PHYSICAL CHANGE vs CHEMICAL CHANGE

Physical change: Physical change is one that is temporary and reversible. No new substance is formed during a physical change.

Example 1 ICE - WATER - STEAM (They are all water!) Ice melting to water or water boiling. In all of these changes, you can get the original materials back! Any change of state is ‘Physical Change’

melting Melting is a change from solid to liquid. During melting heat is added.

evaporation Evaporation is a change from liquid to gas. During evaporation heat is added.

condensation Condensation is a change from gas to liquid. During condensation heat is removed.

freezing Freezing is a change from liquid to solid. During freezing heat is removed.

sublimation Sublimation is a change from solid to gas. During sublimation heat is added.

deposition Deposition is a change from gas to solid. During deposition heat is removed.

Example 2 A physical change may also involve changing the shape of the substance. Paper cut into pieces is still paper Cutting wood into pieces is still wood Molding a sculpture is still cement or marble! Cutting is a ‘Physical Change

Example 3 Dissolving of salt in water is a physical change. Salt can easily be recovered from salt water by evaporation. Similarly mixing of two substances that do not react with each other is an example of physical change. Mixing and dissolving is ‘Physical Change’

Chemical change Chemical change is one where a new substance is produced. The new substance is different from the original - has properties that are different than those of the starting materials.

Chemical change This change is permanent and cannot be easily reversed using simple physical experiments. Original materials cannot be obtained back easily.

Examples of chemical change raw egg becomes cooked egg cake mix becomes cake paper becomes ash steel becomes rust

Evidence of chemical change Colour change Odour Temperature change Production of light Formation of a new solid inside a liquid Production of a new gas – bubbles seen

Heat and chemical change As seen earlier, heat is used to bring about some changes in state. However, applying heat to a substance can also result in chemical changes

When baking powder is heated it undergoes a chemical change that results in the production of carbon dioxide gas (CO2) This is very useful in cooking/baking It is this chemical reaction that allows baked food to become light and fluffy Cooking is an example of chemical change

Combustibility is the ability of a substance to react quickly with oxygen to produce heat and light Burning is another example of a chemical change involving heat

When some substances are mixed, their reaction absorbs heat. a chemical cold pack