Chemical Changes

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Chemical Changes
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What is a chemical change?

• chemical change - matter changes into a new
  substance through a chemical reaction.

• The animation to the right shows a chemical
  change. The blue and white molecule is more
  attracted to the red and white atoms than the
  green atom so they switch, forming a new
  substance.

Animation from http//www.ias.ac.in/initiat/sci_e
d/resources/chemistry/sn2.gif
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Why did baking soda and vinegar bubble?

• A chemical change occurred when you combined
  baking soda and vinegar.
• Vinegar is acidic.
• When the molecules in baking soda mix with the
  acid, some of their atoms are more attracted to
  the acid and they break apart to form new
  molecules.

Baking Soda
Vinegar

• Baking soda and vinegar are like two couples
  dancing. When they are separate from each other
  they stay with their own molecule

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Why did baking soda and vinegar bubble?

• A chemical change occurred when you combined
  baking soda and vinegar.
• Vinegar is acidic.
• When the molecules in baking soda mix with the
  acid, some of their atoms are more attracted to
  the acid and they break apart to form new
  molecules.

Baking Soda
Vinegar

• But when the two molecules mix, atoms from one
  molecule are more attracted to the other
  molecule

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Why did baking soda and vinegar bubble?

• A chemical change occurred when you combined
  baking soda and vinegar.
• Vinegar makes the water more acidic.
• When the molecules in baking soda mix with the
  acid, some of their atoms are more attracted to
  the acid and they break apart to form new
  molecules.

CO2 Carbon dioxide
NaC2H3O2 Sodium Acetate
H2O water

• So they switch dance partners and form new
  substances.
• Notice that nothing was created or destroyed
  just changed.

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Evidence for a Chemical Change

• 1. Formation of a gas
• Seeing bubbles or gas after mixing substances
  together is evidence that a chemical change took
  place.
• When you mixed baking soda or baking powder and
  vinegar, the bubbles you saw were carbon dioxide
  gasa new substance that had formed.
• Bubbles dont always mean that a chemical change
  occurred. Can you think of any examples of
  bubbling that do not create a new substance?

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Why does red cabbage juice turn the powders
different colors?

• A chemical change occurres when red cabbage juice
  is added to cream of tartar and laundry
  detergent.
• Red cabbage juice is an indicator, because it
  contains molecules that change color when an acid
  or base is added to them.
• Cream of tartar is an acid, so it gave particles
  to the indicator, making it turn from blue to
  pink.
• Laundry detergent is a base, so it took particles
  from the indicator, making it turn from pink back
  to blue.

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Evidence for a Chemical Change

• 2. Color Change
• Seeing a color change after mixing substances
  together is evidence that a chemical change took
  place.

From http//www.youtube.com/watch?vyqUghaS5apk

• Color is a property of how the molecule is
  arranged. If there is a change to the molecule,
  the color can change.

• Color change doesnt always mean that a chemical
  change occurred. Can you think of any examples of
  a substance changing color that does not create a
  new substance?

Sometimes it takes a while for the molecules to
rearrange and the color to change.
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Evidence for a Chemical Change

• 3. Temperature Change
• Noticing a temperature change after mixing
  substances together is evidence that a chemical
  change took place.
• The substance can get warmer or cooler, depending
  on whether the molecules give energy while
  swapping places (hot - exothermic), or need
  energy to swap (cold - endothermic).
• Temperature change doesnt always mean that a
  chemical change occurred. Can you think of any
  examples of a substance getting warmer or colder
  that do not create a new substance?

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Evidence for a Chemical Change

• 4. Formation of a Precipitate
• A precipitate forms when a substance comes out
  of solution and forms a solid. Seeing a solid
  substance forming after mixing substances
  together is evidence that a chemical change took
  place.
• Dont confuse freezing a substance to make it a
  solid with forming a solid precipitate. Freezing
  is a physical change that does not change what
  the substance is made of! Can you think of any
  examples of solids forming in a liquid that do
  not create a new substance?

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Is it a Chemical Change?

• Write down YES on your board if it is a
  chemical change, and NO if it isnt.
• If YES, list what evidence supports this.
  (Example gas formation).

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Is it a Chemical Change?

• A forest fire destroys acres of land. There is a
  lot of smoke, and the trees turn to charcoal.

ANSWER YES EVIDENCE - Smoke, - color change,
- temperature change
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Is it a Chemical Change?

• When making hot cocoa, clear water turns brown
  when you mix the cocoa in.

ANSWER No (the water is turning brown because
the cocoa is brown)
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Is it a Chemical Change?

• An old wheelbarrow is left out in the rain and
  rusts.

ANSWER Yes EVIDENCE Color change,
precipitate forming (rust)
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Is it a Chemical Change?

• On a hot day, water forms on the outside of a
  cold glass of water.

ANSWER No (No new substance is forming)
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Is it a Chemical Change?

• Ice cream melts after it falls on the ground.

ANSWER No (No new substance is forming)
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Is it a Chemical Change?

• A raw egg gets cooked.

ANSWER Yes EVIDENCE Color change
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Is it a Chemical Change?

• When Alka Seltzer is dropped into water it fizzes.

ANSWER Yes EVIDENCE gas formation
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Is it a Chemical Change?

• Crayons change white paper to red.

ANSWER No (The paper is turning red because
the crayon is red.)
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Is it a Chemical Change?

• Soda fizzes, and bubbles rise as you pour it in a
  glass.

ANSWER No (The bubbles were already there,
just under pressure in the bottle.)
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Is it a Chemical Change?

• When you crack a glow stick, hydrogen peroxide
  mix with other chemicals, causing it to glow and
  warm up.

ANSWER Yes EVIDENCE - color change -
temperature change