2.1 classifying matter · 2.1 classifying matter a pure substance is matter that always has exactly...

2.1 Classifying Matter

Each piece of your

clothing has a label that

recommends cleaning

methods. A 100%-cotton

shirt may need to be

ironed after washing. A

cotton and polyester

blend fabric may come

out of the dryer wrinkle

free. There is no cleaning

process that works for all

materials.


Why are elements and compounds classified

as pure substances?

Pure Substances


A pure substance is matter that always has

exactly the same composition.

• Table salt and table sugar are two

examples of pure substances.

• Substances can be classified into two

categories—elements and compounds.

Pure Substances


Every sample of a given substance has the

same properties because a substance has a

fixed, uniform composition.

Pure Substances


How do mixtures differ from pure

substances?

An element is a substance that cannot be

broken down into simpler substances.

Elements

An element has a fixed composition because

it contains only one type of atom.

No two elements contain the same type of atom.


Examples of Elements

•Some elements are solids at room

temperature. Most soft drink cans are

made from aluminum. Carbon is the

main element in the marks you make

with a pencil.

•The elements oxygen and nitrogen are

the main gases in the air you breathe.

•Two elements are liquids at room

temperature–bromine and mercury.

Elements


Symbols for Elements

Chemists use symbols of one or two

letters to represent elements. The first

letter is always capitalized. If there is a

second letter, it is not capitalized.

• C represents carbon.

• Al represents aluminum.

• Au represents gold. (The Latin name for

gold is aurum.)

Elements


Symbols for Elements

Symbols allow scientists who speak different

languages to communicate without

confusion. For example, nitrogen is azote in

France, stickstoff in Germany, and nitrógeno

in Mexico. But all scientists use N as the

symbol for the element nitrogen.

Elements


Aluminum, carbon, and gold are elements

that you can see in common objects, such as

cans, pencils, and rings. Mixtures containing

iodine are used to prevent and treat

infections.

Elements

Aluminum Carbon Gold Iodine



substances?

Compounds


A compound always contains two or more

elements joined in a fixed proportion.

Compounds

A compound is a substance that is made from two

or more simpler substances and can be broken

down into those simpler substances.


The properties of a compound differ from

those of the substances from which it is made.

• Water is composed of the elements hydrogen

and oxygen. Oxygen and hydrogen are gases at

room temperature, but water is a liquid.

• Hydrogen can fuel a fire, and oxygen can keep a

fire burning, but water does not burn or help

other substances to burn.

Compounds


Silicon dioxide is a compound found in most light-

colored grains of sand and in crystals of quartz. It is a

colorless, transparent solid. Yet, silicon dioxide is

made from a colorless gas (oxygen) and a gray solid

(silicon). In silicon dioxide, there are always two

oxygen atoms for each silicon atom.

Compounds

Silicon

Oxygen

Silicon dioxide



substances?

Mixtures


If you make salsa, a recipe can guide you. You

can use exactly the amounts in the recipe, or

you can adjust the ingredients to your own

taste. Salsa is a mixture. Each

batch may be slightly different.

Mixtures


The properties of a mixture can vary

because the composition of a mixture is not

fixed.

• Mixtures can retain some of the properties of their

individual substances.

• The properties of a mixture are less constant than the

properties of a substance.

Mixtures


Mixtures can be classified by how well the

parts of the mixture are distributed

throughout the mixture.

Heterogeneous Mixtures

In a heterogeneous mixture, the parts of

the mixture are noticeably different from one

another.

Homogeneous Mixtures

In a homogeneous mixture, the

substances are so evenly distributed that it

is difficult to distinguish one substance in

the mixture from another.

Mixtures


The sand is a

heterogeneous mixture of

different kinds of grains.

The spoon is stainless

steel, a homogeneous

mixture of iron,

chromium, and nickel.

Mixtures


Do the Contents of Two Cans of Mixed Nuts

Meet FDA Regulations?

According to Food and Drug Administration

(FDA) regulations, a can labeled mixed

nuts must contain at least four types of

shelled nuts other than peanuts. The mass

of each type of nut must be not less than 2

percent of the total mass and not more

than 80 percent of the total mass.

Mixtures


1. Comparing and Contrasting

How are the two brands of

mixed nuts alike? How are

they different?

Answer:

Mixtures


1. Comparing and Contrasting

How are the two brands of

mixed nuts alike? How are

they different?

Answer: Both

brands contain

the same types of

nuts but the

amount of each

type differs.

