WHAT ARE PHYSICAL AND CHEMICAL CHANGES?

A CHEMICAL CHANGE IS A CHANGE IN WHICH SOMETHING NEW IS FORMED. THE STARTING MATERIALS CHANGE INTO AN ENTIRELY DIFFERENT SUBSTANCE OR SUBSTANCES. THIS NEW SUBSTANCE HAS A DIFFERENT CHEMICAL COMPOSITION THAN THE STARTING MATERIALS. EXAMPLES OF CHEMICAL CHANGES WOULD BE THE REACTION OF IRON WITH AIR (RUSTING} OR THE REACTION OF A METAL AND ACID.

CERTAIN OBSERVATIONS WILL INDICATE THAT A CHEMICAL CHANGE HAS OCCURRED. THESE ARE:

1. THE REACTION PRODUCED A CHANGE IN TEMPERATURE. THE TEMPERATURE COULD GO UP (GETS HOTTER) OR THE TEMPERATURE COULD GO DOWN (GETS COLDER). NOTE: REACTIONS THAT PRODUCE HEAT ARE KNOWN AS EXOTHERMIC REACTIONS WHEREAS REACTIONS THAT ABSORB HEAT ARE KNOWN AS ENDOTHERMIC REACTIONS.
2. FORMATION OF GAS BUBBLES.
3. FORMATION OF A SOLID (PRECIPITATE).
4. A CHANGE IN COLOR. YOU MAY START WITH TWO COLORLESS SOLUTIONS BUT WHEN THEY ARE MIXED YOU MIGHT SEE A BRIGHT PURPLE COLOR.
5. FORMATION OF A DIFFERENT ODOUR. THE STARTING MATERIALS MAY NOT SMELL AT ALL BUT AS YOU MIX THESE MATERIALS YOU MAY END UP WITH A BAD ODOUR OR A PLEASANT ONE.

SEE THIS CHEMICAL CHANGE!

PHYSICAL CHANGE IS A CHANGE IN WHICH THE SUBSTANCE CHANGES FORM BUT KEEPS ITS SAME CHEMICAL COMPOSITION.

EXAMPLES:

• CHANGES OF STATE ARE CONSIDERED TO BE PHYSICAL CHANGES.LIQUID WATER AND ICE (FROZEN WATER) ARE BOTH THE SAME SUBSTANCE, WATER.
• IF YOU FOLD A PIECE OF PAPER IT IS A PHYSICAL CHANGE.YOU HAVE CHANGED THE FORM OF THE PAPER BUT YOU HAVE NOT CHANGED THE FACT THAT IT IS PAPER.
• IF YOU HEAT AN IRON BAR UNTIL IT GLOWS RED HOT, IT IS STILL CHEMICALLY THE SAME IRON. THE IRON HAS NOT CHANGED INTO SOMETHING ELSE.
• IF YOU DISSOLVE SALT IN WATER YOU HAVE NOT CHANGED THE MATERIALS CHEMICALLY. YOU STILL HAVE SALT AND YOU STILL HAVE WATER. THIS CAN BE SHOWN IF YOU CHOOSE TO SEPARATE THE MIXTURE BY DISTILLATION OR THE SIMPLE EVAPORATION OF THE WATER. THE SALT WOULD BE THE RESIDUE AND THE WATER WOULD BE THE DISTILLATE. (CLICK HERE TO READ ABOUT DISTILLATION.)

ACTIVITY

IT IS IMPORTANT FOR YOU TO BE ABLE TO RECOGNIZE CHEMICAL CHANGES WHEN THEY OCCUR.

PERFORM THE ACTIVITY FOUND ON PAGE 178 AND 179 OF THE CHEMICALS IN ACTION TEXTBOOK.

NOTE: YOU MAY PERFORM THIS EXPERIMENT IN THE COMFORT OF YOUR OWN HOME KITCHEN. INSTEAD OF USING BEAKERS, USE SMALL GLASSES. TO MEASURE THE AMOUNTS OF YOUR MATERIALS YOU CAN USE MEASURING SPOONS. (1 TEASPOON = 4 ML). INSTEAD OF TEST TUBES YOU CAN AGAIN USE SMALL GLASSES. BE SURE TO CLEAN UP AFTER YOU HAVE FINISHED.

ORGANIZE YOUR OBSERVATIONS IN A DATA TABLE. COMPARE YOUR OBSERVATIONS WITH THE OBSERVATIONS MADE BY ANOTHER CLASSMATE. TOGETHER DECIDE WHICH REACTION(S) WERE EXAMPLES OF CHEMICAL CHANGE.

ASSIGNMENT

ON PAGE 178 OF THE CHEMICALS IN ACTION TEXTBOOK DO QUESTIONS 1 AND 2 FROM SECTION 9.1 "CHEMICALS IN ACTION".

COMPARE YOUR ANSWERS WITH THE ANSWERS MADE BY ANOTHER CLASSMATE. DISCUSS ANY DIFFERENCES THE TWO OF YOU HAVE AND DEVELOP A CONCENSUS. IF YOU CANNOT AGREE, THEN CONTACT YOUR TEACHER.

ASSESSMENT

SELF CHECK. CHECK YOUR RESULTS OF THE ACTIVITY AND OF THE ASSIGNMENT WITH MINE. YOU WILL HAVE TO REQUEST TO SEE THE ANSWER KEY. IF YOU HAVE ANY QUESTIONS PLEASE CONTACT YOUR TEACHER. SEE THE ANSWER KEY.

YOUR TEACHER MAY ASK YOU TO HAND IN YOUR ACTIVITY RESULTS OR YOUR ASSIGNMENT RESULTS BEFORE SENDING YOU THE ASNWER KEY. YOUR TEACHER MAY ALSO DECIDE TO EVALUATE YOUR WORK FOR MARKS.

YOU WILL BE ASKED TO CLASSIFY CHANGES AS CHEMICAL CHANGES OR PHYSICAL CHANGES ON YOUR UNIT TEST.