Solutions

Solutions

Middle School

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Learning Objectives

• Define the key terms: solution, solute, and solvent.

• Explain how the process of dissolving works at the particle level.

• Identify examples of solutions on Earth and in living organisms.

• Explain the concept of concentration in solutions.

Key Vocabulary

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Solution

A mixture where one substance, the solute, is completely dissolved in another substance, the solvent.

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Solute

The substance in a solution that gets dissolved by the solvent, such as salt in water.

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Solvent

The substance in a solution that does the dissolving, like water dissolving a cough drop.

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Dissolving

The process where particles of a solute break apart and mix evenly into the solvent particles.

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Mixture

Any combination of two or more materials that are physically put together but not chemically bonded.

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Soluble

A substance that has the ability to dissolve in a solvent, like sugar in water.

Key Vocabulary

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Insoluble

A substance that is unable to dissolve in a particular solvent, such as sand in water.

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Concentration

The measure of the amount of a solute that has been dissolved in a given solvent.

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The Process of Dissolving

• When a solid is in a liquid, it starts to dissolve or break down.

• This is different from melting, which is caused by heat.

• The solid’s particles separate and move into the liquid, spreading out evenly.

• This creates a stable mixture called a solution, like pink-colored water.

Multiple Choice

What happens to a solid when it dissolves in a liquid?

• The solid's particles break down and spread out evenly in the liquid.

• The solid gets hot enough to turn into a liquid.

• The solid sinks to the bottom of the liquid without changing.

• The liquid changes color but the solid remains whole.

Multiple Choice

What is the result of a solid's particles separating and spreading out evenly within a liquid?

• The solid and liquid separate into layers.

• The liquid becomes hotter due to a chemical reaction.

• A stable mixture called a solution is formed.

• The solid melts and disappears completely.

Multiple Choice

If you stir sand into water and the sand settles at the bottom after a minute, why is this not an example of dissolving?

• Because dissolving only happens with colored liquids.

• Because dissolving requires heat, and the water was cold.

• Because dissolving requires particles to spread out evenly, not settle at the bottom.

• Because the sand is a solid and only liquids can dissolve.

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How Does Dissolving Work?

• Particles in a liquid (solvent) are in constant motion, bumping into solids.

• The solid (solute) is held together by attractive forces called bonds.

• Solvent collisions transfer energy, breaking the bonds holding the solute together.

• Freed solute particles mix with the solvent and spread out evenly.

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Multiple Choice

What causes the bonds holding a solid solute together to break during dissolving?

• The solute particles losing their own energy.

• Energy transferred from collisions with solvent particles.

• The attractive forces between solute particles getting stronger.

• The solute particles creating new bonds with each other.

Multiple Choice

Which statement explains the relationship between a liquid solvent and a solid solute at the beginning of the dissolving process?

• The solute particles begin to move around the solvent particles.

• The constant motion of solvent particles causes them to collide with the solute.

• The solute's bonds break apart on their own without any interaction.

• The solvent particles create a new container for the solute.

Multiple Choice

What would likely happen if the attractive forces holding a solid solute together were much weaker than usual?

• The solute would dissolve more quickly because its bonds would require less energy to break.

• The solute would dissolve more slowly because weaker bonds are harder to separate.

• The solute would not dissolve because there are no bonds to break.

• The solute would not spread out evenly after its bonds were broken.

Mixtures and Solutions

• A mixture has parts you can see, but a solution looks like one substance.

• A solution has a solute that dissolves and a solvent that does the dissolving.

• In a solution, solute particles spread out evenly and become invisible.

Multiple Choice

What is the main difference between a mixture and a solution?

• In a solution, the components are not individually visible, while in a mixture they often are.

• A mixture has a solute, while a solution has a solvent.

• Solutions are always liquids, while mixtures can be solids, liquids, or gases.

• Mixtures involve a chemical change, while solutions do not.

Multiple Choice

What is the relationship between a solute and a solvent in a solution?

• The solute is the substance that dissolves, and the solvent is the substance that does the dissolving.

• The solvent is the substance that dissolves, and the solute is the substance that does the dissolving.

• The solute and solvent are both substances that dissolve.

• The solute and solvent are the visible parts of a mixture.

Multiple Choice

When a substance dissolves to form a true solution, what best explains why it seems to disappear?

• The solute particles have settled at the bottom of the solvent.

• The solute particles have chemically transformed into the solvent.

