Describing Chemical Change

1
Describing Chemical Change
2
Signs of a Chemical Change

• Look for evidence of something new having been
  made.
• Common signs include
• Color change
• Temperature change
• Formation of a solid
• Formation of a gas

3
Chemical Reactions

• Write chemical reactions/equations to describe
  chemical change.
• A correctly written chemical reaction will
• Obey the law of conservation of matter
• Show the chemical formula and state of each
  reactant and product

4
Symbols used in reaction writing
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Symbols used in reaction writing
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Correctly written reaction

• Mg(s) 2 HCl(aq) ? MgCl2 (aq) H2
  (g)

7
Writing Chemical Reactions

• Write chemical formula for each reactant and
  product use naming rules to do so
• Often already done for you..except on a take
  home exam.
• Write a symbol after each reactant and product to
  indicate its state.
• You will be given enough information to do this.

8
Writing Chemical Reactions

• Balance the reaction by adding coefficients in
  front of any reactant and/or product.
• The goal is to have the same number of each type
  of atom on the two side of the equation.
• If you have 2 Cl atoms on the product side of the
  equation you must also have 2 Cl atoms on the
  reactant side.

9
Writing Chemical Reactions

• Balance the reaction by adding coefficients in
  front of any reactant and/or product.
• You may NOT change the chemical formula of the
  reactants or products to balance the reaction.