"Chemical Vs Physical Changes" Return to list of pracs

Aim: This activity is used to make demonstrate how physical and chemical changes differ.

Equipment: The equipment includes...  

• container;
• salt;
• water;
• vinegar;
• baking powder;
• source of heat ((a) bunsen burner, tripod, beaker, heat and gause mats; or (b) hot plate).

Method: "Salt solution": Mix a spoon full of salt in half a cup of water (125mL) in the container until it dissolves. Observe that the mixture is now fully liquid. Place it on a hot plate or bunsen burner and bring it to boil. Continue until all the water has evaporated. Observe what remains behind.

"Vinegar & baking powder": Mix together a level spoon full of baking powder and half a cup of vinegar (125mL) in the container until it stops fizzing. Place it on a hot plate or bunsen burner and bring it to boil. Continue until all the liquid has evaporated. Observe what remains behind.

Background knowledge: Salt added to water is only dissolved. This means the particles of salt (eg. table salt is sodium chloride, chemical formula NaCl) are broken up by the interaction with water molecules. Table salt is a combination of a sodium metal atom and a chlorine atom connected together. Anything that dissolves does the same thing. It breaks the molecule up and they then float freely in the water. The word "solution" means a solid that has dissolved in some liquid.

Any liquid can only hold a certain amount of dissolved substances. You can keep mixing salt into water up to a point. Any more than this maximum and any extra salt will remain undissolved. At this point the liquid is termed saturated. The liquid can no longer support any more. If the solution is then heated and the water is slowly evaporated the amount of solid the liquid can hold becomes less and less. As a result, the original dissolved substance starts to become a solid again. If all of the liquid is evaporated the original dissolved substance is completely turned into a solid again. Therefore, this is only a physical change. Other physical changes include turning water into ice or steam and back again. Whenever the original substance can by some process be made again, the change is a physical change.

As for the vinegar and baking powder, a chemical reaction takes place. The vinegar is acetic acid (CH₃COOH : 2 carbons atoms, 2 oxygen atoms and 4 hydrogen atoms grouped together) and the baking powder is sodium bicarbonate (Na₂HCO₃: 2 sodium atoms, 1 hydrogen atom, 1 carbon atom and 3 oxygen atoms). When they are mixed together they react (CH₃COOH + Na₂HCO₃ + 3H2O--> 2NaCO₃ + CO2). The bubbles result from carbon dioxide gas (CO2) being produced in the reaction. This is a chemical reaction which can never return the original chemicals you started with. Therefore, if you try to evaporate the mixture it will completely evaporate.