Physical and Chemical Change Chapter 7

Physical and Chemical Change

Physical and Chemical Change Physical change - a change in shape or state that does not produce a new substance Chemical change - a change that results in a new substance being formed Physical and Chemical Change

Physical Change: Changing State By applying heat, a substance will change from a solid to a liquid (melting) or from a liquid to a gas (evaporation). If cooled, a substance will change from a gas to a liquid (condensation) or from a liquid to a solid (freezing or solidification). A few substances change directly from a solid to a gas (sublimation) or a gas to a solid (deposition) Physical Change: Changing State

Physical Change: Changing State

Examples: melting and freezing Melting – When ice melts in your mouth or when butter is spread on hot toast the butter melts, but no new substances are formed. Freezing – When liquid water is put in the freezer to form ice cubes or when drops of liquid candle wax cool and become solid again, no new substances are formed. Examples: melting and freezing

Examples: evaporation and condensation Evaporation – Puddles of water dry faster in hot weather, but no new substances are formed. Condensation – When you breathe out on a cold day, ‘fog’ comes out of your mouth. This ‘fog’ is water vapour in your breath condensing to form a mist of tiny liquid water droplets. Examples: evaporation and condensation

Dry ice (solid carbon dioxide) undergoes sublimation at temperatures above -78.5oC. Dry ice is used in smoke machines in theatres and concerts. Iodine, graphite and naphthalene (mothballs) also undergo sublimation. Example: Sublimation

States of matter: changing state

Mixing Mixing two substances is a type of physical change. For example, mixing different coloured balls does not form new colours. Mixing

When a solid (solute) is dissolved in a liquid (solvent), they form a solution. A solution is a mixture. By removing the solvent by evaporation, then the solute particles form crystals. For example, sugar dissolved in water can be separated and so no new substances are formed. Dissolving

Dissolving Sugar

Chemical Change Signs of a chemical change include: colour change gas produced (bubbles) a solid (precipitate) forming in a clear solution energy is produced or observed in the form of heat or light. Chemical Change

When something changes colour, a chemical change has occurred. Burning a match, paper or toast produces a black powder, which is carbon. When iron is exposed to water and oxygen, it turns orange-brown because iron oxide (rust) is produced. Green apples ripening to turn red. Bleaching or dying clothes or hair. (Note - Mixing paint is not a chemical change because no new substances are formed). Colour Change

The production of gas indicates a chemical change. This may be in the form of bubbles, a new smell or smoke. Adding vinegar to bicarbonate of soda, bubbles of carbon dioxide are formed. Rotten eggs produce the gas hydrogen sulfide. (Note – Boiling and sublimation are not chemical changes). Formation of gas

Precipitation formation of a solid When two solutions are mixed a new solid is produced, which indicates a chemical change Pipes and drains form a precipitate when scale or lime deposits inside. Kidney stones precipitate in the kidneys and are very painful.

Energy is produced or observed When energy is produced or observed, in the form of heat, light or sound, this is a sign of chemical change. There are two types of reactions: Exothermic – releases energy (hot) Endothermic – absorbs energy (cold)

Exothermic Reactions Fireworks Sparklers Burning magnesium ribbon Lighting a match Respiration (the body converts food into energy) Hot pack Exothermic Reactions

Endothermic Reactions Cold pack Photosynthesis (plants absorb sunlight to produce glucose) Eating sherbet Endothermic Reactions

Exothermic and Endothermic reactions

Module 7.1 Review Questions (page 293) Q1,2,3,4,6,7,8,11,12