2.1 Classifying Matter Each piece of your clothing has a label that recommends cleaning methods. A...

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.


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


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


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


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.

TrueFalse



1. The process of filtration uses the difference in boiling points of substances to separate a mixture.

TrueFalse

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.

The green patina on an old copper roof is a mixture of copper compounds.



Production of a GasWhen 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 PrecipitateAny 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.

Is a Change Chemical or Physical?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.

Is a Change Chemical or Physical?



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

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