Chapter 2

Properties of Matter

Pure Substances

• Def: matter that always has exactly the same composition

• EX: table salt, table sugar• Every sample of a given substance

has the same properties because a substance has a fixed, uniform composition

• 2 categories: Elements, Compounds

Elements

• Millions of substances, about 100 elements

• Def: a substance that cannot be broken down into simpler substances

• Atom: the smallest particle of an element

• An element has a fixed composition because it contains only one type of atom

• No 2 elements contain the same type of atom

Examples of Elements

• Most elements are solid at room temp.• Aluminum: used in foil, soft drink cans• Carbon: used in pencils• Some elements are gases at room

temp. • Oxygen and Nitrogen: gases we

breathe• Only two elements are liquids at room

temp: bromine and mercury (both poisonous)

Symbols for Elements

• Each symbol has either 1 or 2 letters• First letter is capitalized, second is not• Carbon: C• Aluminum: Al• Gold: Au• Symbols were based on the Latin names

of the elements (“gold” is aurum)

Compounds

• Def: a substance that is made from two or more simpler substances and can be broken down into those simpler substances

• EX: H20 (Water)

• A compound always contains two or more elements joined in a fixed proportion.

Properties of Compounds

• The properties of a compound are different from those of the substances from which it is made.

• O2 gas at room temp, can fuel fires

• H gas at room temp, can fuel fires

• H20 liquid at room temp, does not burn

Mixtures

• Mixtures retain some of the properties of their individual substances.

• However, the properties of a mixture are less constant than the properties of a substance.

• The properties of a mixture can vary because the composition of a mixture is not fixed.

Heterogeneous Mixtures

• Hetero: different

• Genus: kind

• In a heterogeneous mixture, the parts of the mixture are noticeably different from one another

• EX: sand (when viewed with lens)

Distinguishing Mixtures

• Sometimes it is difficult to distinguish heterogeneous and homogeneous mixtures

• We may need to observe the properties of the mixture before we decide

• Based on the size of its largest particles, a mixture can be classified as a solution, a suspension, or a colloid.

Solutions

• Def: a mixture that forms when substances dissolve and form a homogenous mixture

• EX: windshield wiper fluid• One: they do not separate into

distinct layers over time• Two: none of the substances can

be trapped in a filter• Three: you can see through them

Suspensions

• Def: a heterogeneous mixture that separates into layers over time (usually cloudy).

• “Shake well before using” indicates a suspension

• EX: a mixture of water and sand• If mixed, they would form a suspension• However, the sand and water could be

separated by a filter

Colloids

• Def: contains some particles that are intermediate in size between the small particles in a solution and the larger particles in a suspension

• In other words, colloids are in-between solutions and suspensions

• Cannot be separated into layers; cannot be filtered

• EX: “store” milk, fog

Homogeneous Mixtures

• Homo: same• Genus: kind • In a homogeneous mixture, the

substances are so evenly distributed that it is difficult to distinguish one substance from another

• In fact, they appear to contain only one type of substance!

• EX: water in a swimming pool

Physical Properties

• Def: Any characteristic of a material that can be observed or measured without changing the composition of the substances in the material .

• EX: Viscosity, conductivity, malleability, hardness, melting point, boiling point, density

Viscosity

• Def: The tendency of a liquid to keep from flowing; resistance to flowing

• The higher the viscosity, the slower the liquid moves

• High viscosity: honey (thick liquids)• Low viscosity: vinegar (thin liquids)• Viscosity decreases as the liquid is

heated (i.e., it becomes thinner).

Conductivity

• Def: a material’s ability to allow heat to flow

• Materials that have a high conductivity are called conductors

• Good conductors are heat are usually good conductors of electricity

• Good conductors: metals• Bad conductors: wood

Malleability

• Def: the ability of a solid to be hammered without shattering

• EX: most metals

• Not malleable: ice cubes when struck by hammer; glass when hit by a baseball

Hardness

• How to compare: see which of the materials can scratch the other

• EX: a kitchen knife can scratch a copper sheet because stainless steel is harder than copper

• Hardest known material: diamond

Melting and Boiling Points

• Melting point: the temperature at which a substance changes from a solid to a liquid

• For water, this normally occurs at 0oC.

• Boiling point: the temperature at which a substance boils

• For water, this normally occurs at 100oC

Density

• Def: the ratio of the mass of a substance to its volume

• Density can be used to test the purity of a substance (silver, methanol, etc.)

Using Properties to Separate Mixtures

• Filtration: separates materials based on the size of their particles

• EX: using a strainer to make tea with tea leaves; drip method for brewing coffee

• Distillation: separates substances in a solution based on their boiling points

• EX: providing fresh water for submarines

Recognizing Physical Changes

• A physical change occurs when some of the properties of a material change, but the substances in the material remain the same.

• EX: water changing from a liquid to a gas

• EX: crumpling a piece of paper• EX: slicing a tomato • Size and shape are changed, but not

composition

Chemical Properties

• Def: any property that produces a change in the composition of matter

• Chemical properties can be observed only when the substances in a sample of matter are changing into different substances

• EX: flammability, reactivity

Flammability and Reactivity

• Flammability: a material’s ability to burn in the presence of oxygen

• EX: Materials that burn can be used as fuel, such as gasoline

• Reactivity: how readily a substance combines chemically with other substances

• EX: oxygen is highly reactive

Recognizing Chemical Changes

• Chemical changes occur when a substance reacts and forms one or more new substances.

• The best way to determine if a chemical change is taking place is to compare materials before and after reactions

• Evidence: (1) change in color; (2) production of a gas; (3) formation of a precipitate

Color Changes

• A change in color is a clue that a chemical change has produced at least one new substance.

• EX: Silver bracelets exposed to air will darken

• EX: As matches burn, they shrivel up and turn black

Gas Production

• EX: Mixing vinegar and baking soda produces carbon dioxide

• EX: Using baking powder in cake recipes—CO2 is produced, causing cake to rise

Formation of a Precipitate

• Any solid that forms and separates from a liquid mixture is called a precipitate.

• EX: curdling of milk• When an acid is added to milk,

proteins in the milk undergo a chemical change that alters their structure, causing them to stick together in clumps.

Chemical or Physical?

• Important Question: Are different substances present after the change takes place?

• If yes, it is a chemical change. • If no, it is a physical change. • In chemical changes, the

composition of matter changes; in physical changes, the composition of matter stays the same.