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The structure of the water molecule

Water is a substance made of molecules. Each molecule consists of two hydrogen atoms connected with one oxygen atom. Oxygen and hydrogen atoms are connected by polar covalent bonds. Hydrogen and oxygen atoms do not lie along one line, the bonds between them form an angle of about 104.5 °.

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Ilustracja przedstawia model cząsteczki wody. Składa się z atomu tlenu, który oznaczony jest czerwoną kulą i symbolem pierwiastka tlenu o oraz dwóch atomów wodoru symbol ha, które są oznaczone jako dwie mniejsze szare kulki połączone z atomem tlenu.

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Ilustracja przedstawia wzór strukturalny cząsteczki wody złożonej z dwóch atomów wodoru i atomu tlenu. Na atomie tlenu jest oznaczony partial negative charge (czyli cząstkowy ładunek ujemny), a na atomach wodoru partial positive charge (czyli cząstkowy ładunek dodatni).

The behaviour of water molecules in various states of matter

As we know, opposite (positive and negative) charges attract each other. This is also the case with water molecules – the hydrogen atom of one molecule can interact electrostatically with an oxygen atom of the other molecule. This phenomenon can be clearly observed in water in liquid and solid state. In water in the liquid phase, apart from free molecules, there are also clusters of molecules that form exactly in the result of electrostatic attraction. These clusters are not permanent. Some molecules are released from them, while others join the system. In the solid state, water molecules, due to electrostatic interactions, form relatively stable structures.

The phenomenon of particles (molecules, ions, atoms) interconnecting to form larger systems composed of two or more particles as a result of electrostatic interactions is called associationassociationassociation.

How does the polar structure of water molecules affect the density of this substance?

The polar structure of water molecules has its consequences – the physical properties of water. Usually, the particles in solids are closer to each other than in liquids, and a substance in the solid state has a higher density than in the liquid state. In the case of water, solid phase molecules form structures that leave a lot of free space; this is why the distances between particles are larger in ice than in water. For this reason ice has a lower density than water in the liquid state.

Task 3

Look at the interactive diagram. What is the importance of the ice density, which is low comparing to the density of water in the liquid state, for the aquatic life on Earth?

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Ilustracja przedstawia rysunek: zimowy krajobraz. Widoczny jest ląd pokryty śniegiem oraz staw przykryty lodem, który jest ukazany tak, że widać jego dno oraz pływające w nim ryby i roślinność. Na ilustracji znajdują się informacje o temperaturze panującej na lądzie i w wodzie na różnych jej głębokościach. Temperatura na lądzie wynosi minus 10 stopni Celsjusza. Niedaleko powierzchni wody temperatura wody wynosi zero stopni Celsjusza. Im niżej, tym temperatura jest wyższa: kolejno wynosi jeden stopień, dwa, trzy. Na samym dnie zbiornika temperatura wynosi cztery stopnie Celsjusza. Na ilustracji interaktywnej umieszczone są cztery cyfry, po których kliknięciu pojawiają się następujące informacje: 1. Ice sheet. Ice that forms on the water surface insulates deeper water layers and protects them against freezing. 2. Ice – why doesn’t it sink? Because water reaches the highest density at 4ºC, its further cooling or heating decreases its density. As a result, ice is light and floats on water, while normally other solids always sink in the liquids forming when they melt. 3. Water density and temperature. Water reaches the highest density at 4ºC – it sinks to the bottom and as a result deep water tanks have a temperature close to 4°C. In winter, water at the bottom of the tank always has a temperature of 4°C, allowing the fish to survive this period. 4. Snow protection Snow deposited on ice may create additional thermal insulation for a lake or pond during winter. This protects the aquatic flora and fauna against excessively low temperatures.

Ilustracja przedstawia rysunek: zimowy krajobraz. Widoczny jest ląd pokryty śniegiem oraz staw przykryty lodem, który jest ukazany tak, że widać jego dno oraz pływające w nim ryby i roślinność. Na ilustracji znajdują się informacje o temperaturze panującej na lądzie i w wodzie na różnych jej głębokościach. Temperatura na lądzie wynosi minus 10 stopni Celsjusza. Niedaleko powierzchni wody temperatura wody wynosi zero stopni Celsjusza. Im niżej, tym temperatura jest wyższa: kolejno wynosi jeden stopień, dwa, trzy. Na samym dnie zbiornika temperatura wynosi cztery stopnie Celsjusza. Na ilustracji interaktywnej umieszczone są cztery cyfry, po których kliknięciu pojawiają się następujące informacje: 1. Ice sheet. Ice that forms on the water surface insulates deeper water layers and protects them against freezing. 2. Ice – why doesn’t it sink? Because water reaches the highest density at 4ºC, its further cooling or heating decreases its density. As a result, ice is light and floats on water, while normally other solids always sink in the liquids forming when they melt. 3. Water density and temperature. Water reaches the highest density at 4ºC – it sinks to the bottom and as a result deep water tanks have a temperature close to 4°C. In winter, water at the bottom of the tank always has a temperature of 4°C, allowing the fish to survive this period. 4. Snow protection Snow deposited on ice may create additional thermal insulation for a lake or pond during winter. This protects the aquatic flora and fauna against excessively low temperatures.

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1. Ice sheet Ice that forms on the water surface insulates deeper water layers and protects them against freezing

2. Ice - why doesn’t it sink? Because water reaches the highest density at 4ºC, its further cooling or heating decreases its density. As a result, ice is light and floats on water, while normally other solids always sink in the liquids forming when they melt.

3. Water density and temperature Water reaches the highest density at 4ºC - it sinks to the bottom and as a result deep water tanks have a temperature close to 4°C. In winter, water at the bottom of the tank always has a temperature of 4°C, allowing the fish to survive this period.

4. Snow protection Snow deposited on ice may create additional thermal insulation for a lake or pond during winter. This protects the aquatic flora and fauna against excessively low temperatures

How does the polar structure of water molecules affect the melting and boiling point of this substance?

Understanding the physical properties of water will be easier if we recall a few facts about the states of matter.

When a substance changes its state from solid to liquid and then gaseous, energy must be supplied so that molecules can move away from each other. The more energy has to be supplied, the higher the melting point and the boiling point of the substance.

As ice passes into liquid, water molecules must free themselves from electrostatic interactions and then obtain energy that will allow them to move. Similarly, during vaporization, water molecules must obtain the energy needed to overcome electrostatic forces and necessary to move away from each other over long distances and to move around. Therefore, water has relatively high melting and boiling point. Much higher than substances with a similar structure, with molecules that do not interact with one another, e.g. hydrogen chloride ($\text{HCl}$) or hydrogen sulfide (${\text{H}}_{\text{2}}\text{S}$).

Summary

• Water is a polar substance. There is a partial negative charge on the oxygen atom, while on hydrogen atoms – positive one.

• Water molecules in water in the liquid phase interact with one another: the hydrogen atom of one molecule interacts electrostatically with the oxygen atom of the other molecule, and next to the free molecules there are clusters of molecules that persist due to electrostatic attraction.

• Water molecules in ice form ordered structures in which they are at greater distances from one another than in water in the liquid state. Ice has lower density than water.

• Water dissolves substances that are also polar. It also dissolves most of the ionic compounds.

Keywords

association, dipole, states of matter, polar substance

Glossary