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4.4 Physical and Chemical Changes

3 min readjanuary 8, 2023

Hope Arnett

Hope Arnett

Hope Arnett

Hope Arnett

Review of Chemical Equations

show the products that the combination of reactants yields, but the equation doesn’t show how the reactants form those products. What we don’t see are the underlying chemical and that allow molecules to rearrange to change properties or create new substances. All it does is represent a chemical reaction!

Chemical Changes

Generally, involve intramolecular (literally meaning “inside molecule”) bonds. This includes breaking and/or forming ionic or between elements during a chemical reaction. Let’s look at a chemical reaction using Lewis Dot Diagrams to visualize it.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-NdJlpeJH6yhR.png?alt=media&token=014adacc-f56f-4d2a-97f7-6d4f34fb5044

The above shows an interaction between two magnesium atoms and an oxygen molecule. The bonds that are broken are red, and the bonds formed are green. Here, we can see that the oxygen molecule's had to break, and then new formed between the magnesium and oxygen to form two molecules of MgO.

Note that are not represented like with lines as shown in the above diagram. They are rather expressed with brackets and charges around the brackets to show the transfer of electrons.

Remember that are usually accompanied by a chemical reaction to show this breaking and forming of bonds. Examples of include:

  • Burning: The chemical reaction that occurs when a substance reacts with oxygen to produce heat and light. Think about !

  • : The chemical reaction that occurs when iron reacts with water and oxygen to form iron oxide (rust).

  • : The chemical reactions that occur in the body to break down food into nutrients that can be absorbed and used by cells.

Physical Changes

are usually intermolecular changes (literally meaning “between molecules”), such as . Some examples are freezing water and cutting paper.

The ice’s molecules maintain the same atomic structure (H2O), but more between each water molecule are formed. Conversely, the paper’s molecules keep the same atomic structure, but the interaction between the paper molecules is altered, or broken.

Since are only a transformation that changes a substance's properties without undergoing a chemical change, they are reversible.

However, some reactions can go either way and sometimes physical processes involve the breaking of chemical bonds. For example, the reaction between salt and water, or the dissolution of salt in water, involves breaking the bonds between ions, but it also creates between the water and ions.

Review Activity

Name whether each scenario describes a chemical or physical change.

  1. Burning a match🔥

  2. Iron over time🚲

  3. Mixing two powders🥣

Answers

  1. Burning a match is a chemical change

  2. Iron is a chemical change

  3. Mixing two powders is a physical change

🎥 Watch AP Chemistry teacher Wes Winter discuss physical and chemical changes, as well as introduce chemical reactions.

Key Terms to Review (15)

Chemical Changes

: Chemical changes involve altering the identity or composition of matter at its molecular level. This results in new substances being formed with different properties than those of original substances.

Chemical Equations

: A chemical equation is a written representation of the process that occurs in a chemical reaction. It shows the reactants (substances that are combined and changed during the reaction) on one side, and the products (the substances that are produced by the reaction) on the other.

Combustion Reactions

: A combustion reaction is an exothermic chemical process where fuel reacts with an oxidant (usually oxygen) producing heat and often light; typically resulting in fire.

Covalent Bonds

: A covalent bond is a type of chemical bond where two atoms share one or more pairs of electrons, resulting in a stable balance of attractive and repulsive forces.

Digestion

: In the context of chemistry, digestion refers to the process by which a substance is broken down into simpler substances or elements.

Hydrogen bonds

: A type of attractive intermolecular force that occurs when a hydrogen atom bonded to a highly electronegative atom exists in the vicinity of another electronegative atom with a lone pair of electrons.

Intermolecular Changes

: Intermolecular changes involve alterations between molecules without changing their internal structure or composition.

Intramolecular Bonds

: Intramolecular bonds are forces that hold atoms together within molecules. They include covalent bonds, ionic bonds, and metallic bonds.

Ion-Dipole Interactions

: Ion-dipole interactions occur when an ion interacts with a polar molecule. The positive end of the polar molecule is attracted to the negative ion and vice versa.

Ionic Bonds

: Ionic bonds are chemical bonds formed through the electrostatic attraction between oppositely charged ions (cations and anions).

Lewis Dot Diagrams

: Lewis dot diagrams are visual representations used in chemistry to show how valence electrons are arranged among atoms in a molecule.

Phase Changes

: Phase changes refer to the transition of matter from one state (solid, liquid, gas) to another. These changes occur when heat (energy) is added or removed.

Physical Changes

: Physical changes refer to changes affecting the form of a chemical substance, but not its chemical composition.

Rusting

: Rusting is an oxidation process where iron reacts with water and oxygen over time, forming rust.

Synthesis Reaction

: A synthesis reaction is a type of chemical reaction in which two or more simple substances combine to form a more complex product.

