​Chemical Changes, Reaction, and Formulas

​Chemical Changes, Reaction, and Formulas

By Victor Castillo

Multiple Choice

Baking soda and vinegar combine to make carbon dioxide

• Chemical Change
• Physical Change

Chemical Change

Middle School

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Learning Objectives

• Define and differentiate between physical and chemical changes.

• Analyze density, melting point, and flammability data to spot chemical reactions.

• Describe how reactant atoms rearrange to create new substances with new properties.

• Identify signs of chemical reactions, like color change, gas bubbles, or heat.

Key Vocabulary

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Physical Change

This is a change in the form or appearance of a substance without creating a new one.

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Chemical Change

A chemical change is a process that results in the formation of new chemical substances.

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Reactants

Reactants are the starting materials or substances that undergo a change during a chemical reaction.

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Products

Products are the new substances that are formed as a result of a chemical reaction.

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Flammability

Flammability describes the ability of a particular substance to burn or ignite, causing fire or combustion.

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Precipitate

A precipitate is a solid substance that forms from a solution during a chemical reaction.

Key Vocabulary

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Density

Density describes the amount of mass packed into a specific, certain, or given amount of volume.

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Melting Point

The melting point is the specific temperature at which a solid substance starts to transform into a liquid.

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Boiling Point

The boiling point is the exact temperature at which a liquid substance begins to turn into a gas.

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Solubility

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

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Odor

Odor is the distinctive or characteristic smell that is given off by a particular chemical substance you encounter.

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Atom

An atom is the smallest and most basic building block of a chemical element, like hydrogen or oxygen.

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Physical vs. Chemical Changes

Physical Change

• ​This change alters a substance's appearance, but does not create a new substance.

• ​​Examples include cutting paper, melting ice, or bending a metal spoon.

• ​Its characteristic properties, like boiling point or density, remain the same.

Chemical Change

• ​This type of change, also known as a chemical reaction, produces new substances.

• ​​The original substances are called reactants, and the new ones are called products.

• ​Examples include burning a piece of wood or an iron nail rusting.

Multiple Choice

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

• A physical change creates a new substance, while a chemical change does not.

• A chemical change creates a new substance, while a physical change only alters a substance's appearance.

• A physical change involves reactants and products, while a chemical change does not.

• A chemical change alters a substance's mass, while a physical change alters its density.

Multiple Choice

During a physical change, what happens to a substance's characteristic properties, such as its boiling point?

• The properties change to match the new substance formed.

• The properties, like boiling point, remain the same.

• The properties become reactants in a chemical reaction.

• The properties are lost during the change.

Multiple Choice

When wood is burned, it turns into ash and smoke. Why is this considered a chemical change?

• Because the wood has changed its appearance.

• Because the change involves a solid turning into a gas.

• Because the wood and oxygen are reactants that form new products like ash and smoke.

• Because the process can be easily reversed.

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Characteristic Properties of Substances

• Every pure substance has unique properties used for its identification.

• Key examples include density, melting point, boiling point, and solubility.

• Other properties are flammability (the ability to burn) and odor.

• Comparing properties tells us if a new substance has formed.

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Multiple Choice

What is the primary purpose of using a substance's characteristic properties?

• To change the color of a substance.

• To measure the mass of a substance.

• To help identify a pure substance.

• To determine the volume of a substance.

Multiple Choice

Which of the following is a characteristic property that can be used to identify a substance?

• Temperature

• Volume

• Flammability

• Mass

Multiple Choice

A scientist measures the boiling point of a liquid as 100°C. After a chemical reaction occurs, the boiling point of the new liquid is 115°C. What is the best conclusion the scientist can draw?

• The substance only changed its state from liquid to gas.

• A new substance was formed because a characteristic property changed.

• The amount of the substance decreased during the experiment.

• The two substances did not actually mix together.

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Atoms in Chemical Reactions

• A chemical reaction rearranges atoms; it does not create or destroy them.

• ​Atoms in reactants break their bonds and form new combinations.

• These new combinations of atoms are called the products.

• Products have different properties, such as a new odor or density.

Multiple Choice

What happens to atoms during a chemical reaction?

• They are destroyed completely.

• They are created from nothing.

• They are rearranged into new substances.

• They are converted into pure energy.

Multiple Choice

What is the relationship between reactants and products in a chemical reaction?

• Reactants are the new substances formed, and products are the starting materials.

• Reactants and products are both destroyed during the reaction.

• Reactants and products are two names for the same substances.

• Reactants are the starting materials, and products are the new substances formed.

Multiple Choice

If a chemical reaction begins with two clear, odorless liquids and results in a yellow solid, what can be concluded?

• The atoms of the reactants were destroyed.

