Why most people don't understand anything about water?
The answer to the question is a little embarrassing. In most science lessons a wrong model for liquid water is taught. So science itself is responsible for our bad understanding.
But let us first start with the good news. The scientific models for the water molecule, ice and water vapour are excellent and a good starting point for a better understanding of liquid water.
The water molecule is the fundamental building block of water. It consists of two small hydrogen atoms bound to a 16-fold heavier oxygen atom. Water is nothing more than burned hydrogen.
The water molecule is electrically neutral but the center of positive and negative charges are located in different places. This fact gives the water molecule its most important property: it is a polar molecule. The center of the negative charge is located more in the oxygen atom leaving the hydrogen atoms positively charged (Fig. 1). The polarity of the water molecule is large compared to other types of molecules. Therefore, water, a very light molecule, is not a gas. Under normal conditions it is a liquid.
When the polar water molecules come together (at normal temperatures) they will interact with each other. The positive part of the hydrogen atoms will be attracted to the negative part of the oxygen atoms, forming what is called 'a hydrogen bond' and resulting in three dimensional structures of water molecules (Fig. 2). In most text books hydrogen bonds are only linked to the water structure but they are also the fundamental bonds in the ice structure. Only in water vapour, the influence of the hydrogen bonds is negligible because water molecules are too far from each other.
The most common crystalline phase of ice has a hexagonal crystal structure (see Fig. 3 left). Each water molecule is fixed and bonded by 4 nearest neighbours. The relation between the macroscopic properties of ice and this microscopic ice model is clear. Ice is rigid and (almost) not compressible and so is its microscopic structure. A similar model for a gas is easily constructed. Water vapour (see Fig. 3 right) has low density and is highly compressible. Our microscopic model has to deal with that. The average distance between the gas molecules is significantly higher than in the solid state and the gas molecules are moving around freely and randomly. No stable hydrogen bonds are possible. So, our gas model explains both the low density and the compressibility of water vapour.
But the liquid state is more challenging. The density and the compressibility of liquid water have the same order of magnitude as ice. So the water molecules have to be close to each other just like in ice. But water also flows and fills a container, just like gases do.
In most textbooks, the flowing properties of liquids are explained describing the motion of the liquid molecules as free and random, ”like a bunch of marbles in a bag”. So, we model the liquid molecules as free and random, just like the gas molecules, and at the same time we assume that the molecules are as close to each other as in a solid. Do you feel the intrinsic contradiction here?
Most science teachers will keep silent about the hydrogen bond interactions in liquids and hope that no clever student will mention them. But why do we neglect them? The water molecules are even at closer distance in water than in ice. Why is the polarity of the molecules, which is strong enough to form rigid ice not playing any role in the water structure? In order to save the marble model, it is sometimes claimed that the water molecules in the liquid have more thermal energy breaking the hydrogen bonds. But this is certainly not a convincing answer. The difference in thermal energy is not large and both the liquid phase and the ice phase coexist at 0°C. At that temperature the thermal energy of the molecules in both phases is the same.
We are in the heart of the water problem. For the sake of didactical simplicity, we have made a mess of the model most people (including scientists) have about liquids. We let them believe that the molecules in a liquid are free, like in a gas, and this only to explain liquidity. But in order to do so, we had to ignore the fundamental interaction between the water molecules (the hydrogen bonds) that can make water solid.
There is hard evidence that this gas-like model doesn't make any sense. One only has to compare the latent heat of fusion of ice (6.0 kJ/mol) with its heat of sublimation (51 kJ/mol) at 0°C. The heat of sublimation is the energy necessary to break all the hydrogen bonds of ice, the heat of fusion is the energy necessary to form liquid water from the ice structure. The values make clear that most hydrogen bonds are still present in liquid water, only 12% of the bondings is broken during the melting process. Liquid water is not a bag of marbles, most molecules are still bonded to each other. It is clear that the liquidity is not related to free molecules, but to a limited amount of free molecules in a rigid structure.
Liquidity is not a gas property, it is a solid property. And this not a strange idea, it is a widespread scientific idea. Solid pitch will flow if you wait long enough and ice glaciers are called solid rivers. The concept of liquidity is nicely addressed by Veritasium: 'Is glass a liquid?'. We are used to the idea that liquids fill containers in a time scale of seconds. But solids will also behave like liquids on a large time scale. The distinction between liquids and solids is only related to our time scale and it is not so fundamentally different as we suppose. Realising this is the only way out of the mess of our gas-like water model. Of course there are much more broken hydrogen bonds in water than in ice, but not enough to break the solid structure. The liquid phase is more similar to the solid state than to the gas state. In order to find a better model for water, we have to start all over again, and to look to water with a new pair of glasses.
