Chemical Reactions

Chemical Reactions

Middle School

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Learning Objectives

• Distinguish between physical and chemical changes by observing properties of substances.

• Use data on properties like density to determine if a chemical reaction occurred.

• Use models to show how atoms are conserved during a chemical reaction.

• Design a device that uses a chemical reaction to release or absorb heat.

Key Vocabulary

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Chemical Reaction

A process that creates new substances with new properties that did not exist before the reaction.

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Reactant

A substance that is present at the start of and undergoes change during a chemical reaction.

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Product

A new substance with different properties that is formed at the end of a chemical reaction.

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Conservation of Mass

This law states that matter cannot be created or destroyed in a closed system chemical reaction.

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Synthetic Material

A material made by humans through chemical reactions that is not found naturally in the environment.

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Polymer

A large molecule that is formed by the chemical joining of many smaller, repeating molecular units.

Key Vocabulary

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Exothermic Reaction

A chemical reaction that releases energy into the surroundings, primarily in the form of heat.

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Endothermic Reaction

A chemical reaction that absorbs energy from its surroundings, making the immediate environment feel colder.

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Solubility

The maximum ability of a substance, the solute, to dissolve in another substance, the solvent.

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Flammability

A measure of how easily a substance will ignite and burn when exposed to flame or heat.

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Density

The measure of how much mass is contained within a specific volume of a given substance.

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Melting Point

The specific temperature at which a solid material will change its state to become a liquid.

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Physical vs. Chemical Changes

Physical Change

• This type of change alters a substance's form or appearance but does not create a new substance.

• Examples include melting, freezing, boiling, bending, crushing, or cutting the substance.

• Even if the substance looks different, its characteristic properties remain the same.

Chemical Change

• This change, also called a chemical reaction, produces one or more new substances with new properties.

• Atoms from the original substances, called reactants, rearrange to form new substances called products.

• Common examples of this type of change are the burning of wood and the rusting of iron.

Multiple Choice

What is the fundamental difference between a physical change and a chemical change?

• Whether the change is reversible or not.

• Whether the substance changes its state of matter.

• Whether or not a new substance is formed.

• Whether the change happens quickly or slowly.

Multiple Choice

Why is boiling a pot of water considered a physical change?

• The water changes its form, but its characteristic properties remain the same.

• The heat causes the water's atoms to rearrange into a new substance.

• Boiling is a chemical reaction that releases a gas.

• The water cannot be changed back to its liquid form.

Multiple Choice

When iron rusts, it combines with oxygen to form a new, reddish-brown substance. What is the best explanation for this process?

• A chemical change occurred, where the original reactants rearranged to form new products.

• A physical change occurred, because the wood simply changed into a different form.

• The change was only physical because the total amount of matter did not change.

• The atoms of the wood were destroyed by the fire, leaving only ash.

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Identifying Reactions with Property Changes

• A chemical reaction forms a new substance with different characteristic properties.

• Key properties to observe are density, melting point, and boiling point.

• Other properties to check for include solubility, flammability, and odor.

• For example, burning sugar creates carbon, with a different taste and density.

Multiple Choice

What is the primary evidence that a chemical reaction has created a new substance?

• By observing that the new substance has different properties.

• By checking if the substance has changed its shape or size.

• By measuring the starting amount of the substance.

• By seeing if the substance is hotter or colder than before.

Multiple Choice

Which of the following is a characteristic property used to determine if a new substance has been formed?

• The volume of the container it is in.

• The temperature of the room.

• The melting point of the substance.

• The time of day the observation is made.

Multiple Choice

A scientist heats a white, odorless powder. After a few minutes, a strong, sweet smell is produced. Based on this observation, what is the most likely conclusion?

• The substance simply changed from a solid to a gas.

• The substance only changed its temperature.

• A chemical reaction occurred because a new substance with a different odor was formed.

• The properties of the substance remained the same.

Energy in Chemical Reactions

• Exothermic reactions release energy into the surroundings as heat or light.

• Endothermic reactions absorb energy from their surroundings, making them feel cold.

