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Composing and Decomposing Matter

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The content and activities in this topic will work towards building an understanding of how the properties of substances change in a chemical reaction.

Chemical Reactions

Compounds are made of atoms of two or more elements joined chemically. This chemical joining is called bonding. There are many types of chemical reactions that result in chemical compounds, some of these are described in Table 1.4. In a chemical reaction, one or more substances undergoes a chemical change to produce a different substance or substances. The original substances are the reactants. The new substance that results from the chemical reaction is the product. A chemical reaction is generally written in this format:


reactant + reactant → product + product


The arrow indicates that a chemical reaction has occurred. The above statement would be read “reactant plus reactant yields product and product.”


An example of a chemical reaction is the neutralizing of stomach acid with an antacid. The chemical formula of an antacid is Ca(OH)2. Stomach acid is hydrochloric acid (HCl). When these two reactants combine, they react to yield calcium chloride (CaCl2) and water (H2O). The water is formed from the OH in Ca(OH)2 and the H in HCl, to make HOH, another way of writing H2O. The chemical reaction is written in this format:


Ca(OH)2 + 2 HCl → CaCl2 + 2 H2O


This reaction reads: calcium hydroxide plus hydrochloric acid yields calcium chloride plus water. Examples of chemical reactions are given in Table 1.4.


Table 1.4. Types of common chemical reactions, with model reactions and example reactions
Type of Reaction Model Reaction Example Reaction

A + B → AB


Two or more reactants combine to form a product

4 Fe + O2 → 2 Fe2O3


iron plus oxygen yields iron oxide

the simplified reaction for rust formation

Single Replacement

A + XY → AY + X


A single switch is made between components of two reactants

3 Ag2S + 2 Al → 6 Ag + Al2S3


silver sulfide plus aluminum yields silver and aluminum sulfide

the reaction for polishing tarnished silver in a bath with aluminum foil

Double Replacement

AB + XY → AY + XB


A double switch is made between components of two reactants

Ca(OH)2 + 2 HCl → CaCl2 + 2 H2O


calcium hydroxide plus hydrochloric acid yields calcium chloride plus water

the reaction for the neutralization of stomach acid with an antacid


CiHj +O2 → CO2 + H2O


A reactant made of carbon and hydrogen (a hydrocarbon) reacts with oxygen to form carbon dioxide and water. The subscripts i and j refer to the number of C and H atoms, which are different in different hydrocarbons.


CH4 + 2 O2 → CO2 + 2 H2O


methane plus oxygen yields carbon dioxide and water

the reaction for burning methane (main component of natural gas)


AB → A + B


A reactant breaks down into two or more products

2 H2O2 → 2 H2O + O2


hydrogen peroxide yields water and oxygen gas

the reaction for the decomposition of hydrogen peroxide



Fig. 1.8. Hydrogen peroxide (H2O2) easily breaks down to H2O and O2 in sunlight, which is why it is stored in an opaque bottle.

Image by Jordan Wang

Compounds can be decomposed into simpler compounds or their elemental components by breaking bonds. Some compounds are decomposed by heating or by exposure to sunlight. For example, calcium carbonate (lime stone) decomposes into calcium oxide (quick lime) and carbon dioxide when heated. Light decomposes hydrogen peroxide into water and oxygen (Fig. 1.8). Because water is an extraordinarily stable compound it does not decompose easily.


The process of electrolysis uses electricity to break bonds, causing decomposition of matter. Electricity is the flow of electrons, the negatively charged particles in atoms. In electrolysis, a chemical change is caused by electricity flowing through a chemical compound.




Hoffman Apparatus



Fig. 1.11. Hoffman apparatus

Image by Jordan Wang


A Hoffman apparatus is used for the electrolysis of water. It is a glass U-tube that is filled by a thistle tube (Fig. 1.11). Table 1.5 explains the parts of the Hoffman apparatus.

Table 1.5. Parts of a Hoffman Apparatus
Labeled parts in Fig. 1.11 Name Description
A Thistle Tube The middle tube has a funnel-like section at the top called a thistle tube that is used for filling the apparatus with the water that will be electrolyzed.
B U-tube The U-tube consists of two upright cylinders joined by a horizontal tube. The markings on the cylinders allow volumes of gases and liquids to be accurately measured.
C Platinum electrodes Platinum electrodes are at the bottom of the cylinders. Platinum electrodes conduct electricity and do not corrode.
D Stopcocks The stopcocks can be opened or closed to allow the gases to be collected.





Fig. 1.12. Diagram of the mechanism of electrolysis of water; the "e-" indicates electrons.

Image by Byron Inouye

Electrolysis uses electricity to split water into hydrogen and oxygen. At the negative terminal of the battery, electrons are produced. Those electrons flow through the wire to the electrode in solution (Fig. 1.12). At the negative (-) electrode, electrons are transferred to the water and picked up by hydrogen atoms in water. This allows hydrogen to split from the rest of the water molecule and form H2 molecules. At the other electrode, the positive (+) electrode, electrons are donated by the oxygen in the water molecule, which allows oxygen to split from the water and form O2 molecules. Electrons flow from this electrode back to the positive terminal of the battery.


The chemical reaction that occurred at the positive electrode was:

2 H2O → O2 + 4H+ + 4e-


The chemical reaction that occurred at the negative electrode was:

2 H2O + 2e- → H2 + 2OH-


Physical and Chemical Changes of Water

When water is heated and boils (Fig. 1.13 A), a physical change takes place as liquid water evaporates into gaseous water vapor. Boiling is a physical change, because even though the water changes state from liquid to gas, the liquid and gas are both made of water molecules. The water molecule has not decomposed.



Image caption

Fig. 1.13. (A) Boiling water is an example of a physical change. (B) Electrolysis is an example of a chemical change.

Image copyright and source

Image A by Jordan Wang; Image B by Byron Inouye

In the electrolysis of water (Fig. 1.13 B), hydrogen and oxygen gases are produced from liquid water. The electrolysis of water causes a chemical change in which water molecules are split to form hydrogen and oxygen—two substances that are chemically different from water. Decomposing water is difficult and cannot be accomplished through heating alone. Water is not broken apart even by the heat of a volcanic eruption!


Conservation of Matter

An important characteristic of matter is that it can never be created or destroyed, only transformed. A physical or chemical change transforms matter from one form of matter to another. John Dalton was one of the first scientists to recognize this property of matter. This idea is called the Law of Conservation of Matter. In any physical or chemical change matter is not created out of nothing. For example, a metal object that rusts will gain a brownish coating and will also increase in mass. This comes from a chemical change, the reaction of iron atoms with oxygen atoms in the air. Matter also does not disappear, although it can appear to. For example, in the electrolysis of water, although the volume of liquid water decreases, the matter that makes up the water is not disappearing. Rather, the hydrogen and oxygen atoms in the liquid water are being converted into molecules of hydrogen gas and oxygen gas. The Law of Conservation of Matter is one part of Dalton’s Atomic Theory, which has five basic parts:

  1. Matter is composed of small particles called atoms.
  2. Atoms of the same element are the same; atoms of different elements are different.
  3. Matter cannot be created or destroyed.
  4. Atoms can combine in different ratios to form compounds.
  5. In chemical reactions, atoms are rearranged to form new compounds.


Dalton’s Atomic Theory forms the basis for modern atomic theory.


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Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.