Physical and Chemical Changes

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Physical and Chemical Changes

• SNC 1D

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A burning candle

• Question When a candle burns, is the event a
  chemical or a physical change?

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Hypothesis

• Answer the question which has just been raised
  through discussions with your neighbours.

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Physical Changes

• Remember from our previous class, that a physical
  change simply changes the state or form of the
  substance, but it is STILL THE SAME substance.

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Physical Changes

• Changes of state include melting, boiling,
  freezing, condensation and sublimation are
  physical changes. This is because the substance
  is still the same the only change is its state.

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Physical Changes
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Physical Changes

• When a substance is dissolved into a solvent, a
  physical change has occurred. When putting salt
  in water, it will cause the salt to dissolve if
  the water is unsaturated. You do not have a new
  substance. You simply have salt water. The salt
  can be removed from the water by simply boiling
  the water out.

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THINK

• Have you ever swam in the ocean? You know that
  the ocean consists of salt water. Were you
  worried about drinking the salt water? After
  swimming, did you ever notice a white film on
  certain parts of your skin? Discuss these
  questions.

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Chemical Changes

• Remember from last class that you know that a
  chemical change has occurred when you have a
  completely new substance with new properties.

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Chlorine

• Chlorine is a gas at room temperature and is
  green in colour. If you breathe it, it will burn
  your eyes and lungs. It is placed in small doses
  in swimming pools to kill bacteria. In the First
  World War, it was used to clear enemy troops from
  their trenches due to its density (which seeps
  into the trenches).

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Chlorine Gas
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Chlorine Gas in WWI
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Chlorine Gas in WWI
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THINK

• Would you consume a product which contains
  chlorine?

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Sodium

• Sodium is a highly explosive metal. If put in
  water, it will break the water molecule and
  ignite the hydrogen gas which is released.
  Imagine if this substance came in contact with
  your tongue? Pure sodium must be stored in oil
  because it will even react with the water in the
  air (humidity).

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Sodium
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THINK

• Would you consume a product which contains
  sodium?

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Sodium chloride

• Imagine if you answered no to any of the previous
  questions. When sodium and chlorine are combined
  together, they form a salt known as sodium
  chloride. No human being can survive without
  this essential life giving substance. How else
  would your body retain water?

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Sodium Chloride

• Unlike chlorine, sodium chloride is not a gas (it
  is a solid), it is white and crystallized (not
  green). Furthermore, it is not poisonous and
  will only harm you if taken in excess. Unlike
  sodium, sodium chloride will not explode in
  water, it will dissolve.

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Whats the point?

• Sodium chloride has completely different physical
  properties than pure sodium and pure chlorine.
  This is one way we know that a chemical change
  has occurred.

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Evidence of a chemical change

• A chemical change has occurred when
• A colour change has occurred
• Heat or light is given off
• Bubbles of gas are given off
• A solid material is created (i.e., two liquids
  form a solid precipitate)
• Odour is produced
• The change likely cannot be to reversed

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Colour changes

• Have you ever had a beautiful shiny pinkish (or
  rosé) penny? Did you ever wonder why that same
  penny turned into a dull dark brownish or even
  green penny? Some metals react with the oxygen
  in the environment. This chemical reaction is
  called an oxidation reaction (as known as
  rusting).

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Heat or Light Emission

• Have you ever had a BBQ? Propane gas within the
  tank is released and exposed (once) to an
  igniter. Once ignited, the propane gas reacts
  with the oxygen to produce carbon dioxide and
  water while emitting both light and heat. This
  is known as a combustion reaction. Much light a
  lighter (butane) or a match (sulfur).

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Bubbles of gas

• When vinegar and baking soda are mixed together,
  bubbles are released. The reaction is releasing
  hydrogen gas into the environment.

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Precipitate formation

• Youve just eaten spaghetti with tomato sauce
  (acidic) with wine (acidic) and dessert is a
  grapefruit in lemon juice (highly acidic). You
  now have acid indigestion or acid reflux. You go
  to the medicine cabinet and drink some milk of
  magnesia (a base).

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Precipitate formation

• When acids are mixed with bases, the end result
  is the formation of water and a salt. The
  creation of a salt is known as the precipitate.
  Careful! If you do this too often you may get
  what are known stones!!!

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Irreversible reactions

• Burning wood is evidence that a chemical reaction
  has occurred as it is next to impossible to
  covert ashes back into woodand new products have
  been created, plus gas and heat have been
  produced

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Burning candle

• Question When a candle burns, is the event a
  chemical or a physical change?

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Burning candle

• If you said that the burning of a candle was a
  chemical change, congratulations! The candle
  which is made of wax burns (things that burn are
  changing chemically), which produces light, heat,
  water and carbon dioxide (or carbon monoxide when
  not enough oxygen is present giving a black
  soot).

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Burning candle

• If you said that the burning of a candle was a
  physical change, congratulations! When the
  candle is lit, the flame but melts the wax.
  Melting a substance is a physical change from a
  solid to a liquid. Sometimes this wax overflows
  down the side of the candle stick and then
  solidifies or freezes (freezing does not always
  happen at 0C).