• A new substance with different properties was formed.

• The reaction created entirely new atoms.

• The properties of the reactants and products are identical.

Evidence of a Chemical Reaction

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Change in Color

• A color change can indicate that a new substance has been formed.

• For example, bread turns brown in a toaster due to chemical reactions.

• The heat causes new compounds to form, changing the bread's color.

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Production of a Gas

• Bubbles forming may signal that a new gas is being made.

• This can happen when solids or liquids react with each other.

• Vinegar and baking soda create bubbles of CO₂ gas.

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Formation of a Solid

• A precipitate is a solid that forms from a liquid solution.

• This can happen when two different liquids are mixed together.

• The new solid is evidence a chemical reaction has occurred.

Multiple Choice

What is the most likely reason for a change in color, the appearance of bubbles, or the formation of a solid in a substance?

• A chemical reaction has occurred

• The substance has gotten hotter

• The substance has dissolved in water

• The substance has changed its state

Multiple Choice

How does the formation of bubbles in a liquid, like when vinegar is added to baking soda, serve as evidence of a chemical reaction?

• It shows that a new gas is being produced

• It means the liquid is starting to boil

• It indicates that a solid is dissolving

• It proves the liquid is getting colder

Multiple Choice

If you mix two clear liquids and the mixture becomes cloudy because a solid substance forms and settles at the bottom, what is the best conclusion you can draw?

• A new solid, called a precipitate, has formed from a chemical reaction

• The two liquids were not able to mix together properly

• One of the liquids has evaporated, leaving a solid behind

• The temperature change caused a solid to form

Energy in Chemical Reactions

• Some reactions release energy as heat or light, making surroundings warmer.

• Some reactions absorb energy from surroundings, making them feel colder.

• Energy moves from chemicals to surroundings, or from surroundings into chemicals.

Multiple Choice

What is the primary role of energy in a chemical reaction?

• It is either released or absorbed.

• It is always destroyed.

• It is always created from nothing.

• It remains unchanged within the chemicals.

Multiple Choice

What happens to the surroundings when a chemical reaction releases energy?

• The surroundings become warmer.

• The surroundings become colder.

• The temperature of the surroundings does not change.

• The reaction stops producing light.

Multiple Choice

If a chemical reaction in a beaker makes the beaker feel cold, what is the best explanation for what is happening?

• The reaction is absorbing energy from its surroundings.

• The reaction is releasing energy into its surroundings.

• The chemicals are creating their own energy.

• The temperature has no connection to the energy of the reaction.

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Common Misconceptions

Misconception	Correction
Bubbles always mean a chemical reaction is happening.	Bubbles can also form from physical changes, like opening a soda can.
When a substance disappears by dissolving, it's a chemical change.	Dissolving is a physical change. The substance is still present, just mixed in.
All changes involving heat are chemical changes.	Physical changes like melting ice also require heat. No new substance is formed.

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Summary

• A physical change alters form; a chemical change creates new substances.

• Characteristic properties like density and flammability are used to identify substances.

• In a reaction, atoms rearrange to form new products with new properties.

• Evidence of a reaction includes color change, gas production, or energy changes.

Multiple Choice

A different chemical substance is formed when:

• a cloth is cut

• a cup breaks

• a nail rusts

• a piece of wood breaks

Multiple Choice

Which object would be most likely to rust?

• a plastic comb

• a glass marble

• a paper clip

• an iron nail

Multiple Choice

Which would most likely cause a substance to melt?

• addition of heat

• removal of heat

• increase in mass

• decrease in mass

Multiple Choice

While completing a science experiment, James notices that he has created a new substance. What type of change took place?

• chemical change

• physical change

• no change

• increase in mass

Multiple Choice

Which of these is a chemical property?

• ability to be broken

• ability to react with acid

• ability to melt when heated

• ability to be cut

Multiple Choice

Which of these describes a physical change in matter?

• milk going sour

• a nail rusting

• a carton of juice freezing

• cooking an egg

Multiple Choice

Which of these describes a chemical change?

• water evaporates

• butter melts on warm toast

• aluminum foil is cut in half

• a paper being burned

Multiple Choice

Determine whether the image depicts a physical change or a chemical change:

• chemical change

• physical change

Multiple Choice

Is this a chemical change or a physical change?

• chemical change

• physical change

Multiple Choice

Is this a chemical change or a physical change?

• chemical change

• physical change

Multiple Choice

Dissolving sugar in your coffee is a ____ change.

• chemical change

• physical change

Multiple Select

Choose the statement which best demonstrates a chemical change. (There are 2 correct answers.)

