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56 Chemical Reactions and Equations
Lesson 1
Scan Lesson 1. Read the lesson titles and bold words. Look at the pictures. Identify three facts you
discovered about chemical reactions. Record your facts in your Science Journal.
Understanding Chemical Reactions
Copyright © McGraw-Hill Education.
Differentiate a physical change from a chemical change.
Physical Change Chemical Change
Identify signs of a chemical reaction.






One way to be
certain:
Four changes
in properties
Two changes
in energy
Signs of
Chemical
Reaction
Sequence changes in atoms during a chemical reaction.
Bonds between
atoms break.
Changes in Matter
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Signs of a Chemical
Reaction
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What happens during a
chemical reaction?
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Recall that matter can undergo two types
of changes-chemical or physical. A physical
change does not produce new substances. The
substances that exist before and after the
change are the same, although they might
have dierent physical properties. This is
what happens when liquid water freezes. Its
physical properties change from a liquid to a
solid, but the water, H,O, does not change
into a dierent substance. Water molecules
are always made up of two hydrogen atoms
bonded to one oxygen atom regardless of
whether they are solid, liquid, or gas.
Recall that during a chemical change, one
or more substances change into new sub-
stances. The starting substances and the sub-
stances produced have dierent physical and
chemical properties. For example, when
brownie batter bakes, a chemical change
OCcurs. Many of the substances in the baked
brownies are dierent from the substances in
the batter. As a result, baked brownies have
physical and chemical properties that are dif-
ferent from those of brownie batter.
alan is called a cheminal
Change of color
Odor
Formation of
bubbles
Formation of
precipitation
Warming or
cooling
Release of
light
Bonds form between
hydrogen atoms
Bonds form between
oxygen atoms
Chemical Reactions and Equations 57
Lesson 1 | Understanding Chemical Reactions (continued)
Copyright © McGraw-Hill Education.
Distinguish the parts of a chemical equation.
Chemical Equation
description of
to represent to represent
subscripts describe the
using
Detail information regarding the writing of chemical equations.
Define reactant.
Define product.
Write the general
structure for a chemical
equation.
How is the arrow sign
read?
Write the equation for
“carbon plus oxygen
produces carbon dioxide.”
Restate the law of conservation of mass.Conservation of Mass
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Chemical Equations
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z z
A reaction
Element symbols Chemical formulas
Elements Compounds
Number of atoms of each element
The starting substances in a chemical reaction
The substances produced by a chemical reaction
Reactant + reactant ——> product + product
As “produces” or “yields”
C + O ———-> CO
The total miss before a chemical reaction is the same as the total mass after a chemical reaction.
58 Chemical Reactions and Equations
Lesson 1 | Understanding Chemical Reactions (continued)
Copyright © McGraw-Hill Education.
Relate atoms to mass in a chemical reaction.
Mass before a chemical
reaction is equal
toNumber of atoms in the
reactants
Paraphrase what it means when you say a chemical equation is
balanced.
Explain the meaning of chemical formulas. Circle the coefficient.
H20 2H20
means means
Order the steps in balancing a chemical equation.
1.
2.
3.
4.
Balance the chemical equation for carbon monoxide.
C + O2 CO
Look back at the picture of the firefly on the first page of Lesson 1. How could
you conclude that the firefly’s blinking is a chemical rather than a physical change simply by
viewing the picture and without reading the text on the page?
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Number of atoms in the products
The specic numbers of types of atoms are the same on both sides of the equation
One water molecule Two water molecules
Write the unbalanced equation
Count atoms of each element in the reactions and product’s
Add coecient to balance the atoms
Write the balanced chemical equation
The rey's blink gives