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Reactions - Physical and Chemical Changes

Chemical changes or chemical reactions are a main part of the study of chemistry. The first question that needs to be asked is, what is a chemical reaction? What makes these changes different from physical changes?

History

Explore the discoverer's biography, including general facts about his life and anecdotes regarding how he made this particular discovery. Also see other significant scientific discoveries built largely on this concept and other real-world applications in history that may not still be relevant.

Use/Application through History

Chemistry is a well developed but very ancient science, and chemical changes have been handled by human beings since 4,000 years ago. The Egyptians pioneered the “art” of chemistry, and since then, people have explored and continue to explore the many different aspects of chemistry. Around 1000 BCE, ancient societies discovered and mastered many chemical techniques such as how to extract metal from alloy, to make wine out of grapes or rice, to make pottery or china, and to make pigment for drawing. Throughout the rest of human history, chemistry has been used in the invention of paper, gun powder, ink, and more recently, gasoline, plastic, soap etc. Today, chemistry is used in a variey of ways across the globe.

 

Concept Definition

Study the primary definition of this concept, broken into general, basic, and advanced English definitions. Also see the mathematical definition and any requisite background information, such as conditions or previous definitions.

General Science

In a chemical change, the substances are altered chemically and display different physical and chemical properties after the change.

Basic

Chemical changes are often accompanied by color change, precipitation, generation of a gas, or a combination of the three.

Advanced

Substances react chemically in characteristic ways with other substances to form new substances with different characteristic properties. Chemical equations describe the overall outcomes of chemical reactions by identifying the beginning substances (reactants) and ending substances (products).

Mathematical Definition

$\text{Reactants} \to \text{Products} $

$\text{Beginning Substances} \to \text{Ending Substances} $

 

Real World Application

Discover processes or disciplines in the natural or man-made worlds that employ the concept.

The oil industry uses chemical reactions such as cracking of petroleum.

The military uses chemical reactions in the gas bomb.

Plastic making involves chemical changes with the composition of polymers.

Turning fat or oil and alkaline solution into a bar of soap, widely used in soap industry, is a chemical change.

Reactions in the human body such as digestion, turning food into glucose, ATP, ADP formation, etc., are chemical changes.

Chemical changes even occur in art. For example the obscure oxidation drawing by Andy Warhol, was completed by using the chemical reaction of copper metallic paint oxidized by urine.

Chemical changes are everywhere!

 

Vocabulary

Learn important vocabulary for this concept, including words that might appear in assessments (tests, quizzes, homework, etc.) that indicate the use of this concept.
Important Vocabulary
Term Context
and, +
  • Oxygen and hydrogen react to yield water.
balanced chemical equation
  • The chemical change is described by the balanced chemical equation.
catalyst
  • Manganese is a catalyst in the decomposition of H2O2.
equilibrium
  • The equilibrium in chemical reactions within the body is very delicate.
product
  • The products of any combustion reaction (burning) are CO2 and H2O.
react
  • Noble gases do not react with anything; they remain unchanged.
reactant
  • The reactants in a chemical reaction must be mixed in order to react.
yield
  • Many substances, when mixed with oxygen, will yield new products.

 

Videos

Browse relevant videos from the Journal of Chemical Education's (JCE) Chemistry Comes Alive! library and other video sources.

Potassium with Water

Potassium reacts vigorously with water, producing a flame in this exothermic reaction.

2K + 2H2O --> H2 + 2KOH

Cold Enough to Freeze Water

Here, the freezing of water is caused by the temperature change in a chemical reaction.

$ 2\text{NH}_4\text{(SCN)} \ + \ \text{Ba(OH)}_2 \bullet 8 \text{H}_2\text{O} \ + \ heat \ \to \ \text{Ba(SCN)}_2 \ + \ 10\text{H}_2\text{O} \ + \ 2\text{NH}_3$

Iodine Clock

Observe color-change in a chemical reaction between two colorless solutions.

Genie in a Bottle

The classic demo: Genie in a Botttle . It demonstrates the generating of gas in some chemical changes.

Sodium Iodide and Lead(II) Nitrate

Adding very pale yellow sodium iodide to colorless lead(II) nitrate produces a yellow precipitate.

