Physical Science 8th grade (1).pdf

Figure 102 the chemical reaction between baking soda

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Figure 10.2: The chemical reaction between baking soda and and vinegar. reactants - the substances which are combined and changed in the chemical reaction. products - the new substances which result from a chemical reaction.
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204 U NIT 4 M ATTER AND C HANGE A chemical reaction rearranges the atoms of the reactants to form the new compounds of the products No new atoms are created! A similar reaction occurs when an effervescent tablet is dropped into water. The effervescent tablet also contains sodium bicarbonate. The reactant side of the reaction is: H 2 O + NaHCO 3 Can you figure out what the products are? Hint: this reaction creates a bubbling gas, too. The products of the reaction Figuring out the products The chemical reaction rearranges the same atoms in the reactants to become new compounds in the products. In this case, the 3 carbon, 5 oxygen, 5 hydrogen, and 1 sodium atoms are rearranged to become sodium acetate (NaC 2 H 3 O 2 ), water (H 2 O) and carbon dioxide (CO 2 ). Note that the same exact atoms in the reactants are rearranged to make the products . No new atoms are created! The whole reaction We can now see the whole reaction clearly. Only three chemical bonds actually change. The sodium ion jumps to the acetic acid to make sodium acetate. The rest of the bicarbonate breaks up into water and carbon dioxide. Since carbon dioxide is a gas, that explains the bubbles observed during the reaction.
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205 10.1 U NDERSTANDING C HEMICAL R EACTIONS C HAPTER 10: C HEMICAL R EACTIONS Chemical equations Understanding a chemical equation A chemical equation is an abbreviated way to show the exact numbers of atoms and compounds in a chemical reaction. Without drawing elaborate diagrams, we can write the baking soda and vinegar reaction as a chemical equation. The arrow shows the direction the reaction goes, from reactants to products. Conservation of mass Notice that there are the exact same number of each type of atom on the reactant side of the equation as there are on the product side. There are three carbon atoms in the reactants and three carbon atoms in the products. This demonstrates that chemical reactions conserve mass . The total mass of the reactants is equal to the total mass of the products because they are the same atoms! They have just been rearranged into new compounds. Demonstrating conservation of mass Once you understand atoms and reactions, conservation of mass is a perfectly obvious result. The mass is the same because the atoms are the same atoms . Of course, demonstrating the conservation of mass in this reaction is tricky because one of the products is a gas! It took a long time before people realized that mass is conserved because they were fooled by their own measurements. If you compared the mass of the reactants and products as shown in Figure 10.3, what do you think you will find? Can you think of a way to do the experiment so that no mass escapes being measured?
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