Observing Signs of a Chemical Reaction Helpful Hints Definitions l Physical Changes – do not alter the chemical composition of the components – Example – change of state (boiling, evaporating, sublimation, condensing, etc), dissolving, etc. l Chemical Changes – do alter the chemical composition Observing Signs of a Chemical Reaction Evidence that a chemical change has taken place 1. Formation of a Precipitate – solid formed when 2 liquids are added together l Some precipitates will be very obvious and sink to the bottom while other precipitates may only turn a clear solution cloudy Observing a Chemical Reaction 2. Color Change l Make sure that there is a definite color change l If the color of a solution fades as more liquid is added – it is not a definite color change l If 2 different colored solutions are mixed the color change can simply be the result of mixing Yellow and Blue make green – not a chemical change l Example Answer : Solution turned from colorless to yellow Observing Signs of a Chemical Reaction 3. Temperature Change l All chemical changes result in a change in energy. The temperature changes indicate the direction of the energy – released or absorbed. l Sometimes the temperature changes are too small to be easily noticed by touch, then thermometers can be used to accurately measure temperature changes and to calculate the energy change Observing Signs of a Chemical Reaction l Exothermic Reactions give off energy and generally cause the surroundings to get hotter (feels hotter) l Endothermic Reactions require energy and generally cause the surroundings to become cooler (feels cooler) Observing Signs of a Chemical Reaction 4.Formation of a Gas Bubbling and/or fizzing without the addition of heat l Sometimes bubbles are slow to appear l Do not confuse trapped air bubbles with the production of a gas l Trapped air bubbles can be tapped away but the bubbles will reappear if a gas is being produced Example Answer: bubbling occurred so gas given off Observing Signs of a Chemical Reaction 5. Odor Change The production of a distinctly different odor than the original materials is an indication of a chemical change.
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