• iron rusts

• sodium hydroxide dissolves in water

• a safety match ignites and burns

• a cube of ice melts to form a puddle of water

Multiple Select

Choose the statement which best demonstrates a physical change. (There are 2 correct answers)

• sodium hydroxide dissolves in water

• a chocolate bar melts in the sun

• pancakes are cooking on the griddle

• wood and leaves rot to form humus

Multiple Choice

When water changes from a gas to a liquid, this is called?

• evaporation

• freezing

• condensation

• melting

Multiple Choice

At daybreak, a wet field is warmed by the sun. What phase change is occurring in this photo?

• evaporation

• freezing

• condensation

• melting

Multiple Choice

The left side of the equation below is called

• Products
• Reactants

Multiple Choice

The right side of the equation below is called the

• products
• reactants

Multiple Choice

Which observation(s) might be used as evidence that a chemical reaction occurred?

• Color change
• Odor change
• Properties change
• All of the above

Multiple Choice

________ result(s) in no new substances.

• A physical change
• A chemical change
• Physical and chemical changes
• None of the above

Multiple Choice

Which is an example of a physical change?

• Burning wood
• Freezing water
• Rust on a car
• Cooking food

Multiple Choice

Which is an example of a chemical change?

• Burning wood
• Rust on a car
• Cooking food
• All of the above

Multiple Choice

Evidence of a chemical reactions is anything that shows - 

• a new substance has formed
• a solid dissolved in a liquid
• interaction between 2 gases
• a change in the state of matter

Multiple Choice

A precipitate is a 

• liquid that forms when two solids are reacted
• a solid that forms when two liquids are mixed
• a gas that forms when vinegar and baking soda react
• a change of color in a solution

Multiple Choice

If reaction starts with 20g of reactants it should produce 

• a total of 40g of products
• a total of 10g of products
• a total of 80 g of products
• a total of 20g of products

Multiple Choice

The substances at the beginning of a chemical equation are called the ____

• product
• yield
• chemical symbol
• reactants

Multiple Choice

In a chemical reaction, everything to the right of the arrow is called ______________.

• Products
• Yields
• Molecules
• Reactants

Multiple Choice

In the equation,          2Mg + O₂ ---> 2MgO, which are the reactants?

• Mg and O
• MgO
• Mg and MgO
• O and MgO

Multiple Choice

An gel ice pack getting cold after you put it in the freezer for an hour.

• Chemical Change
• Physical Change

Multiple Choice

Rotting Fruit (Like a brown banana)

• Physical Change

• Chemical Change

Multiple Choice

Burning Wood

• Physical Change

• Chemical Change

Multiple Choice

Toasting Bread

• Physical Change

• Chemical Change

Multiple Choice

What kind of change occurs if matter changes in amount, size, or shape?

• substance change

• chemical change

• physical change

• elemental change

Multiple Choice

Glass bottle that is shattered

• Chemical Change
• Physical Change

Chemical Formulas

Middle School

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Learning Objectives

• Define what chemical symbols and chemical formulas are and what they represent.

• Identify elements and their proportions in a compound using its chemical formula.

• Explain the roles of subscripts and coefficients in a chemical formula.

• Count the total number of atoms for each element in a chemical formula.

Key Vocabulary

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Chemical Symbol

A one- or two-letter designation for a chemical element, with the first letter always capitalized.

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Chemical Formula

An expression showing the elements in a compound and the relative proportions of those elements.

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Compound

A substance formed from the chemical combination of two or more different elements.

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Subscript

A number written to the lower right of a symbol showing the atom count for that element.

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Coefficient

A number placed in front of a chemical formula that multiplies every atom in the formula.

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Chemical Symbols and Formulas

Chemical Symbols

• A chemical symbol is a shorthand used to represent a specific element, like O for oxygen.

• The first letter is always capitalized, while the second letter, if present, is always lowercase.

• Some symbols are from English names (Zn for zinc), while others are from Latin (Fe for iron).

Chemical Formulas

• A chemical formula represents a compound by showing the elements and the number of atoms of each.

• For example, the formula for water is H₂O, indicating it has two hydrogen atoms and one oxygen atom.

• The formula H₂SO₄ shows that sulfuric acid contains hydrogen, sulfur, and oxygen atoms.

Multiple Choice

What is a chemical formula?

• An expression that shows the elements in a compound and their relative proportions.

• A one- or two-letter designation of an element.

• A symbol for an element that is derived from its Latin name.

• The name of a compound based on the elements it contains.

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Reading Formulas: What Are Subscripts?

• Chemical formulas use symbols and subscripts to show a compound's elements.

• A subscript shows the number of atoms for a specific element.

• For example, in H₂O, the ₂ means two hydrogen atoms.

• No subscript means one atom, like the S in H₂SO₄.