Mixtures


2. Calculating What is the percent by mass of each

type of nut in each can?

Answer:

Mixtures


2. Calculating What is the percent by mass of each

type of nut in each can?

Answer:

Brand A: 44.2% peanuts, 13.64% almonds,

16.79% Brazil nuts, 13.4% cashews, 5.77%

hazelnuts, and 6.21% pecans

Brand B: 54.8% peanuts, 8.90% almonds, 5.59%

Brazil nuts, 21.06% cashews, 4.82% hazelnuts,

and 4.82% pecans

Mixtures


3. Drawing Conclusions Do the contents of each

can meet the FDA regulations? Explain.

Answer:

Mixtures


3. Drawing Conclusions Do the contents of each

can meet the FDA regulations? Explain.

Answer:

Yes. Both brands contain more than four types of

nuts other than peanuts. The masses are within

the 2% to 80% range.

Mixtures


4. Inferring On the Brand A label, the nuts are listed

in this order: peanuts, Brazil nuts, almonds,

cashews, pecans, and hazelnuts. What do you

think determines the order?

Answer:

Mixtures


4. Inferring On the Brand A label, the nuts are listed

in this order: peanuts, Brazil nuts, almonds,

cashews, pecans, and hazelnuts. What do you

think determines the order?

Answer: The ingredients are listed in order by total

mass. The ingredient with the largest mass is

listed first.

Mixtures


What is the main difference among

solutions, suspensions, and colloids?

Solutions, Suspensions, and Colloids

The size of the particles in a mixture has an

effect on the properties of the mixture.

Based on the size of its largest particles, a

mixture can be classified as a solution, a

suspension, or a colloid.


Solutions

When substances dissolve and form a

homogeneous mixture, the mixture that forms

is called a solution.

Suspensions

A suspension is a heterogeneous mixture

that separates into layers over time.



Colloids

A colloid contains some particles that are

intermediate in size between the small

particles in a solution and the larger particles

in a suspension.

• Like solutions, colloids do not separate into

layers.

• You cannot use a filter to separate the parts of a

colloid.



These liquids represent three categories of

mixtures.

• Windshield wiper fluid is a solution.

• Muddy water collected from a swamp is a

suspension.

• Milk is a colloid.



Assessment Questions

1. Which of these substances is a compound?

a. copper

b. water

c. oxygen

d. carbon



1. Which of these substances is a compound?

a. copper

b. water

c. oxygen

d. carbon

ANS: B



2. Which of these groups of letters could be used as

a symbol for an element?

a. HF

b. Cm

c. Car

d. fe



2. Which of these groups of letters could be used as

a symbol for an element?

a. HF

b. Cm

c. Car

d. fe

ANS: B



3. Which of the following statements does not apply

to a compound?

a. It is made of two or more elements.

b. It has components that are joined in fixed proportions.

c. It can be separated into components by physical

methods.

d. It can be broken down into elements or other

compounds.



3. Which of the following statements does not apply

to a compound?

a. It is made of two or more elements.

b. It has components that are joined in fixed proportions.

c. It can be separated into components by physical

methods.

d. It can be broken down into elements or other

compounds.

ANS: C



4. How does a compound differ from a mixture?

a. A compound cannot be broken down into simpler

substances.

b. Compounds can be separated by physical processes

and mixtures cannot.

c. The composition of a mixture cannot vary.

d. A compound is made of two or more elements in fixed

proportion.



4. How does a compound differ from a mixture?

a. A compound cannot be broken down into simpler

substances.

b. Compounds can be separated by physical processes

and mixtures cannot.

c. The composition of a mixture cannot vary.

d. A compound is made of two or more elements in fixed

proportion.

ANS: D



5. Which of these materials is a heterogeneous

mixture?

a. air

b. seawater

c. sand

d. steel



5. Which of these materials is a heterogeneous

mixture?

a. air

b. seawater

c. sand

d. steel

ANS: C



6. Which of the following can be separated with a

filter?

a. colloids

b. compounds

c. solutions

d. suspensions



6. Which of the following can be separated with a

filter?

a. colloids

b. compounds

c. solutions

d. suspensions

ANS: D

2.2 Physical Properties

There are pitchers of ice water and lemonade

on a picnic table. How do you know which

liquid is in each pitcher? It’s easy! Lemonade

is yellow and has a tart taste that is hard to

miss. A yellow color and a tart taste are two

properties of lemonade.


What are some examples of physical

properties?

A physical property is any characteristic of a

material that can be observed or measured

without changing the composition of the

substances in the material.

