Properties of Water

Properties of Water

Middle School

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Learning Objectives

• Define the structure of a water molecule and explain its polarity.

• Describe the properties of cohesion, adhesion, and surface tension.

• Explain why water is called the universal solvent for dissolving many substances.

• Understand water's high specific heat and its three states of matter.

Key Vocabulary

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Polar Molecule

A polar molecule has an uneven electrical charge, creating separate positive and negative areas.

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Cohesion

Cohesion is the attraction between molecules of the same kind, causing them to stick together.

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Adhesion

Adhesion is the attraction between molecules of different substances, like water sticking to glass.

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Surface Tension

Surface tension measures how hard it is to stretch or break the surface of a liquid.

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Universal Solvent

A universal solvent, like water, is a substance that can dissolve many other substances.

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The Water Molecule and Polarity

• A water molecule has two hydrogen atoms and one oxygen atom (H₂O).

• It is a polar molecule, with a positive charge on the hydrogen side.

• The oxygen side of the molecule has a corresponding negative charge.

• This polarity makes water molecules attract each other, forming hydrogen bonds.

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Multiple Choice

Why is a water molecule considered polar?

• It is made of two hydrogen atoms and one oxygen atom.

• It has an uneven electrical charge, with a positive side and a negative side.

• It has an even electrical charge distributed throughout the molecule.

• It forms strong covalent bonds between its atoms.

Cohesion, Adhesion, and Surface Tension

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Cohesion

• Cohesion is the attraction between molecules of the same substance.

• This property makes water molecules stick tightly together.

• This is why water forms into small, round droplets.

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Adhesion

• Adhesion is the attraction between molecules of different substances.

• It allows water to stick firmly to other surfaces.

• It helps create the curve of a meniscus in tubes.

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Surface Tension

• Surface tension makes it hard to break a liquid's surface.

• It is caused by the strong cohesion between water molecules.

• This force allows some small insects to walk on water.

Multiple Choice

Which property allows an insect to walk on the surface of water?

• Surface Tension

• Adhesion

• Universal Solvent

• Polarity

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Water: The Universal Solvent

• Water is the universal solvent because it dissolves many different types of substances.

• The ability of a substance to dissolve in another is called solubility.

• Water's polarity helps it pull apart other particles, causing them to dissolve.

• This dissolving ability is tasteless, odorless, and essential for most life processes.

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Multiple Choice

What characteristic of water allows it to be called the 'universal solvent'?

• Its ability to exist in three states of matter.

• Its high surface tension.

• Its polarity, which allows it to dissolve many substances.

• It is tasteless and odorless.

The Three States of Water

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Solid (Ice)

• ​Ice is the solid form of water, found all over Earth.

• ​It has a definite, fixed shape and its own constant volume.

• Water becomes solid ice at its melting point, which is 0 °C (32 °F).

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Liquid (Water)

• ​This is the most common state of water we often see.

• ​It takes the shape of the container that it is held in.

• ​Water needs lots of energy to raise its own temperature.

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Gas (Water Vapor)

• ​Water vapor is the invisible gas form of the water.

• ​It will spread out to completely fill any available space.

• Water becomes a gas (water vapor) at its boiling point, which is 100 °C (212 °F).

Multiple Choice

Which statement correctly describes ice as the solid form of water?

• Ice changes shape to fit any container.

• Ice has a definite shape and a constant volume.

• Ice forms only below –10 °C.

• Ice melts at 100 °C.

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Evaporation and Vapor Pressure

• Evaporation is a liquid turning to gas; in plants, it's called transpiration.

• Vapor pressure is the pressure from a liquid's vapor in a closed space.

• The strong hydrogen bonding between water molecules keeps most from escaping, giving water a very low vapor pressure.

• Water's low vapor pressure prevents large water bodies from evaporating too quickly.

• As a liquid's temperature increases, its vapor pressure also increases.

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Multiple Choice

What is the primary reason for water's unusually low vapor pressure?

• The process of transpiration in plants.

• The strong hydrogen bonding between water molecules.

• The high temperature of large bodies of water.

• Its ability to dissolve many substances.

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Common Misconceptions About Water

Misconception	Correction
Water is only a liquid.	Water naturally exists on Earth as a solid, liquid, and gas.
Anything can dissolve in water.	It dissolves many substances, but not all.
Boiling water is a quick process.	Water has a high specific heat and heats up slowly.
Water molecules repel each other.	Water molecules are strongly attracted to each other.

Multiple Choice

How does the polarity of a water molecule contribute to its ability to dissolve a substance like salt?

• The positive and negative areas of water molecules attract and pull apart the salt ions.

• The hydrogen atoms in water bond with each other to trap the salt.

• The water molecule's covalent bonds break the salt apart.

• The oxygen in water reacts chemically with the salt.

Multiple Choice

A water droplet hangs from a leaf before falling. Which two properties are primarily responsible for this phenomenon?

• Cohesion and Adhesion

• Surface Tension and Evaporation

• High Specific Heat and Vapor Pressure

• Solubility and Polarity

Multiple Choice

If a liquid had much weaker bonds between its molecules compared to water, what could you predict about its properties?

• It would have a higher boiling point and higher surface tension.

• It would have a higher specific heat and lower vapor pressure.

• It would be a better universal solvent.

• It would have a lower boiling point and higher vapor pressure.

Multiple Choice

Hydrogen bonds are the connections between water molecules. How do these bonds affect the amount of heat needed to raise water's temperature and to make it boil?

• Significant energy is needed to overcome strong hydrogen bonds, both to increase molecule movement (heat) and to allow them to escape as a gas (boiling).

• Hydrogen bonds cause water to repel heat, requiring more energy to raise its temperature.

• Hydrogen bonds store heat, which is released during boiling, lowering the energy needed.

• Hydrogen bonds are weak, making it easy to change water's temperature and state.

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Summary

• A water molecule (H₂O) is polar, causing hydrogen bonds to form.

• Water molecules stick to each other and to other surfaces.

• Cohesion between water molecules results in high surface tension.

• Due to its polarity, water can dissolve many different substances.

• Water resists temperature changes because of its high specific heat.

• Water is found naturally on Earth as a solid, liquid, and gas.

Poll

On a scale of 1-4, how confident are you about the properties of water covered in today's review?

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