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Published byAmy Carson Modified over 9 years ago
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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
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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!!!
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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
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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?
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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:
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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
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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.
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Homework Page 99 Questions 2 and 3 Congrats! The Chemistry part of Science 9 is done
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