1/3/2012 Chemical and physical changes

1. 1/3/2012 Chemical and physical changes • Write the date and title in your books and unscramble the learning objective. • LO: ot wonk eht ecnereffid neewteb lacisyhp segnahc dna lacimehc snoitcaer.

2. Chemical and physical changes Learning objective: To Know the difference between a chemical reaction and a physical change Success Criteria Level 6 – Explains observations Level 5 – Observes and records chemical and physical changes Level 4 – Describes the differences between reactants and products Level 3 – Can recall the words reactants and products

3. Physical changes and chemical reactions • Physical change: a change that occurs that does not change the identity of the substance. • Chemical reaction: a change that occurs causing the identity of the substance to change, so reactants form products.

4. How do we know a chemical reaction has happened? • Indicators of a chemical change: • Evolution of light • Evolution of heat • Evolution of a gas • Color change • Formation of a precipitate

6. Chemical reactions Physical changes V Chemical reactions Physical changes New substances are formed No new substances are formed Changes are usually NOT permanent ie Reversible Changes are usually permanent ie Irreversible Summary of differences between them

7. O  Mg O O O Mg Chemical Reactions – not easy to reverse • During chemical changes (reactions) different atoms become bonded. • Chemical bonds are not easy to break. • Because of this chemical changes are usually difficult to reverse. (It is difficult to change products back into reactants.) E.g. We cannot “un-burn” magnesium. Mg Mg magnesium oxide magnesium oxygen • Note: although many reactions give out heat once started they often need energy to get them going.

8. Description of a chemical reaction. • Using all of the key words in the box below, write a description of a chemical reaction. • Once you have finished try and think what we call total mass of reactants = total mass of products. Products reactants irreversible Bonds colour change bubbles Observation energy

9. Chemical and physical changes Learning objective: To Know the difference between a chemical reaction and a physical change Success Criteria Level 6 – Explains observations Level 5 – Observes and records chemical and physical changes Level 4 – Describes the differences between reactants and products Level 3 – Can recall the words reactants and products

10. Adding acid to alkali Fireworks Driving a car    Boiling the kettle frying an egg Ice cream melting  x x Dissolving sugar in tea Burning a candle Melting candle wax which of these are chemical reactions?  x x Lighting a match Making ice cubes Ironing clothes  x x Launching a space rocket Printing Baking a cake x  