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Describing Chemical Reactions Controlling Chemical Reactions Fire and Fire Safety

Properties and Changes in Matter
- Observing Chemical Change Properties and Changes in Matter Matter can undergo both physical change and chemical change.

Properties and Changes in Matter
- Observing Chemical Change Properties and Changes in Matter Chemical changes occur when bonds break and new bonds form.

Energy in Chemical Changes
- Observing Chemical Change Energy in Chemical Changes A student places two substances in a flask and measures the temperature once per minute while the substances react. The student plots the time and temperature data and creates the graph at left.

Energy in Chemical Changes
- Observing Chemical Change Energy in Chemical Changes Reading Graphs: What was the temperature in the flask at 4 minutes? When was the first time the temperature was at 6ºC? At 4 minutes the temperature in the flask was about 23ºC. The first time the temperature was 6ºC was at about 7 minutes.

Energy in Chemical Changes
- Observing Chemical Change Energy in Chemical Changes Calculating: How many degrees did the temperature drop between 2 minutes and 5 minutes? 20ºC

Energy in Chemical Changes
- Observing Chemical Change Energy in Chemical Changes Interpreting Data: Is the reaction endothermic or exothermic? Explain. The reaction was endothermic; it absorbed thermal energy from the reaction mixture, causing the temperature to drop.

Energy in Chemical Changes
- Observing Chemical Change Energy in Chemical Changes Inferring: At what temperature did the reaction stop? How can you tell? The reaction stopped at about 2ºC. You can tell because that is the lowest temperature reached.

Energy in Chemical Changes
- Observing Chemical Change Energy in Chemical Changes Drawing Conclusions: Suppose the temperature in the flask increased instead of decreased as the reaction occurred. In terms of energy, what kind of reaction would it be? Explain. If the temperature increased, the reaction would be exothermic; an exothermic reaction is one in which energy is released.

Properties and Changes of Matter
- Observing Chemical Change Asking Questions Before you read, preview the red headings. In a graphic organizer like the one below, ask a what or how question for each heading. As you read, write answers to your questions. Properties and Changes of Matter Question Answer What are physical properties of matter? Physical properties are characteristics that can be observed without changing one substance into another. What is the evidence for chemical reactions? The evidence for chemical reactions is the formation of new substances and changes in energy.

- Observing Chemical Change Links on Chemical Changes Click the SciLinks button for links on chemical changes.

End of Section: Observing Chemical Change

Describing Chemical Reactions
Cellular phone messages make use of symbols and abbreviations to express ideas in shorter form. Similarly, chemists often use chemical equations in place of words.

What Are Chemical Equations?
- Describing Chemical Reactions What Are Chemical Equations? Chemical equations use chemical formulas and other symbols instead of words to summarize a reaction.

Chemical Formulas - Describing Chemical Reactions
The formula of a compound identifies the elements in the compound and the ratios in which their atoms are present.

Conservation of Mass - Describing Chemical Reactions
The principle of conservation of mass states that in a chemical reaction, the total mass of the reactants must equal the total mass of the products.

Balancing Chemical Equations
- Describing Chemical Reactions Balancing Chemical Equations To describe a reaction accurately, a chemical equation must show the same number of each type of atom on both sides of the equation.

Balancing Equations Activity
- Describing Chemical Reactions Balancing Equations Activity Click the Active Art button to open a browser window and access Active Art about balancing equations.

Balancing Chemical Equations
- Describing Chemical Reactions Balancing Chemical Equations Magnesium metal (Mg) reacts with oxygen gas (O2), forming magnesium oxide (MgO). To write a balanced equation for this reaction, first write the equation using the formulas of the reactants and products, then count the number of atoms of each element.

Balancing Chemical Equations
- Describing Chemical Reactions Balancing Chemical Equations Balancing Chemical Equations: Balance the equation for the reaction of sodium metal (Na) with oxygen gas (O2), forming sodium oxide (Na2O).

Balancing Chemical Equations
- Describing Chemical Reactions Balancing Chemical Equations Balancing Chemical Equations: Balance the equation for the reaction of tin (Sn) with chlorine gas (Cl2), forming tin chloride (SnCl2).

Classifying Chemical Reactions
- Describing Chemical Reactions Classifying Chemical Reactions Many chemical reactions can be classified in one of three categories: synthesis, decomposition, or replacement.

Building Vocabulary - Describing Chemical Reactions
Using a word in a sentence helps you think about how best to explain the word. After you read the section, reread the paragraphs that contain definitions of Key Terms. Use the information you have learned to write a meaningful sentence using each Key Term. Key Terms: Examples: replacement Key Terms: Key Terms: Examples: closed system coefficient synthesis decomposition Examples: chemical equation In a closed system, matter is not allowed to enter or leave. A chemical equation is a short, easy way to show a chemical reaction. Replacement is the process in which one element replaces another in a compound or two elements in different compounds trade places. reactant A substance you have at the beginning of a reaction is a reactant. A coefficient is a number in a chemical equation telling you how many atoms or molecules of a reactant or product take part in the reaction. product A new substance produced in a reaction is a product. conservation of mass Conservation of mass means that during a chemical reaction, matter is not created or destroyed. Synthesis means combining two or more elements or compounds to make a more complex substance. open system In an open system, matter can enter from or escape to the surroundings. Decomposition means breaking down compounds into simpler products.

End of Section: Describing Chemical Reactions

Energy and Reactions - Controlling Chemical Reactions
All chemical reactions need a certain amount of activation energy to get things started.

Energy and Reactions - Controlling Chemical Reactions
Both exothermic and endothermic reactions need energy to get started.

Rates of Chemical Reactions
- Controlling Chemical Reactions Rates of Chemical Reactions The cells in your body (as in all living things) contain biological catalysts called enzymes. At the end of a reaction, an enzyme molecule is unchanged.

Relating Cause and Effect
- Controlling Chemical Reactions Relating Cause and Effect As you read, identify the factors that can cause the rate of a chemical reaction to increase. Write the information in a graphic organizer like the one below. Causes Increase in surface area Effect Increase in temperature Increased rate of reaction Increase in concentration of reactants Use of a catalyst

Data Sharing Lab Click the PHSchool.com button for an activity about sharing data for the Skills Lab Temperature and Enzyme Activity.

Exothermic and Endothermic Reactions
- Controlling Chemical Reactions Exothermic and Endothermic Reactions Click the Video button to watch a movie about exothermic and endothermic reactions.

End of Section: Controlling Chemical Reactions

Understanding Fire - Fire and Fire Safety
Three things are necessary to start and maintain a fire–fuel, oxygen, and heat.

Understanding Fire - Fire and Fire Safety
The fire triangle can be controlled in the grill. If any part of the fire triangle is missing, the fire will not continue.

Home Fire Safety - Fire and Fire Safety
If you know how to prevent fires in your home and what to do if a fire starts, you are better prepared to take action.

Using Prior Knowledge - Fire and Fire Safety
Before you read, write what you know about fire safety in a graphic organizer like the one below. As you read, write what you learn. What You Know A fire needs fuel to burn. A fire need oxygen to burn. All homes should have smoke detectors. What You Learned Fire is a result of a combustion reaction. The most deadly fires start with cigarettes. Baking soda can be used to put out small fires.

- Fire and Fire Safety Links on Fire Safety Click the SciLinks button for links on fire safety.

End of Section: Fire and Fire Safety

Endothermic reactions
Graphic Organizer Chemical reactions can be are written as Endothermic reactions Exothermic reactions Equations balanced with which require written using Coefficients Activation energy Chemical symbols Arrows and plus signs

End of Section: Graphic Organizer