Physical and Chemical Changes

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TABLE OF CONTENTS

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The Law of matter conservation states that matter cannot be created or destroyed, only changes its form. It can go through either a physical or chemical change. In this article, we will learn what these changes, what they look like, and how they help us identify substances

This article is about physical and chemical changes
Firstly, we will define physical and chemical properties
We will look at examples of these properties and how we can utilize them to identify unknown substances
Then we will dive into the definition of physical and chemical changes
We'll identify examples of each of these changes
Lastly, we'll learn the key differences between these two

What are physical and chemical properties?

Let's say you had to pick your friend out of a crowd. How would you find them? You can identify them by things like height, hair color, and how talkative they are. Elements and chemical compounds work the same way. Everyone has their unique properties that help us identify them. A substance can have two main types of properties: chemical and physical. So what are these properties exactly?

Physical Properties

A physical property is a trait that can be seen and/or measured without changing the identity of the substance. Some examples include color, density, and mass.

Physical properties can help identify substances without needing to use them in a reaction. There are two types of physical properties: intensive and extensive.

Intensive physical properties do not depend on the amount of material, such as boiling point, density, or temperature.

Extensive physical properties do depend on the amount, such as mass, length, and shape.

Other examples of physical properties are malleability (the ability of a substance to be molded into thin sheets), hardness, solubility, and electrical conductivity. We can use these properties to determine the identity of unknown compounds or elements.

Let's say you were handed three metal balls. They are all the same size and shape, and all of them are silver-colored. Now, how can we tell them apart? One way is to determine their densities.

Metal Name	Density (g/cm³)
Aluminum	2.70
Tin	5.75
Iron	7.86
Silver	10.5
Zinc	7.14

All of these metals are similar in color, but as you can see, their densities vary enough that we can tell them apart. After measuring the density of each ball three times for the measure of accuracy, we get these results: ball 1 (2.72 g/cm³), ball 2 (7.87 g/cm³), and ball 3 (10.3 g/cm³). Based on our chart, we see that ball 1 is aluminum, ball 2 is iron, and ball 3 is silver.

The formula for density is \(D = \frac{mass}{volume}\). So how would we calculate that using our ball example?

First, we would weigh our balls using a scale to determine mass. Next, we would determine volume using a method called water displacement. You would fill a beaker with a known amount of water (let's say 50 mL), then drop the ball inside this beaker. You would then measure the new volume of the water and subtract that from the initial to get the volume of the ball. This method is best used for irregularly shaped objects whose volume cannot be measured easily using a ruler or measuring tape.

Even though density is defined as mass/volume, we typically always use weight per volume. Since we are always affected by gravity (on earth anyway), the distinction between the two is somewhat meaningless.

Chemical Properties

A chemical property is the potential of a substance to undergo a chemical change. It describes how it would behave in certain scenarios. For example, flammability determines how likely something will catch fire.

A key difference between a chemical and physical property is that a physical property describes what a substance currently is while a chemical property describes what a substance can do. This can be a little confusing, so let's break it down. Chemical properties are all about potential: the potential to catch fire, corrode, oxidize, etc. Once a substance actually does any of that, its identity (chemical composition) changes. For example, Iron (Fe) can oxidize, but once it does, it's no longer iron! It is instead iron oxide (Fe(OH)₃).

Now, what about physical properties like a boiling point? Boiling is something liquid can do, right? Not quite, by "can do", we mean "can do a chemical change". The key difference here is that once a liquid boils, it still is the same molecule. Water, no matter what state it's in, has the same boiling point. So by "currently is", we mean "this property can be observed without a chemical change taking place".

Physical and Chemical Changes A rusted chain Vaia Fig. 1. When this iron chain was exposed to moisture, it oxidized and became rusted. The ability to rust is a chemical property of iron. Source: Pixabay.

The capability of iron to rust is a chemical property. So, how can we distinguish substances using chemical properties?

Let's say you are given two silvery lumps of metal and challenged to determine what they are based only on chemical properties. To make it easy, we are told that one is tin and one is potassium. Potassium reacts violently when dropped in water due to the creation of hydrogen gas. Tin, however, doesn't react with water under normal conditions. So we can take a small piece of each of our metals and place them in water (while wearing safety protection!) and whichever one reacts is the potassium.

What are chemical and physical changes?

Now that we've covered chemical and physical properties, let's look at physical and chemical changes.

Physical changes

A physical change is any change that does not change the identity of the substance.

There are several kinds of physical changes, some common ones are changes in the state of matter (boiling, freezing, etc.), cutting/bending/breaking, separation of a mixture, and creation of a mixture.

When we melt ice, it becomes water. Even though the name of the substance is changed, the atomic structure remains the same.

Mixtures versus chemical reactions

Let's look at another physical change: dissolving salt in water. While the salt is now separated into ions, it can be returned to solid salt if the water is boiled off. Saltwater is a mixture, more specifically, a homogeneous mixture. A homogeneous mixture is a substance made up of two or more compounds or elements that is uniform. If I took a 1 mL sample of 50 mL solution of salt water, that 1 mL would be representative of the whole thing.