• The solute particles have spread out evenly within the solvent and become too small to see.

• The solvent has caused the solute particles to evaporate from the solution.

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Solutions on Earth

The Ocean

• ​The ocean is Earth's most massive solution, with pure water acting as the solvent.

• ​​The primary solute dissolved in the seawater is sodium chloride, also known as salt.

• ​Thousands of other elements and minerals are also dissolved in the ocean water.

The Atmosphere

• ​The atmosphere is the largest solution by volume, composed of a mixture of gases.

• ​​The most abundant gas, nitrogen, makes up 78% of the air and acts as the solvent.

• ​All other gases, like oxygen and carbon dioxide, are considered solutes in the air.

Multiple Choice

What is the primary solute dissolved in seawater?

• Oxygen

• Sodium chloride

• Carbon dioxide

• Nitrogen

Multiple Choice

How are the solutions of the ocean and the atmosphere similar?

• Both have a solvent that is more abundant than the solutes.

• Both are made primarily of salt and minerals.

• Both have a liquid as the primary solvent.

• Both are composed of the same elements.

Multiple Choice

In a mixture of gases that makes up air, which gas is considered the solvent, and why?

• Oxygen, because it is needed for respiration

• Carbon dioxide, because it participates in photosynthesis

• Nitrogen, because it is present in the largest proportion

• Water vapor, because it can condense into clouds

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Solutions for Life

In Animals

• ​Many essential fluids in your body are solutions with water as the solvent.

• ​​Saliva is a solution containing enzymes, while blood plasma is a solution of proteins.

• ​Whole blood is a mixture because solid cells are suspended in the plasma solution.

In Plants

• ​Plants use solutions to transport nutrients and also to attract helpful pollinators.

• ​​The sweet nectar that attracts insects and birds is a sugar-water solution.

• ​The sap that flows through stems is a solution of sugars and salts.

Multiple Choice

Based on the examples of saliva, blood plasma, and sap, what is the primary role of solutions in living things?

• To transport important substances for life processes

• To create solid structures for physical support

• To provide a source of heat for the body

• To make an organism appear more colorful

Multiple Choice

How do the functions of nectar and sap differ in plants?

• Nectar is used to attract pollinators, while sap is used to transport nutrients.

• Nectar is a solution of proteins, while sap is a solution of enzymes.

• Nectar flows through the stem, while sap is found in flowers.

• Nectar provides energy for the plant, while sap provides water.

Multiple Choice

Why is whole blood classified as a mixture, while the blood plasma it contains is classified as a solution?

• Because it contains solid cells that are suspended in the liquid plasma.

• Because blood plasma is a solution of proteins, not just water.

• Because whole blood is thicker and redder than blood plasma.

• Because it also contains saliva, which has enzymes.

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What Is Concentration?

• Concentration is the amount of solute that is dissolved in a solvent.

• A solution with a high amount of solute is called a concentrated solution.

• A solution with a low amount of solute is called a dilute solution.

• For example, adding water to orange juice concentrate dilutes the solution.

Multiple Choice

What does it mean for a solution to be described as concentrated?

• It contains a high amount of solute dissolved in a solvent.

• It contains a low amount of solute dissolved in a solvent.

• It has had all of the solute removed from the solvent.

• It has an equal amount of solute and solvent.

Multiple Choice

How can you make a solution more dilute?

• By adding more solute to the solution.

• By adding more solvent to the solution.

• By removing the solvent from the solution.

• By making the solution thicker.

Multiple Choice

If a student has a glass of orange juice with a very strong, intense flavor, what would be the most likely result of adding more water?

• The flavor will become more intense.

• The flavor will become less intense.

• The amount of solute will increase.

• The solution will become thicker.

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Common Misconceptions

Misconception	Correction
Dissolving is the same as melting.	Dissolving is breaking down in a liquid. Melting is a solid changing to liquid.
When a solute dissolves, it disappears.	The solute's particles become too small to see and spread out evenly.
All mixtures are solutions.	A solution is a mixture where a solute dissolves completely, like salt in water.
Blood is a pure solution.	Blood is a mixture containing plasma (a solution) and undissolved cells.

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Summary

• A solution is a uniform mixture of a solute and a solvent.

• The concentration of a solution measures the amount of solute in the solvent.

• Solutions can be separated by methods like evaporating the solvent.

• Soluble substances dissolve to form solutions, but insoluble substances do not.

Poll

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