2 Chemical Reactions2.4Iron 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. You will learn to recognize chemical changes and to distinguish them from physical changes.
3 Chemical Changes 2.43 What always happens during a chemical change?
4 2.4Chemical ChangesThe 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.
5 During a chemical change, the composition of matter always changes. 2.4Chemical ChangesDuring a chemical change, the composition of matter always changes.Recall that during a physical change, the composition of matter never changes.
6 2.4Chemical ChangesA magnet separates iron from sulfur. This is an example of a physical change.A mixture of iron filings and sulfur can be changed. a) A magnet separates the iron from the sulfur. b) Heat combines iron and sulfur in a compound. Classifying Which change is a chemical change? Explain.
7 2.4Chemical ChangesA mixture of iron and sulfur is heated. The iron and sulfur react and form iron sulfide. This is an example of a chemical change.A mixture of iron filings and sulfur can be changed. a) A magnet separates the iron from the sulfur. b) Heat combines iron and sulfur in a compound. Classifying Which change is a chemical change? Explain.
8 A chemical change is also called a chemical reaction. 2.4Chemical ChangesA 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.
9 Question!Is a change in phase a physical or a chemical change? Explain ans. It Is a physical change the composition does not change.
10 Iron and oxygen combine to form iron oxide Question!Iron and oxygen combine to form iron oxideList the reactants and the productsAns. Fe + O Fe2O3reactants Products
11 Recognizing Chemical Changes 2.4Recognizing Chemical ChangesRecognizing Chemical ChangesWhat are four possible clues that a chemical change has taken place?
12 Recognizing Chemical Changes 2.4Recognizing Chemical ChangesPossible clues to chemical change include:a transfer of energya change in colorthe production of a gasthe formation of a precipitate.
13 Clues to chemical changes have practical applications. 2.4Chemical ChangesA precipitate is a solid that forms and settles out of a liquid mixture.Clues to chemical changes have practical applications.Clues to chemical change often have practical applications. a) Bubbles of carbon dioxide gas form when a geologist puts acid on a rock that contains compounds called carbonates. b) When a test strip is dipped in urine, the color change is used to estimate the level of the sugar glucose in urine. c) One step in the production of cheese is a reaction that causes milk to separate into solid curds and liquid whey.
14 2.4Conservation of MassConservation of MassHow are the mass of the reactants and the mass of the products of a chemical reaction related?
15 2.4Conservation of MassDuring any chemical reaction, the mass of the products is always equal to the mass of the reactants.
16 2.4Conservation of MassThe 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.When the liquids in photograph A are mixed, they react. None of the products are gases. Analyzing Data How do you know that a reaction took place and that mass was conserved during the reaction?
17 Question!A 13.0 g sample of X combines with a g sample of Y to form the compound XY2.What is the mass of the reactants?Ans g
18 Question!Sodium Chloride can be formed by the reaction of sodium metal and chlorine gas. If 45.98g of sodium combines with an excess of chlorine gas to form g sodium chloride, what mass of chlorine gas is used in the reaction?Ans: 2Na + Cl2 2NaCl45.98g X gg g = 70.91g of chlorine
19 Which of the following are the same and which are different? Question!Which of the following are the same and which are different?A substance and a pure substancea heterogeneous mixture and a solutiona substance and a mixtureA homogeneous mixture and a solutionAns. A and d same b and c different
21 2.4 Section Quiz.1. Which of the following is a chemical reaction?melting of leaddissolving sugar in waterrusting of ironcrushing of stone
22 2.4 Section Quiz.2. Which of the following is NOT a possible clue that a chemical change is taking place?a change of statea change in colorproduction of a gasformation of a precipitate
23 2.4 Section Quiz.3. During any chemical change, the mass of the products isalways equal to the mass of the reactants.always greater than the mass of the reactants.always less than the mass of the reactants.sometimes different than the mass of the reactants.
24 Concept Map 2Concept Map 2 Solve the Concept Map with the help of an interactive guided tutorial.