Physical and Chemical Change

Chemical Changes Versus Physical Changes It is important to understand the difference between chemical and physical changes. No Change to Molecules When you step on a can and crush it, you have forced a physical change. However, you only changed the shape of the can. The molecules in the can are still the same molecules. No chemical bonds were created or broken. When you melt an ice cube (H2O), you have a physical change. In this example, you added enough energy to create a phase change from solid to liquid. Physical actions, such as changing temperature or pressure, can cause physical changes. No chemical changes took place when you melted the ice. The water molecules are still water molecules. A simple list of Physical Changes: -Any change of state -Dissolving to form a homogenous mixture, such as a solution -Crushing, crumpling, breaking, or other physical manipulation of the matter that does not change the molecules into new matter Changing the Molecules Chemical changes happen on a much smaller scale. While some experiments show obvious chemical changes, such as a color change, most chemical changes are not visible. The chemical change of hydrogen peroxide (H2O2) becoming water cannot be seen since both liquids are clear. However, behind the scenes, billions of chemical bonds are being created and destroyed. In this example, you may see bubbles of oxygen (O2) gas. Those bubbles are evidence of the chemical changes. Melting a sugar cube is a physical change because the substance is still sugar. Burning a sugar cube is a chemical change. Fire activates a chemical reaction between sugar and oxygen. The oxygen in the air reacts with the sugar and the chemical bonds are broken. Iron (Fe) rusts when it is exposed to oxygen gas in the air. You can watch the process happen over a long period of time. The molecules change their structure as the iron is oxidized, eventually becoming iron oxide (Fe2O3). Rusty pipes in abandoned buildings are real world examples of the oxidation process. A list - of observations that would indicate a Chemical Change: bubbles of gas are formed precipitate forms (a solid “suddenly” appears in a liquid) Colour changes (be careful…this can happen in physical changes as well) An odour is emitted - Release of energy as heat or light or sound   For  each  of  the  following,  write  “P”  for  physical  change  and  “C”  for  chemical   change.                                           1.    Crushing  a  can   2.    Iron  rusting   3.    The  combustion  (or  burning)  of  wood   4.    Dissolving  sugar  in  coffee   5.    Metabolism  of  food  in  your  body   6.    Melting  an  ice  cube   7.    Boiling  water   8.    Mixing  and  acid  such  as  HCl  and  a  base  such  as  NaOH   9.    Cooking  an  egg   10.    Mixing  sand  and  water   11.    Breaking  a  beaker   12.    The  digestion  of  carbohydrates  by  the  enzyme  amylase   13.    Shredding  paper   14.    Chopping  wood   15.    Mixing  baking  soda  and  vinegar   16.    Mixing  red  and  green  M&M’s   17.    Dry  ice  sublimating  to  create  a  spooky  Halloween  “fog”   18.    Baking  a  cake   19.    Electroplating  a  metal  (eg.    Silverware)   20.    Using  a  chemical  Battery         PHYSICAL  CHANGES   1. crushing a can 2. melting an ice cube 3. boiling water 4. mixing sand and water 5. breaking a glass 6. dissolving sugar and water 7. shredding paper 8. chopping wood 9. mixing red and green marbles 10. sublimating dry ice CHEMICAL CHANGES 11. Rusting of iron 12. combustion (burning) of wood 13. metabolism of food in the body 14. mixing an acid and a base, such as hydrochloric acid (HCl) and sodium hydroxide (NaOH) 15. cooking an egg 16. digesting sugar with the amylase in saliva 17. mixing baking soda and vinegar to produce carbon dioxide gas 18. baking a cake 19. electroplating a metal 20. using a chemical battery