In the other type of mixture, a heterogeneous mixture, the composition is not uniform. An example of this is oil and water. If I poured out some of the liquid, I would only get oil (since it is less dense).

The key difference between a mixture and a chemical reaction is that a mixture is reversible. Most importantly, it is reversible using only physical changes such as boiling, sifting, or separating by density.

Chemical Changes

A chemical change is any change that does change the identity of the substance. This change may be reversible, but only through another chemical change, A chemical change can also be called a chemical reaction

The key component of a chemical change is that the identity of the reactant(s) is changing. The molecule(s) we start with are different from our end product.

When we freeze water, we still have water molecules! In a chemical change, bonds are broken/formed, so we end up with different molecules.

Let's look at the reaction of butane and oxygen as an example.

\(2C_4H_{10} + 13O_2 \rightarrow 8CO_2 + 10H_2O\)

In this reaction, butane is converted into carbon dioxide and water. The only way we get butane back is through more chemical reactions. Butane is commonly used as a lighter fluid due to its flammability ( chemical property). The reaction itself is the change.

Physical and Chemical Changes A lighter Vaia Fig. 2. The combustion of butane releases heat energy in the form of fire, so it is commonly used in lighter fluid. Source: Pixabay.

Examples of Physical and Chemical Changes

To understand physical and chemical changes, we'll have a look at some examples of both:

Examples of Physical Changes:

Melting: When ice melts, it changes from a solid to a liquid state, but its chemical composition remains the same.
Cutting: When a piece of paper is cut into smaller pieces, its physical appearance changes, but it is still made up of the same materials.
Boiling: When water boils, it changes from a liquid to a gas state, but it is still made up of the same molecules.

Examples of Chemical Changes:

Combustion: When wood burns, it undergoes a chemical reaction with oxygen in the air to produce carbon dioxide, water vapour, and ash.
Oxidation: When iron rusts, it undergoes a chemical reaction with oxygen in the air to form iron oxide.
Digestion: When food is digested in the stomach, it undergoes a series of chemical reactions to break down complex molecules into simpler ones that can be absorbed by the body.

Compare and contrast physical and chemical changes

The differences between these two types of changes can be summarized in the table below:

Physical Change	Chemical Change
Identity stays the same	Identity changes
Combinations of molecules result in mixtures	Combinations/Reactions of molecules result in products
Combinations can be reversed only using physical changes	Combinations can be reversed only using chemical changes
Easily reversible	Difficult or impossible to reverse
Change is physical properties only	Change in physical and chemical properties

Physical and Chemical Changes - Key takeaways

A physical property is a trait that can be seen and/or measured without changing the identity of the substance. Some examples include density, mass, and melting point.
A chemical property is a potential for that substance to undergo a chemical change. It describes how it would behave in certain scenarios. For example, flammability determines how likely something will catch fire.
A physical change is any change that does not change the identity of the substance.
A chemical change is any change that does change the identity of the substance. This change may be reversible, but only through another chemical change, A chemical change can also be called a chemical reaction.

Frequently Asked Questions about Physical and Chemical Changes

The difference between a physical and a chemical change is that a physical change doesn't alter the identity of the substance, while a chemical change does.

A physical change is any change that doesn't alter the identity of a substance. It can be reversed by another physical change. A chemical change is any change that does alter the identity of a substance. It may be reversed by another chemical change, but not always.

Energy can be released or required for both physical and chemical changes.

The only property that remains the same regardless of the type of physical or chemical change is that the amount of matter stays the same. In other words, the amount of matter before the change and after it is the same. Matter is conserved.

An example of a physical change/process is ice melting. For a chemical change/process, an example would be a log burning.

Final Physical and Chemical Changes Quiz

Physical and Chemical Changes Quiz - Teste dein Wissen

Question

What is a physical property?

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Answer

A physical property is a trait that can be seen and/or measured without changing the identity of the substance.

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What are intensive physical properties?

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Physical properties that do not depend on the amount of substance

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What are extensive physical properties?

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Physical properties that do depend on the amount of substance

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Which of the following are considered physical properties?

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Shape, malleability, density, and mass

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What is a chemical property?

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A chemical property is the potential for that substance to undergo a chemical change. It describes how it would behave in certain scenarios.

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Which of the following is an example of a chemical property?

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Iron can rust

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What is a physical change?

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A physical change is any change that does not change the identity of the substance. Physical changes can be reversed to regain some or all of the original material

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What is the difference between a homogeneous and a heterogeneous mixture?

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A homogeneous mixture is uniform while a heterogeneous mixture is not. Both are a result of a physical change

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True or False: Salt dissolving in water is a physical change

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True

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What is a chemical change?

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Which of the following are indicators of a chemical change?

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All of the above

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True or false: A chemical change results in only chemical properties being changed

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False

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True or False: Chemical changes are never reversible

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False

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True or False: A substance bubbling always indicates a chemical change

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False

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Which of the following is an example of a physical change?

