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Physical and Chemical Changes

Physical and Chemical Changes. Definitions. Physical changes: Alters neither the nature nor the characteristic properties of matter. The atoms and molecules of the substance do not change. Physical change.

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Physical and Chemical Changes

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  1. Physical and Chemical Changes

  2. Definitions • Physical changes: • Alters neither the nature nor the characteristic properties of matter. The atoms and molecules of the substance do not change.

  3. Physical change • When salts dissolve in water. This is a physical change because this reversible. We can evaporate the water to collect the salt. • When a liquid is boiled or frozen, this is a physical change because it is reversible. We only change the state of the solution (gas, liquid, solid). • Can change the appearance but not the identity of matter.

  4. Definitions • Chemical changes: • Alters the nature and characteristic properties of matter. The bonds between atoms are rearranged, and new molecules are formed. Not reversible.

  5. Chemical changes • Tend to be • exothermic: release heat/energy when they occur (ex.: combustion) • endothermic: absorb heat/energy to occur (ex.: photosynthesis)

  6. Remember Dalton’s atomic theory: Rule #5 Atoms are indivisible in chemical processes. That is, atoms are not created or destroyed in chemical reactions. A chemical reaction changes the way the atoms are grouped together • During a chemical reaction, atoms are neither created nor destroyed. This means the mass of the entire reaction does not change. Everything needs to be accounted for. • http://www.youtube.com/watch?v=J5hM1DxaPLw

  7. Types of chemical changes • Synthesis and decomposition • Precipitation • Acid-base neutralization • Oxidation • Combustion • Cellular respiration • Photosynthesis

  8. 1. Synthesis and decomposition • Synthesis: where two or more reactants combine to form a new product. N2 + 2O2 2NO2 • Decomposition: where a compound separates into two or more compounds or elements. • 2H2O  2H2 + O2

  9. What is this type of chemical change? C2H6 + O2 CO2 + H2O

  10. 2. Precipitation • When two solutions are combined, an insoluble (or slightly insoluble) substance sometimes forms. • NaCl(aq) + AgNO3(aq)AgCl(s) + NaNO3(aq) • http://www.youtube.com/watch?v=DITY2rXYU-I&feature=related

  11. 3. Acid-Base Neutralization • A reaction in which an acid and base react to form a salt and water. • Acid(aq) + Base(aq) salt(aq) + water(l)

  12. Examples • HF(aq) + KOH(aq) KF(aq) + H2O(l) • H2SO4(aq) + Mg(OH)2(aq) MgSO4(aq) + 2H2O(l) • 2HBr(aq) + Ca(OH)2(aq) CaBr2(aq) + 2H2O(l)

  13. 4. Oxidation • A chemical change involving oxygen or a substance with properties similar to those of oxygen. These reactions occur faster in light. Ex.: light causes the oxidation of oil, altering their taste. So we can’t eat them.

  14. Corrosion is an example of oxidation. Iron reacts with oxygen to produce rust.

  15. 5. Combustion • This is a form of oxidation that releases a lot of energy. • Fossil fuel + oxygen  carbon dioxide + water + energy • Excessive burning of fossil fuels releases a lot of carbon dioxide (CO2), which is primarily responsible for the greenhouse effect. • Some fossil fuels contain sulfur. When we burn those we release sulfur dioxide (SO2) which is partially responsible for acid rain.

  16. There are three conditions (triangle) for a combustion reaction to take place: • Fuel • Oxidizing agent (oxygen) • Ignition temperature • http://www.youtube.com/watch?v=GzQtEJboYaw

  17. Types of combustion • Rapid combustion: releases energy in a short period of time. • Ex.: log fire, burning candle, explosion of gasoline. • Spontaneous combustion: occurs with low ignition temperature. • Ex.: forest fires, very hot and dry. • Slow combustion: occurs over a very long time. Small amounts of energy are emitted over longer periods of time. • Ex.: fermentation, cellular respiration, metal corrosion…

  18. 6. Cellular respiration • Cellular respiration is an example of slow combustion. • Cellular respiration is a chemical change in which glucose and oxygen are used to generate energy. • This keeps our body temperature at around 37°C.

  19. 7. Photosynthesis • It is the opposite reaction to cellular respiration. • Plants consume water and carbon dioxide and energy (light) to form sugar (glucose) and oxygen. • This maintains the level of oxygen in the atmosphere. Why bother replanting plants? Any ideas?

  20. Easy rules… • A number of signs point to the occurrence of a chemical change: • Release of a gas (could also affect the smell) • Emission or absorption of heat • Emission of light • Change in colour • Formation of a precipitate (a solid in a solution)

  21. Sodium http://www.youtube.com/watch?v=Mx5JJWI2aaw&feature=player_embedded • Oscilating reaction: http://www.youtube.com/watch?v=Ch93AKJm9os&feature=player_embedded

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