L4: Physical and Chemical Changes

Lesson 4: Physical and Chemical Changes (Sophomore Level Chemistry)

Bell Work/Intro Activity:

List as many ways as possible that you could change this ball of Play Dough.

Give 2-4 mins for students to come up with as many as possible then call on them to share their answers.  List their answers on the overhead.  When listing answers, I will try to partition physical changes and chemical changes into different sections.   Most likely, most if not all answers will be physical changes.

Introduce the topic:  Today we will be discussing physical and chemical changes in matter.  While there are distinct differences between the two, the one thing they share in common is a change in energy.  Some changes require an input in energy to occur.  Ask for possible examples: melting ice, evaporating water→ adding heat causes the (physical) change.  Other changes give energy off, often in the form of heat:  burning a candle→ heat given off during this (chemical) change

So if both physical and chemical changes involve the change in energy, what makes them different?

A physical change changes matter from one form to another without changing its chemical properties. 

Ask the students to list some examples.  Make sure the once they are had ample time to make suggestions the list includes:  any change in physical state such as cutting, mashing, bending etc.;  changing the phase from solid to liquid, liquid to gas; dissolving something in water or other solvent.

Demo of physical change: Ammonium Nitrate in Water = Ice Pack.   Place 25 g of ammonium nitrate in an empty zip lock sandwich bag.  Pour in 50 mL of water and seal the bag attempting to remove as much air as possible.  The ammonium nitrate dissolves in water, and will feel cool like an ice pack, though no chemical changes have occurred. 

What about chemical changes?  If physical changes do not change chemical properties then….? A chemical change is a change that occurs when one or more substances change into entirely new substances with different properties.  Ask for examples and make sure the list includes: chemical reactions, burning, new colors or odors, production of gasses and solids.

Demo of chemical change:  Reaction of baking soda and vinegar.  Place about 30 mL of vinegar in an empty clean soda bottle.  Place a tsp of baking soda into a balloon.  Place end of balloon over soda bottle without pouring in baking soda until ready.  When ready, lift balloon so baking soda falls into the soda bottle.  Watch as balloon inflates from the release of CO₂.

Once demos are done, review examples by using power point slides.  (When viewing the actual slide show, you can’t see the answers til they have had a chance to answer) Can ask individual students or split the class into groups or teams.  Have students answer whether the example shown is physical or chemical changes.