WHAT IS CHEMISTRY? Chapter Preview Questions

Transcription

1 WHAT IS CHEMISTRY? 1. A piece of iron is made up of a. iron molecules. b. iron compounds. c. iron atoms. d. iron salts. 1

2 1. A piece of iron is made up of a. iron molecules. b. iron compounds. c. iron atoms. d. iron salts. 2. By heat flow or by waves, energy can be a. carried from one place to another. b. created or destroyed. c. changed to light and heat. d. used to create nuclear reactions. 2

3 2. By heat flow or by waves, energy can be a. carried from one place to another. b. created or destroyed. c. changed to light and heat. d. used to create nuclear reactions. 3. To answer questions about the material world, scientists use math skills such as a. reproducibility. b. accuracy. c. precision. d. estimating. 3

4 3. To answer questions about the material world, scientists use math skills such as a. reproducibility. b. accuracy. c. precision. d. estimating. 4. The mass of an object can be measured using a a. meter stick. b. balance. c. graduated cylinder. d. thermometer. 4

5 4. The mass of an object can be measured using a a. meter stick. b. balance. c. graduated cylinder. d. thermometer. What is chemistry? Suppose you have a whole cookie. You break the cookie into tiny pieces and crumbs. Then, you weigh all the pieces and crumbs. How do you think the weight of the whole cookie compares to the total weight of all the cookie crumbs? 5

6 Chapter 2 Section 1 Describing Matter Section 1: Describing Matter What kinds of properties are used to describe matter? What are elements, and how do they relate to compounds? What are the properties of a mixture? 6

7 Matter Anything that has mass and takes up space Properties of Matter Hard, soft, rough, smooth, flammable, hot cold, liquid, solid, gas, etc. The properties and changes of any type of matter depend on its makeup Every form of matter has two kinds of properties: physical and chemical Physical Properties A property of a substance that can change without involving a chemical change: Table Talk: How many of these physical properties describe water? Density Hardness Texture Color Solid Liquid Gas Flexibility Luster Ability to conduct heat or electricity Magnetism Breaking into pieces 7

8 Chemical Properties A characteristic that describes a substances ability to change into different substances Examples: Natural gas can catch fire and burn in air Iron can combine with oxygen and form rust Silver will react with sulfur in the air and tarnish Baking bread (the yeast creates gas which causes bread to rise) Table Talk: Describe the difference between a physical property and and chemical property; use the picture provided to help you. 8

9 Elements What matter is made of The simplest substances An element is a pure substance that cannot be broken down into any other substance Early Greeks believed everything on Earth was composed of 4 elements : earth, wind, fire, water We now know that there are slightly more than 100 elements! Particles of Elements - Atoms Table Talk: How do you think the periodic table might be arranged? Elements are usually represented by one- or two- letter symbols: H = hydrogen Z = zinc He = helium Al = aluminum, etc. Atoms are the smallest possible pieces of elements Different elements have different properties because their atoms are different 9

10 Modeling an Atom Pencil lead is made of mostly graphite, a form of carbon. Two ways to model atoms used in this presentation are shown here for carbon. Modeling Molecules Models of molecules often consist of colored spheres that stand for different kinds of atoms. 10

11 Chemical Bond Water H 2 O Oxygen O 2 Table Talk: How many atoms are in each of these molecules? When atoms combine, this is a force of attraction. Many atoms combine to form molecules (groups of 2 or more atoms held together by chemical bonds) Carbon dioxide CO 2 Ratios A ratio compares two numbers. It tells you how much you have of one item compared to how much you have of another. For example, a cookie recipe calls for 2 cups of flour to every 1 cup of sugar. You can write the ratio of flour to sugar as 2 to 1, or 2:1. The chemical formula for rust, a compound made from the elements iron (Fe) and oxygen (O), may be written as Fe 2 O 3. In this compound, the ratio of iron atoms to oxygen atoms is 2:3. This compound is different from FeO, a compound in which the ratio of iron atoms to oxygen atoms is 1:1. 11

12 Practice Problem Ratios What is the ratio of nitrogen atoms (N) to oxygen atoms (O) in a compound with the formula N 2 O 5? Is it the same as the compound NO 2? Explain. N 2 O 5 contains two nitrogen atoms for every five oxygen atoms. Both N 2 O 5 and NO 2 are made up of only nitrogen atoms and oxygen atoms. However, the two compounds are different because NO 2 contains one nitrogen atom for every two oxygen atoms. Compounds Table Talk: Table sugar C 12 H 22 O 11 is made up of carbon, hydrogen, and oxygen. What is the ratio of elements in this compound? Most elements in nature are found combined with other elements A compound = a pure substance made of two or more elements chemically combined in a set ratio A compound is represented by a chemical formula. For example, CO 2 is carbon dioxide. There are 2 oxygen atoms combined with every carbon atom in this compound When elements are combined, they form compounds having properties that are different from the uncombined elements 12

13 Mixture Elements and compounds are pure substances Most materials we see are not pure Table Talk: What materials might you find in a potting soil mixture? A mixture is two or more substances (elements, compounds or both) that are together in the same place but not chemically combined Heterogenous mixtures Homogenous mixtures Table Talk: When you boil salt water, the water evaporates, leaving salt behind. Is salt water a compound or a mixture? Why? hetero means different In these mixtures, you can see the different parts homo means same In these mixtures, you can t see the different parts A solution is an example of a homogenous mixture See BrainPop compounds & Mixtures end 13

14 End of Section: Describing Matter 14