Physical and Chemical Changes

Physical and Chemical Changes SNC1P Mr. M. Couturier

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

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

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.

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.

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.

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.

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.

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).

THINK • Would you consume a product which contains chlorine?

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).

THINK • Would you consume a product which contains sodium?

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?

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.

What’s 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.

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 (precipitate) • The change is difficult to reverse

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).

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).

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

Precipitate formation • You’ve 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).

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!!!

Irreversible reactions • Cases like burning wood is evidence that a chemical reaction has occurred as it is next to impossible to covert ashes back into wood.

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

Burning candle • If you said that the burning of a candle was a chemical change, then 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).

Burning candle • If you said that the burning of a candle was a physical change, then 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 0°C).

Physical and Chemical Changes