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: H 2 0 (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. O 2  gas at room temp, can fuel fires H  gas at room temp, can fuel fires H 2 0  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 0 o C. Boiling point: the temperature at which a substance boils For water, this normally occurs at 100 o C

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—CO 2 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.