• This principle is used to design devices like hand warmers and cold packs.

Multiple Choice

What is the primary difference between an exothermic and an endothermic reaction?

• Exothermic reactions release energy, while endothermic reactions absorb energy.

• Exothermic reactions get colder, while endothermic reactions get hotter.

• Exothermic reactions only happen in hand warmers, while endothermic reactions only happen in cold packs.

• Exothermic reactions create light, while endothermic reactions create heat.

Multiple Choice

Why does an endothermic reaction make its surroundings feel cold?

• It absorbs energy from the surroundings, making them feel cooler.

• It releases cold into the surroundings, lowering the temperature.

• It stops the movement of molecules in the surroundings completely.

• It converts the surrounding material into a colder substance.

Multiple Choice

A chemical cold pack is used to reduce swelling by making the area feel cold. What type of reaction is most likely occurring inside the cold pack?

• An exothermic reaction, because it is designed to release heat.

• An endothermic reaction, because it absorbs heat from its surroundings.

• An exothermic reaction, because it consumes all the oxygen around it.

• An endothermic reaction, because it must be activated to start working.

Modeling How Atoms Rearrange

• Atoms in reactants rearrange themselves to form new products.

• This creates new substances with different properties.

• For example, C and O₂ regroup to form CO₂.

Multiple Choice

What happens to atoms in reactants during a chemical reaction?

• They are destroyed and no longer exist.

• They remain in their original form.

• They are replaced by entirely new atoms.

• They rearrange to form new substances.

Multiple Choice

What is the direct result of atoms rearranging during a chemical reaction?

• The original substances keep their properties.

• The total number of atoms decreases.

• The atoms change their size and shape.

• New substances with different properties are formed.

Multiple Choice

When carbon (C) and oxygen (O₂) react to form carbon dioxide (CO₂), what conclusion can be drawn about the atoms involved?

• The CO₂ is made of completely new atoms not present before.

• The properties of CO₂ are identical to the properties of C and O₂.

• Only the oxygen atoms are used to form the new substance.

• The CO₂ is made of the same carbon and oxygen atoms from the reactants.

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The Law of Conservation of Mass

Closed System

• In a closed system, matter cannot enter or escape, like in a sealed container for experiments.

• The total mass of substances before a reaction is equal to the total mass of substances afterwards.

• Because atoms are only rearranged and not lost, the total mass stays exactly the same.

Open System

• In an open system, matter can enter or escape, such as smoke from a campfire.

• The mass might appear to change because some products, like gases, can leave the system.

• However, mass is still conserved, but it is just not all contained in one place anymore.

Multiple Choice

Which statement best describes the Law of Conservation of Mass?

• The total mass of substances before a reaction equals the total mass after the reaction.

• The total mass of substances can only increase during a reaction.

• The total mass of substances always decreases in a closed container.

• The total mass of substances changes depending on the type of atoms.

Multiple Choice

In an open system, why might the mass seem to decrease during a reaction?

• The reaction destroys some of the atoms.

• Matter, such as a gas, can escape into the surroundings.

• The container adds mass to the substances.

• Energy is converted into mass.

Multiple Choice

A scientist mixes two chemicals in a sealed jar. The total mass of the jar and its contents is 50 grams. After a reaction occurs and a gas is produced, what will the total mass be?

• The total mass will be less than 50 grams because a gas was formed.

• The total mass will be more than 50 grams because the reaction creates matter.

• The total mass will be exactly 50 grams because atoms are only rearranged, not lost.

• The total mass cannot be known without knowing what substances were used.

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Synthetic Materials from Natural Resources

• Synthetic materials are man-made from natural resources through chemical reactions.

• Chemistry helps create new materials like medicines, foods, and fuels.

• For example, crude oil is used to create plastics and nylon.

• Another example is using sand, a natural resource, to make glass.

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Multiple Choice

What is the definition of a synthetic material?

• They are man-made materials created from natural resources.

• They are materials that occur naturally in the environment.

• They are resources like crude oil and sand in their original form.

• They are materials that cannot be changed by chemical reactions.