Solution A: 0.5 M sodium iodide (very pale yellow) 
Solution B: 0.2 M lead(II) nitrate (colorless) 
Precipitate: (yellow) 
Pb(NO3)2(aq) + 2 NaI(aq) --> PbI2(s) + 2 NaNO3(aq)

 

Classroom Demonstations

Investigate lab procedures suitable for live classroom demonstrations or guided student exploration.

Students requiring adaptations to gain the full benefit of a demonstration may find a worksheet with guided observations useful. Alternatively, a teacher may wish to use a worksheet with guided observations to model what observations all students should be making during a demonstration.

The Demonstration Observation Worksheet is available in

  • PDF [ready to print]
  • Word 2007/2008 DOCX [free to edit/adapt further]
Demos

Burning Iron



Demonstrations
   
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Burning Iron

Burning Iron

Credit: Yan Luo
 
Author: Yan Luo
Description:

Iron will burn in the presence of oxygen if energy is added and the iron has a large surface area.
Usage:

A cool demonstration for combination reactions and the effects of surface area on reaction rates. IMPORTANT: This demonstration should not be used for labs in middle schools.
 
Futher Images
tn_burning-iron-8-p2-87.png tn_burning-iron-8-p3-87.png tn_burning-iron-8-p4-87.png  
Credits, from left: , Yan Luo, Yan Luo, Yan Luo
 
Safety:

IMPORTANT: Wear gloves and goggles at all times. The instructor should stay away from the reaction and students should be kept at least 1 m away from the demonstration.
 
Equipment:
  • A 9V battery with both terminals on the same side (to complete the electric circuit)
  • Recommended: objects made of iron or steel
 
Materials:

A piece of fine steel wool (#0 or finer recommended)
 
Procedure:
  1. (Recommended) For Comparison: Touch different types of iron/steel-made objects (e.g. steel ruler, clamp, ring stand) by the terminals of a 9V battery. Show the students that both terminals of the battery are in touch with the object. (No physical change can be observed).
  2. Put a piece of fine steel wool (#0 or finer recommended, make into a loose sphere with ~1" radius) on an inflammable surface with no flammable objects near by (to prevent possible damage to surface).
  3. Touch the steel wool with a 9V battery. (The steel wool should start burning immediately).
 
Notes:

What to observe: What is different when the same 9V battery is touching steel wool in Part 3 compared to other steel-made objects in Part 1? What causes the differences? Of the objects that were touched to the 9V battery, all of similar mass and made of steel, the steel wool has the largest surface area. More oxygen molecules are exposed to the steel surface in the steel wool, and therefore the energy provided by the 9V battery (which is not a lot) can only burn the steel wool but not the other objects. The chemical equation for the reaction is: $\text{3Fe}_{(s)} + \text{2O}_2_{(g)} \to \text{Fe}_3\text{O}_4_{(s)}$
 
Disposal:

Used batteries should be recycled or disposed properally. The steel wool may be thrown in the garbage as long as it has stopped burning.
 
Difficulty:No specific experience required
Preparation Time:15-20 minutes Demonstration Time: 1 minutes
Availability of Materials:Local grocery store
 Cost of materials:$3
Last Updated:Wed 15 Jun 2011 14:34:05 EDT Viewed:153367 times viewed
Source:Rarren Fix, ScienceFix.com, June 14, 2009, ScienceFix.com

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Iodine Clock



Demonstrations
   
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Iodine Clock

Iodine Clock

Credit: CCAlive!
 
Author: Claire Liu
Description:

Most chemical changes involves color changes. The demo, "Iodine clock", is a classic color-changing reaction.

A solution of hydrogen peroxide and sulfuric acid is added to a solution of iodide ion, thiosulfate ion, and starch. A slow reaction forms triiodide, which reacts rapidly with thiosulfate, until the thiosulfate is consumed. The excess triiodide then reacts with the starch to form a blue complex.
Usage:
No Description available
 
Futher Images
tn_iodine-clock-to-obs-13-p1-120.jpg tn_iodine-clock-to-obs-13-p2-120.jpg tn_iodine-clock-to-obs-13-p3-120.jpg tn_iodine-clock-to-obs-13-p4-120.jpg  
Credits, from left: CCAlive!, CCAlive!, CCAlive!, CCAlive!
 