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Mixing oil and water

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Question

During the elephant toothpaste experiment, a catalyst converts 2 H₂O₂ into 2 H₂O and O₂.

What kind of change is this?

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Answer

Chemical, since one compound is converted into two others!

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Question

During the "golden rain" experiment lead iodide (flakes looking like gold) is produced from a solution containing lead iodided. They heat up the solution to dissolve all the yellow residue and after that, they cool it down to form back the crystals.

What type of change is this?

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Answer

This is a physical change since the identity of the compound does not change. It was lead iodide and stay so. This is btw what is called recrystallization.

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During electrolysis metal cations and electrons combine to form metal atoms.

What type of change is this?

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Answer

Since we are changing the chemical identity ( from ion to atom) we are making a chemical change.

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IF you put potassium metal in water it catches on fire.

Is this a physical or a chemical change?

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This is a chemical change.

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What is a limiting reactant?

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A limiting reactant is a reactant that will be used up first in a reaction. It, therefore, limits how much product can be made.

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What is the actual yield?

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The actual or experimental yield is how much product is actually made during an experiment. This can differ from the theoretical yield for several reasons. Some examples are experimental error, side reactions, difficult/wasteful extraction methods, and inefficient reactions.

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What are the steps for determining the limiting reactant?

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Step 1: Write out the chemical equation

Step 2: Balance the equation

Step 3: Convert all reactant mass amounts to moles

Step 4: Use the molar ratio to calculate the product made for both reactants

Step 5: The reactant that makes less product is the limiting reactant

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Question

The percent yield of a reaction is 112%, which of the following is a possible explanation?

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Improper drying

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True or False: The limiting reactant is the reactant we have the least amount of (in grams)

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False

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True or False: Percent yield is the same as percent error

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False

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A process with a 76% yield can still be inefficient, why or why not?

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It can still be inefficient, since the larger yield may be due to impurities.

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What does percent yield measure?

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The efficiency of a reaction

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The percent yield of a reaction is 42.2%, which of the following is a possible explanation?

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All of the above

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Which of the following statements are true?

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The reactants are on the left side of the chemical equation

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Which type of yield do we calculate by using the molar ratio?

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Theoretical yield

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How do you balance an equation?

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You count the number of atoms of each element on both sides. You multiply whichever species needs to be balanced by a coefficient to get the balanced reaction.

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What is the molar ratio?

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The molar ratio is the ratio of reactants to products in a reaction, which is shown by the coefficients (number in front).

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What is molar mass?

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The molar mass is the weight of a compound. It is the number of grams present in 1 mol.

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Why do we use the molar mass to calculate reactants/products?

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The molar ratio is in units of mols, so we need to use the molar mass to convert from grams to mols

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What is a limiting reactant?

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The limiting reactant is the species that will be consumed first in a reaction.

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How can you tell which reactant is limiting?

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You calculate the theoretical yield for each reactant, whichever produces less product is limiting.

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What is an ideal gas?

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An ideal gas is a theoretical gas whose particles exhibit no intermolecular forces and have a negligible volume.

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What is the ideal gas equation?

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PV=nRT

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A 23.4 L container of methane is at STP, how many moles of methane are present in the container?

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Answer

1.044 mol

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What are the conditions at STP (Standard Temperature and Pressure)

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1 atm, 273 K

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What is stoichiometry?

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Stoichiometry is the ratio of products to reactants in a reaction. This ratio is labeled by the stoichiometric coefficients, which are the numbers in front of an element/molecule in a chemical equation.

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What is a limiting reactant?

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The limiting reactant is the reactant that is consumed first in a reaction

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True or False: 1 mol of gas at STP has a volume of 22.4 L

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True

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If a container of 27.8 L of N₂ gas is at STP and is heated to 500 K, what is the new pressure of the gas? (Assume the volume stays the same)

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1.83 atm

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True or False: The gas constant (R) is always equal to 0.0821

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False

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Why do we measure gas in liters instead of by mass?

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Gas is difficult to way and will always take up whatever container it is in.

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Physical and Chemical Changes

Physical and Chemical Changes

Physical and Chemical Changes

What are physical and chemical properties?

Physical Properties

Chemical Properties

What are chemical and physical changes?

Physical changes

Mixtures versus chemical reactions

Chemical Changes

Examples of Physical and Chemical Changes

Examples of Physical Changes:

Examples of Chemical Changes:

Compare and contrast physical and chemical changes

Physical and Chemical Changes - Key takeaways

Frequently Asked Questions about Physical and Chemical Changes

What is the difference between a physical and chemical change?

What are physical and chemical changes?

How is energy involved in physical and chemical changes?

What properties stay the same during physical and chemical changes?

Which processes are an example of a physical and chemical change?

Final Physical and Chemical Changes Quiz

Physical and Chemical Changes Quiz - Teste dein Wissen

More explanations about Chemical Reactions

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