, leaves little room for doubt that this substance, which was previously regarded as an element, is a compound substance. "

A groundbreaking discovery! To prove this, Lavoisier had passed water vapor through a red-hot iron pipe and thus obtained hydrogen and oxygen. The oxygen reacted with the red-hot iron to form iron oxide. In turn, the pure hydrogen collected in a pig's bladder!

Comment: Aristotle was wrong here!

Fire, water, earth, air - if you follow the teachings of the ancient Greeks, then every material is made up of these four elements. Until recently, we believed that water was an element that cannot be divided. But with his experiment, Antoine Laurent de Lavoisier brought down the beautiful theory of antiquity: water can very well be divided into its two components, hydrogen and oxygen. The four-element theory is thus a thing of the past! And that is why the experiment by the French Lavoisier means nothing less than a revolution for science!

What water can do

No matter whether we drink tap water, jump into a lake or are surprised by a downpour - we are constantly in contact with water. And not only that: we are made of water ourselves, around two-thirds of it. Without question, water is part of our everyday life. But what seems completely normal to us has all kinds of peculiarities. And the water owes this primarily to its structure.

Everything that exists on this earth is made up of tiny building blocks, the atoms. This is also the case with pure water: It is a combination of two hydrogen atoms and one oxygen atom. These combine to form a water molecule, H for short2O. The individual water molecules are only loosely connected to one another.

This loose cohesion ensures that the connection between the molecules breaks at high temperatures: the water evaporates. If, on the other hand, it cools down significantly, the molecules organize themselves into a solid, regular lattice, the ice. The special thing about it: In its solid form, water has a larger volume than in its liquid state.

The arrangement of the water molecules also ensures another property: the surface tension of the water. Because of this tension, water spiders and water striders can easily walk on a pond. But water can do even more: it is able to dissolve substances. Small grains of salt or sugar dissolve completely in water. Sea water, for example, contains large amounts of salt that we can taste but not see.

We owe the fact that lemons ripen on the island of Mainau on Lake Constance to another ability of water: it can store heat. Lakes or seas heat up in summer and keep the heat for a long time. That is why the temperatures on the coast fluctuate less than inland. Far from the coast, the temperature differences between day and night and between summer and winter are much greater than near the sea.

Quick-change artist water

It is well known that water is liquid. However, this is not always the case. In nature, water occurs in three states: as liquid water, as gaseous water vapor or as solid ice. Depending on the external conditions, it changes from one state to the other.

The state of the water depends on the pressure and its temperature. If liquid water exceeds the boiling point, it evaporates and floats in the air as gaseous water vapor. Even when evaporating at room temperature, water changes into a gaseous state. However, this happens more slowly than when evaporating. If, on the other hand, the temperature drops below 0 ° Celsius, the water freezes to ice. As soon as water changes its state between liquid, gaseous or frozen, it changes its properties.

The special thing about water is that it has its greatest density at 4 ° Celsius and takes up little space. When it freezes into solid ice, it expands and increases in volume. At the same time, its density decreases. That is why ice is lighter than water with the same volume. That is why icebergs can drift in the sea. For the same reason, a lake freezes over from above and not from below in winter. That's a good thing, because otherwise we wouldn't be able to skate until the lake was completely frozen from the bottom to the surface.

So water expands when it freezes. If you prevent it from doing so, there is tremendous pressure on it. Anyone who has ever forgotten a bottle of water in the freezing cold outside knows the consequences: after a while the bottle bursts and the ice oozes out. In this way, ice can also break up stone. This happens when water flows into cracks in the rock, freezes there and pushes outwards due to the expansion. If pieces of stone flake off as a result of this force, it is called frost splitting. Anyone who has ever driven into a pothole knows the consequences. Here the constant alternation of wetness and frost have really damaged the asphalt.

The blue planet

Seen from space, the globe appears in a strong blue. This is because almost three quarters of the earth is covered with water. Small amounts of water are transparent, but from a certain depth onwards it becomes more and more blue. Because we see the mighty oceans blue, the earth is also called "the blue planet". The term south of the equator is particularly applicable. Because the southern hemisphere is almost completely covered by the sea, because a large part of the continents have migrated to the north due to plate movement.

The vast oceans contain almost all of the water on earth. There is a lot of salt dissolved in sea water, which is why it is not suitable as drinking water. The little fresh water on earth is frozen mainly in glaciers and ice caps. Only a tiny fraction of freshwater is found in groundwater, in lakes and rivers, or in the air.

But the view from the outside is deceptive: the earth's surface is largely covered by water, but measured by the diameter of the earth, the oceans are only a very thin layer. Therefore, the water makes up only a fraction of the earth's mass. For comparison: if the earth were the size of a basketball, all of the water on earth would fit into a table tennis ball. And the drinking water would be proportionally even smaller than a single popcorn.

How did the water come to earth?

About two thirds of the earth is covered with water - a unique selling point: the earth is the only planet in the solar system on which there is liquid water. Life originated in water, and water is also vital for us humans. But where does the water actually come from on earth?

Scientists suspect that the water comes from comets. These lumps of ice and dust originally formed on the edge of the solar system. But some also got into the interior of the solar system on orbits and became part of the newly emerging planets.

Initially, the young planets were very hot - so hot that the rock melted and formed a liquid ball. And the ice on the comets not only melted, it even evaporated. Because the water vapor was much lighter than the molten rock, it bubbled up towards the surface. There it escaped into the atmosphere through volcanoes.

As the earth slowly cooled, the steam turned back to liquid water. To put it more clearly: It started to rain. Those first downpours must have been stronger than any thunderstorm we can imagine today. And it must have rained for a very long time - tens of thousands of years. Large parts of the young earth's surface were flooded - in some places up to ten kilometers high. This is how the oceans came into being.

And what happened to the water on the other planets? Why are there no oceans there? Mercury doesn't have enough gravity to hold an atmosphere at all - like all gases, water vapor simply escaped into space. The same thing happened on the moon. The solar radiation on Venus is so strong that the water has also evaporated into space. On the other hand, it is too cold on Mars, but there are suspected large deposits of ice under the surface. And the gas planets have no solid surface on which seas could form. One suspects an ocean of water on Jupiter's moon Europa, but the surface is frozen. So the earth remains the only celestial body in the solar system with seas.