Safety:

Wear goggles and gloves.
 
Equipment:

Small beaker or glass bottle;
 
Materials:

hydrogen peroxide and sulfuric acid solution;

iodide ion, thiosulfate ion, and starch solution;
 
Procedure:

First put some of the solution of iodide ion, thiosulfate ion, and starch in a small beaker or glass bottle and then add the solution of hydrogen peroxide and sulfuric acid.
 
Notes:
No Description available
 
Disposal:
No Description available
 
Difficulty:Some laboratory experience required
Preparation Time:  minutes Demonstration Time: 1 minutes
Availability of Materials:Not specified Cost of materials:$ 
Last Updated:Tue 21 Jun 2011 12:49:01 EDT Viewed:65734 times viewed
Source:University of Wisconsin press, Shakhashiri; UW-madison chemistry department; JCE CC Alive!, Shakhashiri, B.Z. Chemical Demonstrations: A Handbook for Teachers of Chemistry

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Chemical Genie in the Bottle



Demonstrations
   
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Chemical Genie in the Bottle

Chemical Genie in the Bottle

Credit: UW-Madison Department of Chemistry
 
Author: Claire Liu
Description:

When a stopper is removed from an opaque bottle, an increasingly vigorous cloud of white vapor emanates from the bottle. This action results from the catalytic decomposition of 30% hydrogen peroxide by manganese dioxide.

Original Demonstration
Usage:
No Description available
 
 
Safety:
No Description available
 
Equipment:

an opaque bottle
 
Materials:

30% hydrogen peroxide and manganese dioxide.
 
Procedure:

put the two material together.
 
Notes:

2 H2O2 (aq) O2 (g) + 2 H2O(l)
 
Disposal:
No Description available
 
Difficulty:Not specified
Preparation Time:  minutes Demonstration Time: 1 minutes
Availability of Materials:Not specified Cost of materials:$ 
Last Updated:Tue 21 Jun 2011 12:50:27 EDT Viewed:65672 times viewed
Source:1991, Fun With Chemistry; A Guidebook of K-12 Activities. vol 1

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Precipitation in Chemical Changes



Demonstrations
   
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Precipitation in Chemical Changes

Precipitation in Chemical Changes

Credit: CCAlive!
 
Author: Claire Liu
Description:

Adding very pale yellow sodium iodide to colorless lead(II) nitrate produces a yellow precipitate.

Usage:
No Description available
 
Futher Images
tn_precipitation-in-che-15-p1-120.jpg tn_precipitation-in-che-15-p2-120.jpg tn_precipitation-in-che-15-p3-120.jpg tn_precipitation-in-che-15-p4-120.jpg  
Credits, from left: CCAlive!, CCAlive!, CCAlive!, CCAlive!
 
Safety:
No Description available
 
Equipment:
No Description available
 
Materials:

Solution A: 0.5 M sodium iodide
Solution B: 0.2 M lead(II) nitrate
 
Procedure:

Adding sodium iodide to colorless lead(II) nitrate.
 
Notes:

Solution A: 0.5 M sodium iodide (very pale yellow)
Solution B: 0.2 M lead(II) nitrate (colorless)
Precipitate: (yellow)
Pb(NO3)2(aq) + 2 NaI(aq) --> PbI2(s) + 2 NaNO3(aq)
 
Disposal:
No Description available
 
Difficulty:Some laboratory experience required
Preparation Time:  minutes Demonstration Time: 1 minutes
Availability of Materials:Not specified Cost of materials:$ 
Last Updated:Fri 01 Apr 2011 17:05:51 EDT Viewed:65761 times viewed
Source:UW-madison chemistry department; JCE CC Alive!, UW card catalog

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Summary

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Summary

The key to a chemical change is that a new substance with different chemical properties than the starting material(s) must be generated after the change. Chemical changes are most commonly indicated by changing color, the formation of a new state of matter (i.e. solid or gas in a reaction with only liquids), or flame.

 

Works Cited

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Works Cited

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