Multiple Choice

In the chemical formula C₁₂H₂₂O₁₁, how many atoms of hydrogen (H) are present?

• 22

• 12

• 11

• 1

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Coefficients and Parentheses

Coefficients

• A coefficient is a number in front of a chemical formula showing how many molecules there are.

• For example, in 3Na₂SO₄, the coefficient 3 multiplies every atom in the formula.

• This gives you 6 sodium atoms, 3 sulfur atoms, and 12 oxygen atoms.

Parentheses

• A subscript outside parentheses multiplies all the elements inside the parentheses.

• In Ca₃(PO₄)₂, the subscript 2 multiplies the phosphorus and oxygen atoms.

• This means the formula has 2 phosphorus atoms and 8 oxygen atoms.

Multiple Choice

What does the coefficient '2' in the expression '2H₂O' signify?

• There are two molecules of water.

• There are two atoms of hydrogen.

• There are two atoms of oxygen.

• The molecule is twice as large.

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Common Misconceptions

Misconception	Correction
'Co' and 'CO' are the same thing.	'Co' is the symbol for cobalt; 'CO' is the formula for carbon monoxide.
A subscript and a coefficient do the same job.	A subscript (e.g., H
No number by an element means there are zero atoms.	No subscript after a symbol means there is one atom of that element.

Multiple Choice

Which of the following is the correct chemical formula for a molecule containing three hydrogen (H) atoms, one sulfur (S) atom, and four oxygen (O) atoms?

• H₃SO₄

• HSO₄

• 3HSO₄

• H₃S₁O₄

Multiple Choice

How is the element 'iron' represented by its chemical symbol, and what is the origin of this symbol?

• Fe, from its Latin name 'ferrum'

• Ir, from its English name 'iron'

• I, from its English name 'iron'

• Fe, from its Greek name 'ferros'

Multiple Choice

Calculate the total number of atoms in the compound represented by the formula Pb(NO₃)₂.

• 9

• 6

• 7

• 5

Multiple Choice

Analyze the expression 4CaCO₃. What is the total number of oxygen atoms present?

• 12

• 7

• 4

• 3

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Summary

• Chemical symbols are one or two-letter codes for elements.

• Chemical formulas show the elements and their proportions in a compound.

• Subscripts indicate the number of atoms; no subscript means one atom.

• Coefficients multiply all atoms in a formula; parentheses group atoms.

Multiple Choice

The _______ describes the atoms in a compound.

• chemical equation
• chemical formula
• chemical symbol
• coefficient

Multiple Choice

How many elements are in C₆H₁₂O₆?

• 1
• 2
• 3
• 4

Multiple Choice

How many atoms of carbon (C) are in C₆H₁₂O₆?

• 3
• 6
• 12
• 24

Multiple Choice

Element or compound?
H₂O

• element
• compound

Multiple Choice

Which of the following is a compound?

• Co
• O₂
• C
• CO₂

Multiple Choice

Which is the correct formula for:
 three hydrogen (H)
one sulfur (S)
four oxygen (O)

• H₃SO₄
• HSO₄
• H₄S₃O
• H₂O

Multiple Choice

The blue numbers in the image below represent ________

• I don't know, and don't want to try
• coefficients
• subscripts
• none of the answers are correct

Multiple Choice

How many atoms are there TOTAL in:
H₂SO₄

• 6
• 5
• 7
• 3

Multiple Choice

How many Sodium (Na) are in 6NaCl?

• 1
• 12
• 6

Multiple Choice

The substances at the beginning of a chemical equation are called the ____

• product
• yield
• chemical symbol
• reactants

Multiple Choice

In this image, the symbols to the right of the arrow (in black) are called the_______.

• Products
• Reactants
• Subscripts
• Coefficients

Multiple Choice

Na + MgF₂ -->  NaF + Mg
What are the reactants?

• Na + MgF₂
• Na
• NaF + Mg
• NaF

Multiple Choice

NH₃ ---> N₂ + H₂
What are the products?

• N₂ + H₂
• N₂
• H₂
• NH₃

Multiple Choice

All of the following are chemical changes except

• iron rusting
• water evaporating
• wood burning
• food rotting

Multiple Choice

How many hydrogen atoms are there?

• 1

• 4

• 5

• 6

Multiple Choice

How many nitrogen atoms are there?

• 4

• 8

• 10

• 20

Multiple Choice

How many phosphorus atoms are there?

• 0

• 1

• 3

• 4

Multiple Choice

How many molecules of methane are there?

• 0

• 1

• 2

• 4

Multiple Choice

How many atoms of carbon are there?

• 0

• 1

• 2

• 3

Multiple Choice

CH₄ is an example of a —

• chemical equation

• chemical formula

• chemical reaction

• chemical symbol