what is chemistry?tognettisclass.weebly.com/.../9/5/57954417/8th_ch2_sect1.pdf1 chapter 2 the nature...

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Chapter 2 The Nature of Matter

WHAT IS CHEMISTRY?


Chapter Preview Questions

1. A piece of iron is made up ofa. iron molecules.b. iron compounds.c. iron atoms.d. iron salts.

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1. A piece of iron is made up ofa. iron molecules.b. iron compounds.c. iron atoms.d. iron salts.



2. By heat flow or by waves, energy can bea. carried from one place to another.b. created or destroyed.c. changed to light and heat.d. used to create nuclear reactions.

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2. By heat flow or by waves, energy can bea. 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.

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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 aa. meter stick.b. balance.c. graduated cylinder.d. thermometer.

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4. The mass of an object can be measured using aa. meter stick.b. balance.c. graduated cylinder.d. thermometer.


Suppose you have a wholecookie. You break the cookieinto tiny pieces and crumbs.Then, you weigh all the piecesand crumbs. How do you thinkthe weight of the whole cookiecompares to the total weight ofall the cookie crumbs?

What is chemistry?

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Chapter 2 Section 1Describing 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?

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Matter

Properties of Matter

Anything that has mass and takes up space

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:

DensityHardnessTextureColorSolidLiquidGasFlexibilityLusterAbility to conduct heat or

electricityMagnetismBreaking into pieces

Table Talk: How many of these physical properties describe water?

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Chemical PropertiesA characteristic that describes a substances ability to change into different substances

Examples:Natural gas can catch fire and

burn in airIron can combine with oxygen

and form rustSilver will react with sulfur in

the air and tarnishBaking 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.

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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

Elements are usually represented by one- or two- letter symbols:H = hydrogenZ = zincHe = heliumAl = aluminum, etc.

Atoms are the smallest possible pieces of elements

Different elements have different properties because their atoms are differentTable Talk: How do you

think the periodic table might be arranged?

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Modeling an AtomPencil “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 MoleculesModels of molecules often consist of colored spheres that stand for different kinds of atoms.

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Chemical Bond 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 CO2

Oxygen O2

Water H2O Table Talk:

How many atoms are in each of these molecules?


RatiosA 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 Fe2O3. 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.

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RatiosPractice Problem

What is the ratio of nitrogen atoms (N) to oxygen atoms (O) in a compound with the formula N2O5? Is it the same as the compound NO2? Explain.

N2O5 contains two nitrogen atoms for every five oxygen atoms. Both N2O5 and NO2 are made up of only nitrogen atoms and oxygen atoms. However, the two compounds are different because NO2 contains one nitrogen atom for every two oxygen atoms.


CompoundsMost 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, CO2 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

Table Talk: Table sugar C12H22O11 is made up of carbon, hydrogen, and oxygen. What is the ratio of elements in this compound?

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MixtureElements and compounds are pure substances

Most materials we see are not pure

A mixture is two or more substances (elements, compounds or both) that are together in the same place but not chemically combined

Table Talk: What materials might you find in a potting soil mixture?


Heterogenous mixtures

Homogenous mixtures

“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

Table Talk: When you boil salt water, the water evaporates, leaving salt behind. Is salt water a compound or a mixture? Why?

See BrainPop “compounds & Mixtures” end

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End of Section: Describing Matter

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