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Chemical Reactions Section 2.4 2.4 Chemical Reactions Iron is abundant, easy to shape when heated, and relatively strong, especially when mixed with carbon in steel. Over time, objects made of iron will rust if they are left exposed to air. Rusting is a chemical change. You will learn to recognize chemical changes and to distinguish them from physical changes in this section. Chemical Properties and Chemical Changes The ability of a substance to undergo a specific chemical change is called a chemical property. Chemical properties can be used to identify a substance. But chemical properties can be observed only when a substance undergoes a chemical change. Chemical Change- the composition or identity of the matter changes. A new substance or new substances are formed. This is different from a physical change because during a physical change the composition of the matter remains the same. Physical Change A magnet separates iron from sulfur. This is an example of a physical change 2.4 Chemical Changes A mixture of iron and sulfur is heated. The iron and sulfur react and form iron sulfide. This is an example of a chemical change. Chemical Changes A chemical change is also called a chemical reaction. One or more substances change into one or more new substances during a chemical reaction. A substance present at the start of the reaction is a reactant. A substance produced in the reaction is a product. Reactant Product Chemical Changes Possible clues to chemical change include: a transfer of energy the production of a gas a change in color the formation of a precipitate: http://www.google.com/imgres?imgurl=http://www.crscientific.com/article-thiosulfate10.jpg&imgrefurl=http://www.crscientific.com/article- redox3.html&h=497&w=396&sz=24&tbnid=KdghgpU_cncJ::&tbnh=130&tbnw=104&prev=/images%3Fq%3Dprecipitate%2Bimages&sa=X&oi=image _result&resnum=3&ct=image&cd=1 A precipitate is a solid that forms and settles out of a liquid mixture. Do these 4 indication of chemical change ALWAYS mean chemical changes?? Transfer of energy- Fire always means chemical change Heat, cooling, and light can indicate either type of change. To be certain of the type of change, the identity of the substance must be examined. Production of a gas- A gas produced when heat is added, such as during boiling is likely to be a physical change The production of gas without the addition of heat indicates a chemical change Do these 4 indication of chemical change ALWAYS mean chemical changes?? Change in color If blue and yellow are mixed and form green, this is likely a physical change. If 2 clear solutions are combined and a colored solution forms, this is a chemical change. In other words, if an unexpected color change occurs, a chemical change occurred. Precipitate If dirt settles out of a mixture of dirty water, this is not a true precipitate- this is a physical change. A true precipitate- one that forms from 2 solutions- always indicates a chemical change. Conservation of Mass The law of conservation of mass states that in any physical change or chemical reaction, mass is conserved. The conservation of mass is easily observed when a change occurs in a closed container.
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