Examples of Physical Properties

Viscosity, conductivity, malleability,

hardness, melting point, boiling point, and

density are examples of physical properties.


Viscosity

The tendency of a liquid to

keep from flowing is called its

viscosity.

• Thick liquids, such as corn

syrup and honey, have a high

viscosity.

• Thin liquids, such as vinegar

and water, have a low

viscosity.



Conductivity

A material’s ability to allow heat to flow

is called conductivity.

• Materials that have a high conductivity,

such as metals, are called conductors.

• Good conductors of heat are usually also

good conductors of electricity.



Malleability

The ability of a solid to be hammered without shattering is

malleability.

• Most metals, such as gold, are malleable.

• An ice cube or piece of glass breaks into small pieces when

struck with a hammer. Solids that shatter when struck are brittle,

not malleable.



Hardness

One material can scratch another material if it

is harder than the other material.

• A kitchen knife can scratch a copper sheet

because stainless steel is harder than copper.

• The material used to sharpen the knife blade

must be harder than stainless steel. Diamond is

the hardest known material.



This Tlingit carver is using an adze to carve a canoe from

Western red cedar. Red cedar is a relatively soft wood.

The adze is hard.



Melting and Boiling Points

The temperature at which a material changes

state is a physical property.

• The temperature at which a substance changes

from solid to liquid (melts) is its melting point.

• The temperature at which a substance changes

from liquid to gas (boils) is its boiling point.




Which of these substances are liquids

at room temperature (20C, or 68F)?

Answer:




Which of these substances are liquids

at room temperature (20C, or 68F)?

Answer: octane,

water, and

acetic acid



Density

The ratio of the mass of a substance to its

volume is its density.

• Density can be used to test the purity of a

substance.

• Silver has a density of 10.5 g/cm3. A coin with a

density of 9.9 g/cm3 is not made from silver, or it

contains substances in addition to silver.



How can knowing the physical properties of

matter be useful?

Physical properties are used to identify a

material, to choose a material for a specific

purpose, or to separate the substances in a

mixture.

Using Physical Properties


Using Properties to Identify Materials

A material can be identified by its

properties.

• Decide which properties to test.

• Do tests on a sample of the unknown

material.

• Compare the results with the data reported

for known materials.



Using Properties to Choose Materials

Properties determine which materials are

chosen for which uses.

• For example, shoelaces must be flexible,

that is they must be able to bend without

breaking.

• They must also be durable, that is, they

must be able to withstand repeated use.



Laces in hiking boots are usually made of

nylon or leather, not from wood.



What processes are used to separate

mixtures?

Filtration and distillation are two common

separation methods.

Using Properties to Separate Mixtures


Filtration

You can separate hot tea from loose tea

leaves by pouring the mixture through a

strainer. Filtration is a process that

separates materials based on the size of

their particles.



These students filter (sift) dirt through a

wire screen to locate small objects.

Particles of dirt are small enough to pass

through the holes, but objects such as

broken bits of pottery are too large.



Distillation

Sometimes all the particles in a solution

are small enough to pass through a filter.

Distillation is a process that separates

the substances in a solution based on

their boiling points.



The change of water from a liquid to a gas

during boiling is a physical change. A

physical change occurs when some of the

properties of a material change, but the

substances in the material remain the same.

Recognizing Physical Changes


During a physical change, the size and

shape of a material can change but not the

composition. Some examples include

• melting butter in a pan

• crumpling a piece of paper

• slicing a tomato



Some but not all physical changes can be

reversed. Braiding hair is a reversible

change. Cutting hair cannot be reversed.




1. Which of the following is not a physical property?

a. density

b. boiling point

c. flammability

d. conductivity



1. Which of the following is not a physical property?

a. density

b. boiling point

c. flammability

d. conductivity

ANS: C



2. Which of these materials is not malleable?

a. copper

b. aluminum

c. glass

d. gold



2. Which of these materials is not malleable?

a. copper

b. aluminum

c. glass

d. gold

ANS: C



3. In choosing a material for use as a wire to carry

electric current, which physical property would be

most important?

a. conductivity

b. malleability

c. hardness

d. boiling point



3. In choosing a material for use as a wire to carry

electric current, which physical property would be

most important?

a. conductivity

b. malleability

c. hardness

d. boiling point

ANS: A



4. Which of these statements best describes a physical

change in a pure substance?

a. The substance changes into one or more new substances.

b. Some of the properties of the substance change, but the material

remains the same.

c. The properties of the material do not change, and the material

remains the same.

d. The substance is separated into two or more simpler substances.