Multiple Choice

What is the relationship between a natural resource, like sand, and a synthetic material, like glass?

• A natural resource is chemically changed to form a new, man-made product.

• The synthetic material is found naturally within the natural resource.

• The natural resource and the synthetic product are the same thing.

• The synthetic product is used to help extract the natural resource.

Multiple Choice

Based on the information, what is a reasonable conclusion about the creation of new materials?

• That new medicines and fuels are likely created by chemically changing natural resources.

• That plastics and glass are the only synthetic materials that can be made.

• That chemistry can only be used to create materials like foods and fuels.

• That all natural resources will eventually turn into synthetic materials on their own.

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What Are Polymers?

• Polymers are large molecules made of smaller, repeating units called monomers.

• These monomers link together in long chains, much like beads on a string.

• They can be natural like wool, or synthetic like plastic made from petroleum.

• This chain structure gives polymers useful properties like strength and flexibility.

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Multiple Choice

What is the definition of a polymer?

• A large molecule made of smaller, repeating units.

• A type of synthetic plastic made only from petroleum.

• A natural material, such as wool or cotton.

• A single, small molecule that cannot be linked.

Multiple Choice

What explains why polymers have useful properties like strength and flexibility?

• Their structure consists of long chains of linked monomers.

• They are always made from natural materials like wool.

• They are always synthetic and derived from petroleum.

• Their monomers are very large and heavy molecules.

Multiple Choice

A scientist is developing a new synthetic material, but the monomers are failing to link into long chains. What is the most likely outcome for this material's properties?

• The material would likely become weak and brittle.

• The material would become a natural polymer like wool.

• The monomers would transform into petroleum.

• The material's properties would not change at all.

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Impact of Synthetic Materials on Society

Benefits

• Synthetic medicines can be mass-produced, making them widely available to treat many different diseases.

• Fibers like nylon are strong and often cheaper than natural options like cotton or silk.

• Special preservatives are used to extend the shelf life of food, which helps in reducing overall waste.

Drawbacks

• Many common plastics are not biodegradable and can pile up in landfills, leading to long-term pollution.

• Some synthetic additives found in food and other products have been linked to negative health problems.

• The process of getting raw materials from the Earth can cause significant damage to the environment.

Multiple Choice

Which statement best summarizes the overall impact of synthetic materials on society?

• They provide significant benefits but also come with serious disadvantages.

• They are primarily a source of pollution and offer few real advantages.

• They are always cheaper and safer than natural materials.

• Their main purpose is to replace all natural resources on Earth.

Multiple Choice

What is a primary reason synthetic materials like medicines and fibers are considered beneficial?

• They can be mass-produced, making them widely available and often cheaper.

• They are always biodegradable and help protect the environment.

• They are known to improve human health with no negative side effects.

• They help to preserve natural resources by not using them.

Multiple Choice

A company creates a new food additive that prevents spoilage. However, its production harms the environment and some studies suggest it may have health risks. Which statement best evaluates this situation?

• The benefit of reducing food waste must be weighed against potential harm to human health and the environment.

• The only important factor is that the food will last longer on shelves.

• The environmental damage is acceptable if the food additive is cheap to produce.

• The additive should be considered a total success because it reduces landfill waste from plastics.

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Common Misconceptions

Misconception	Correction
A substance disappearing, like dissolving, is a chemical change.	Dissolving is a physical change; the original molecules are still present.
Changing shape or state creates a new substance.	Physical changes do not alter a substance's characteristic properties.
Mass is lost in reactions that produce gas.	Mass is always conserved; gas just escapes in an open container.
'Synthetic' automatically means it is bad or harmful.	Synthetic materials include beneficial products like medicines.
All chemical reactions are fast and dramatic.	Many reactions, like the rusting of iron, are slow chemical changes.

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Summary

• Chemical changes create new substances, while physical changes only alter form.

• Property changes like color or flammability signal a chemical reaction.

• Mass is always conserved in reactions because atoms are only rearranged.

• Synthetic materials are created from natural resources using chemical reactions.

Poll

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

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