4.4 Physical and Chemical Changes

3 min readjanuary 8, 2023

Hope Arnett

Hope Arnett

Hope Arnett

Hope Arnett

Review of Chemical Equations

show the products that the combination of reactants yields, but the equation doesn’t show how the reactants form those products. What we don’t see are the underlying chemical and that allow molecules to rearrange to change properties or create new substances. All it does is represent a chemical reaction!

Chemical Changes

Generally, involve intramolecular (literally meaning “inside molecule”) bonds. This includes breaking and/or forming ionic or between elements during a chemical reaction. Let’s look at a chemical reaction using Lewis Dot Diagrams to visualize it.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-NdJlpeJH6yhR.png?alt=media&token=014adacc-f56f-4d2a-97f7-6d4f34fb5044

The above shows an interaction between two magnesium atoms and an oxygen molecule. The bonds that are broken are red, and the bonds formed are green. Here, we can see that the oxygen molecule's had to break, and then new formed between the magnesium and oxygen to form two molecules of MgO.

Note that are not represented like with lines as shown in the above diagram. They are rather expressed with brackets and charges around the brackets to show the transfer of electrons.

Remember that are usually accompanied by a chemical reaction to show this breaking and forming of bonds. Examples of include:

  • Burning: The chemical reaction that occurs when a substance reacts with oxygen to produce heat and light. Think about !

  • : The chemical reaction that occurs when iron reacts with water and oxygen to form iron oxide (rust).

  • : The chemical reactions that occur in the body to break down food into nutrients that can be absorbed and used by cells.

Physical Changes

are usually intermolecular changes (literally meaning “between molecules”), such as . Some examples are freezing water and cutting paper.

The ice’s molecules maintain the same atomic structure (H2O), but more between each water molecule are formed. Conversely, the paper’s molecules keep the same atomic structure, but the interaction between the paper molecules is altered, or broken.

Since are only a transformation that changes a substance's properties without undergoing a chemical change, they are reversible.

However, some reactions can go either way and sometimes physical processes involve the breaking of chemical bonds. For example, the reaction between salt and water, or the dissolution of salt in water, involves breaking the bonds between ions, but it also creates between the water and ions.

Review Activity

Name whether each scenario describes a chemical or physical change.

  1. Burning a match🔥

  2. Iron over time🚲

  3. Mixing two powders🥣

Answers

  1. Burning a match is a chemical change

  2. Iron is a chemical change

  3. Mixing two powders is a physical change

🎥 Watch AP Chemistry teacher Wes Winter discuss physical and chemical changes, as well as introduce chemical reactions.

Key Terms to Review (15)

Chemical Changes

: Chemical changes involve altering the identity or composition of matter at its molecular level. This results in new substances being formed with different properties than those of original substances.

Chemical Equations

: A chemical equation is a written representation of the process that occurs in a chemical reaction. It shows the reactants (substances that are combined and changed during the reaction) on one side, and the products (the substances that are produced by the reaction) on the other.

Combustion Reactions

: A combustion reaction is an exothermic chemical process where fuel reacts with an oxidant (usually oxygen) producing heat and often light; typically resulting in fire.

Covalent Bonds

: A covalent bond is a type of chemical bond where two atoms share one or more pairs of electrons, resulting in a stable balance of attractive and repulsive forces.

Digestion

: In the context of chemistry, digestion refers to the process by which a substance is broken down into simpler substances or elements.

Hydrogen bonds

: A type of attractive intermolecular force that occurs when a hydrogen atom bonded to a highly electronegative atom exists in the vicinity of another electronegative atom with a lone pair of electrons.

Intermolecular Changes

: Intermolecular changes involve alterations between molecules without changing their internal structure or composition.

Intramolecular Bonds

: Intramolecular bonds are forces that hold atoms together within molecules. They include covalent bonds, ionic bonds, and metallic bonds.

Ion-Dipole Interactions

: Ion-dipole interactions occur when an ion interacts with a polar molecule. The positive end of the polar molecule is attracted to the negative ion and vice versa.

Ionic Bonds

: Ionic bonds are chemical bonds formed through the electrostatic attraction between oppositely charged ions (cations and anions).

Lewis Dot Diagrams

: Lewis dot diagrams are visual representations used in chemistry to show how valence electrons are arranged among atoms in a molecule.

Phase Changes

: Phase changes refer to the transition of matter from one state (solid, liquid, gas) to another. These changes occur when heat (energy) is added or removed.

Physical Changes

: Physical changes refer to changes affecting the form of a chemical substance, but not its chemical composition.

Rusting

: Rusting is an oxidation process where iron reacts with water and oxygen over time, forming rust.

Synthesis Reaction

: A synthesis reaction is a type of chemical reaction in which two or more simple substances combine to form a more complex product.


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© 2024 Fiveable Inc. All rights reserved.

AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.