4. Which of these statements best describes a physical

change in a pure substance?

a. The substance changes into one or more new substances.

b. Some of the properties of the substance change, but the material

remains the same.

c. The properties of the material do not change, and the material

remains the same.

d. The substance is separated into two or more simpler substances.

ANS: B



1. The process of filtration uses the difference in

boiling points of substances to separate a mixture.

True

False



1. The process of filtration uses the difference in

boiling points of substances to separate a mixture.

True

False

ANS: F, distillation

2.3 Chemical Properties

How would you describe

these candles? Color,

hardness, and density

are physical properties

that you can use in the

description. You can also

say that the candles are

burning. The ability to

burn is not a physical

property. As a candle

burns, new substances

form.


When can chemical properties be observed?

Observing Chemical Properties


As a candle burns, its compounds

combine with oxygen in the air to form

water and carbon dioxide.

A chemical property is any ability to

produce a change in the composition of

matter. Flammability and reactivity are

two examples of chemical properties.



Chemical properties can be observed only

when the substances in a sample of matter

are changing into different substances.



Flammability

Materials that burn can be used as fuel.

Flammability is a material’s ability to burn in

the presence of oxygen.

Reactivity

The property that describes how readily a

substance combines chemically with other

substances is reactivity.



Rust forms when oxygen reacts with iron and water.

Rust is a brittle, reddish-brown compound. Because

iron is highly reactive, you would not choose iron to

make jewelry or coins.



Nitrogen has many uses that depend on

its low reactivity.

Researchers in Japan pump nitrogen

gas into the steel tanks that hold

seawater in ships. The nitrogen

displaces the oxygen dissolved in the

water and prevents rusting.



What observations might indicate that a

chemical change has occurred?

Three common types of evidence for a

chemical change are a change in color, the

production of a gas, and the formation of a

precipitate.

Recognizing Chemical Changes


The color change in a banana peel is caused by

chemical changes that are taking place in the cells

of the banana. A chemical change occurs when a

substance reacts and forms one or more new

substances.



A Change in Color

A change in color is a clue that a

chemical change has produced at least

one new substance.

• A shiny silver bracelet that is exposed to

air will darken.

• As a match burns, it shrivels up and turns

black.

• A new copper roof and an old copper roof

have different colors.



A new copper roof has a reddish color.



A new copper roof has a reddish color.

The green patina on an old copper roof is

a mixture of copper compounds.



Production of a Gas

When you mix vinegar with baking soda, bubbles of

carbon dioxide form immediately. A similar chemical

change happens when you use baking powder as an

ingredient in a cake recipe. Bubble of carbon dioxide

expand and cause the cake to rise.



Formation of a Precipitate

Any solid that forms and separates from a liquid

mixture is called a precipitate. When an acid is

added to milk, proteins in the milk undergo a

chemical change that causes them to stick together

in clumps and form a precipitate–cottage cheese.



What is the difference between chemical and

physical changes?

Is a Change Chemical or Physical?


When matter undergoes a chemical change,

the composition of the matter changes.

When matter undergoes a physical change,

the composition of the matter remains the

same.


Are different substances present after a

change takes place? If not, then the

change is physical, not chemical.


Even if you observe a color change, a gas, or a

precipitate, you cannot be sure that a chemical change

has taken place. When an iron horseshoe is heated, its

color changes from gray to red, but the iron is still iron.

That means the change is physical, not chemical.




1. Which of these properties is a chemical property

of sulfur?

a. yellow

b. flammable

c. brittle

d. soft



1. Which of these properties is a chemical property

of sulfur?

a. yellow

b. flammable

c. brittle

d. soft

ANS: B



2. Which of the following is not a common type of

evidence for a chemical change?

a. a change of state

b. a color change

c. a gas produced

d. a precipitate formed



2. Which of the following is not a common type of

evidence for a chemical change?

a. a change of state

b. a color change

c. a gas produced

d. a precipitate formed

ANS: A



3. You can be certain that a chemical change has

occurred when

a. there is a visible change.

b. the change is irreversible.

c. the temperature changes.

d. a new substance is formed.



3. You can be certain that a chemical change has

occurred when

a. there is a visible change.

b. the change is irreversible.

c. the temperature changes.

d. a new substance is formed.

ANS: D

2.1 classifying matter · 2.1 classifying matter a pure substance is matter that always has exactly...

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