This blog will help you to obtain a fundamentally new and more rigid water model. Please subscribe to this blog if you are eager to learn more. If you want to read the hard data behind this new model, please click on the page 'Scientific work'.
But let us first start with the good news. The scientific models for the water molecule, ice and water vapour are excellent and a good starting point for a better understanding of liquid water.
 |
| FIg. 1 The water molecule is polar. |
The water molecule is electrically neutral but the center of positive and negative charges are located in different places. This fact gives the water molecule its most important property: it is a polar molecule. The center of the negative charge is located more in the oxygen atom leaving the hydrogen atoms positively charged (Fig. 1). The polarity of the water molecule is large compared to other types of molecules. Therefore, water, a very light molecule, is not a gas. Under normal conditions it is a liquid.
 |
| Fig. 2 One water molecule is connected to four other molecules by hydrogen bonds. |
When the polar water molecules come together (at normal temperatures) they will interact with each other. The positive part of the hydrogen atoms will be attracted to the negative part of the oxygen atoms, forming what is called 'a hydrogen bond' and resulting in three dimensional structures of water molecules (Fig. 2). In most text books hydrogen bonds are only linked to the water structure but they are also the fundamental bonds in the ice structure. Only in water vapour, the influence of the hydrogen bonds is negligible because water molecules are too far from each other.
The most common crystalline phase of ice has a hexagonal crystal structure (see Fig. 3 left). Each water molecule is fixed and bonded by 4 nearest neighbours. The relation between the macroscopic properties of ice and this microscopic ice model is clear. Ice is rigid and (almost) not compressible and so is its microscopic structure. A similar model for a gas is easily constructed. Water vapour (see Fig. 3 right) has low density and is highly compressible. Our microscopic model has to deal with that. The average distance between the gas molecules is significantly higher than in the solid state and the gas molecules are moving around freely and randomly. No stable hydrogen bonds are possible. So, our gas model explains both the low density and the compressibility of water vapour.
 |
| Fig. 3 Left: Hexagonal ice and its microscopic model Right: An artistic impression of water vapour (which is invisible) and its microscopic model |
 |
| Fig. 4 The bad model for water (marbles in a box) that you find in most textbooks. |
In most textbooks, the flowing properties of liquids are explained describing the motion of the liquid molecules as free and random, ”like a bunch of marbles in a bag”. So, we model the liquid molecules as free and random, just like the gas molecules, and at the same time we assume that the molecules are as close to each other as in a solid. Do you feel the intrinsic contradiction here?
Most science teachers will keep silent about the hydrogen bond interactions in liquids and hope that no clever student will mention them. But why do we neglect them? The water molecules are even at closer distance in water than in ice. Why is the polarity of the molecules, which is strong enough to form rigid ice not playing any role in the water structure? In order to save the marble model, it is sometimes claimed that the water molecules in the liquid have more thermal energy breaking the hydrogen bonds. But this is certainly not a convincing answer. The difference in thermal energy is not large and both the liquid phase and the ice phase coexist at 0°C. At that temperature the thermal energy of the molecules in both phases is the same.
We are in the heart of the water problem. For the sake of didactical simplicity, we have made a mess of the model most people (including scientists) have about liquids. We let them believe that the molecules in a liquid are free, like in a gas, and this only to explain liquidity. But in order to do so, we had to ignore the fundamental interaction between the water molecules (the hydrogen bonds) that can make water solid.
There is hard evidence that this gas-like model doesn't make any sense. One only has to compare the latent heat of fusion of ice (6.0 kJ/mol) with its heat of sublimation (51 kJ/mol) at 0°C. The heat of sublimation is the energy necessary to break all the hydrogen bonds of ice, the heat of fusion is the energy necessary to form liquid water from the ice structure. The values make clear that most hydrogen bonds are still present in liquid water, only 12% of the bondings is broken during the melting process. Liquid water is not a bag of marbles, most molecules are still bonded to each other. It is clear that the liquidity is not related to free molecules, but to a limited amount of free molecules in a rigid structure.
 |
| Fig. 5 Pitch will flow if you are prepared to wait long enough. |
This blog will help you to obtain a fundamentally new and more rigid water model. Please subscribe to this blog if you are eager to learn more. If you want to read the hard data behind this new model, please click on the page 'Scientific work'.
Comments
Post a Comment