3.3 Physical and Chemical Changes In physical changes: The appearance of a substance changes, but the chemical bonds holding the substance together do.

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Presentation transcript:

3.3 Physical and Chemical Changes In physical changes: The appearance of a substance changes, but the chemical bonds holding the substance together do not change. No new substance is formed ( it doesn’t change the identity of the substance) It is also fairly easy to reverse. Examples: melting, freezing, boiling Water, whether in the form of liquid or ice is still H 2 O. The chemical formula has not changed even if the appearance is different. See page 98

In chemical changes: New substances are produced in the process of breaking chemical bonds and forming new ones. They are difficult to reverse. ex: cooking is a chemical change can’t reserve an omelet egg back into original egg!!!

Evidence of chemical change: a)Colour change b)Heat, light, sound produced or consumed c)Bubbles of gas form d)Formation of a precipitate e)The change is difficult to reverse

Interesting example Rotting Apples When apples go bad (rot) this is a chemical change, scientists find that many new chemical substances are produced, such as a gas called “ethene” which is a chemical messenger that signals to other neighboring apples to start the process of rotting too! So one bad apple could spoil the bunch ! This is both a chemical and clearly a physical change Can you guess if it is endothermic or exothermic?

Energy Changes in chemical reactions In both physical and chemical changes, energy changes take place. Energy can either be released/given off or absorbed/taken in. Depending on the energy changes, we can have two types of reactions:

Two types of reactions Exothermic reactions involve the overall ___________ of energy in the form of heat and light. Energy exits Endothermic reactions involve the overall ___________ of energy Energy enters Campfire: exothermic Instant Cold Pack: Endothermic

Applications of Chemical Changes Some chemical changes present problems, while others provide opportunities and advantages Corrosion(rust): is a major problem for steel structures - by protecting or coating steel surfaces with a protective layer, the chemical reaction of iron with oxygen/water can be prevented. First nations people of the Pacific Coast have used smoking as a means of preserving food. Smoke causes chemical changes in meat that kill bacteria.

Homework Page 99 Questions 2 and 3 Congrats! The Chemistry part of Science 9 is done