Chemical and physical changes Lesson 15 Chemical and physical changes

Key Terms Chemical property Physical property Chemical change Physical change Endothermic Exothermic Reactant Product Precipitate Combustion reaction

What You Will Learn: Let’s say you have two containers. They have the same volume, color, consistency, etc. How can you tell them apart? One way would be to examine their properties. A property is any characteristic that can be used to identify and describe matter. Every substance has a group of properties that you can observe or measure. Substances also have properties that determine how they will interact with other materials. In this lesson, you will learn about some of these different properties.

Physical Properties A physical property is a characteristic that can be observed or measured without changing the identity of a substance. Mass, volume, color, shape, hardness, magnetism, weight, and state are all physical properties. Every substance has its own set of physical properties, so physical properties can help you identify a substance. Density, melting point, and boiling points are examples of this.

Chemical Properties A chemical property is a characteristic that determines how a substance will interact with other substances during a chemical reaction. Every substance has its own set of chemical properties that can help identify the substance. However, chemical properties are not as easy to observe. A substance changes when its chemical properties are measured or observed. For example, flammability is a chemical property. To see how flammable a substance is, you have to burn it.

Physical vs. Chemical Changes Physical changes occur when substances or objects undergo a change without changing into another substance. Physical changes cause a change in properties such as volume, mass, or state. Physical changes can be reversed. Chemical changes are changes substances undergo when they become new or different substances. The identity of the substance changes. The new substance is known as the product. The substance in the reaction are the reactants. The new substance will have different chemical properties such as a different density, boiling point, color, or reactivity. For example, hydrogen + oxygen.

Physical vs. Chemical Changes Remember that after a: Physical change – object’s identity remains the same Usually involves energy (like heat) and states of matter. Chemical change – object’s identity changes Chemical changes happen on a molecular level

Chemical or Physical?

Chemical or Physical?

Chemical or Physical?

Chemical Changes Chemical changes are more difficult to identify than physical changes. We can’t see changes in molecules, but we can look for observable signs: color change bubbling and fizzing light production smoke, and presence of heat/change in temperature

Precipitate Another sign of a chemical change is the formation of a precipitate. A precipitate is a solid formed by a chemical reaction that takes place in a solution. A precipitate is a solid formed in a chemical reaction that is different from either of the reactants.

Combustion Reaction A combustion reaction is a chemical reaction in which oxygen combines with certain other substances, and heat and light are given off. Burning a log in a fireplace is a combustion reaction. When the wood burns, it is in the presence of oxygen in the air, and it gives off heat and light. Two new substances are formed-carbon dioxide and water. Ash, which is mostly carbon, is left behind. DO NOT TRY to create a combustion reaction on your own.

Two Types of Heat Reactions Thermal energy is energy generated and measured by heat. It is caused by the increased activity or velocity of molecules in a substance, which in turn causes temperature to rise accordingly. If thermal energy is released during a reaction, it is said to be exothermic. If thermal energy is absorbed during a reaction, it is said to be endothermic.

Law of Conservation of Energy Energy cannot be created nor destroyed. Energy can diminish in quality from useful to less useful. Energy can be transformed from one form to another, Heat goes from hot to cold.

Exothermic Reactions Chemical reactions that release energy in the form of heat, light, or sound. The products contain less energy than the reactants. Heat is lost to the surroundings.

Endothermic Reactions Absorbs energy Products contain more energy than the reactants Heat is absorbed from the surroundings A temperature drop is measured during the reaction

Let’s Watch-Do Not Try ANY of This at Home https://www.youtube.com/watch?v=UkBhW8Kj3r8 http://www.middleschoolchemistry.com/multimedia/chapter6/lesson7#thermite Or http://www.middleschoolchemistry.com/lessonplans/chapter6/lesson7

Can you identify the following changes as physical or chemical? Crushing a plastic bottle Melting of ice Digestion of food Formation of frost on your window Milk turns sour Fireworks

Let’s Try It! Baking Soda/Vinegar Lab Follow these directions as I put them on the board. You will measure the change in temperature of the reaction between baking soda and vinegar. You will take notes and turn them in for a grade. Question to Investigate: Does the temperature increase, decrease, or stay the same in the reaction between baking soda and vinegar? Form your hypothesis. Materials Vinegar in a cup Baking soda in a cup Thermometer

Materials Note: The amount of the solutions must be enough to cover the bulb of the thermometer. If they aren’t, use a smaller cup or clip the end of a plastic-backed thermometer so that the backing is flush with the bottom of the bulb. If you change the amount of anything, you must make a note of it to me on your notes. Place about 10 mL of vinegar in a small plastic cup. See note above. Place a thermometer in the vinegar. Read the thermometer and record the temperature on your lab notes. While the thermometer is in the cup, add a half a teaspoon of baking soda. Watch the thermometer to observe any change in temperature. Record the temperature after it has stopped changing.

Add this to your notes: What were the reactants and the products in this lab? What occurred? What type of heat reaction was produced? Complete your notes and turn in your paper. Expected Results If you begin with room-temperature vinegar, the temperature will decrease by about 7 °C. The amount of temperature decrease will vary. Carbon dioxide gas is also produced. Did the temperature increase, decrease, or stay the same when you combined baking soda and vinegar? Hopefully, the temperature decreased. What is the lowest temperature reached during your group’s reaction? When the temperature of a chemical reaction decreases, the reaction is called an endothermic reaction. The first part of the word, endo, means in or into and thermic has to do with heat or energy. So an endothermic reaction means that more energy goes into making the reaction happen than is released by the reaction. This leaves the reaction mixture at a lower temperature.

Review! Review these terms with your lab partner: Chemical property Physical property Chemical change Physical change Endothermic Exothermic Reactant Product Precipitate Combustion reaction