Chemisty_Honors_Ch._2

Section 1: -scientific method is a logical approach to solving problems by observing and collecting data, formulating hypotheses, testing hypotheses, and formulating theories that are supported by data I.Observing and collecting data -Observing is the use of the senses to obtain information. -descriptive (qualitative) or numerical (quantitative) -A system is a specific portion of matter in a given region of space that has been selected for study during an experiment or observation II. Formulating Hypotheses -Generalizations are statements that apply to a range of information -hypothesis, or testable statement. -The hypothesis serves as a basis for making predictions and for carrying out further experiments III. Testing Hypotheses -During testing, the experimental conditions that remain constant are called controls, and any condition that changes is called a variable IV. Theorizing -A model in science is more than a physical object; it is often an explanation of how phenomena occur and how data or events are related. -If a model successfully explains many phenomena, it may become part of a theory. -A theory is a broad generalization that explains a body of facts or phenomena. -Theories are considered successful if they can predict the results of many new experiments. Section 2: -Measurements represent quantities. -A quantity is something that has magnitude, size, or amount. A quantity is not the same as a measurement. I. SI Measurments -SI units are defined in terms of standards of measurement. The standards are objects or natural phenomena that are of constant value, easy to preserve and reproduce, and practical in size. II. SI Base Units -Quantities: A. Mass -mass is a measure of the quantity of matter -SI standard unit for mass is the kilogram -The gram (1/1000 of kilogram) ise useful for measuring mases of small objects -milligram (1/1000 of gram & 1/000 000 of kilogram) used for even smaller objects -Mass is often confused with weight -Mass is determined by comparing the mass of an object with a set of standard masses that are apart of the balance; does not depend on gravity -Weight is a measure of the gravitational pull on matter B. Length -SI standard unit for length is the meter. -To express longer distances, the kilometer, km, is used. One kilometer equals 1000 m. -To express shorter distances, the centimeter, is often used. one centimeter equals 1/100 of a meter. III. Derived SI Units -Derived units are produced by multiplying or dividing standard units. A. Volume -Volume is the amount of space occupied by an object -The derived SI unit of volume is cubic meters 1cm3=1mL -the cubic centimeter, cm3, is often used. 1m^3*100cm/1m*100cm/1m*100cm/1m=1 000 000cm^3 -When chemists measure the volumes of liquids and gases, they often use a non-SI unit called the liter. B.Density -Density is the ratio of mass to volume, or mass divided by volume. -Mathmatically: density= mass/volume or D=m/v -The SI unit for density is derived from the base units for mass and volume—the kilogram and the cubic meter; expressed kg/m3. -You will often see density expressed in grams per cubic centimeter, g/cm3, or grams per milliliter, g/mL -Density is a characteristic physical property of a substance. -It does not depend on the size of the sample because as the sample’s mass increases, its volume increases proportionately, and the ratio of mass to volume is constant. Therefore, density can be used as one property to help identify a substance. -That is because density varies with temperature. -Most objects expand as temperature increases, thereby increasing in volume. Because density is mass divided by volume, density usually decreases with increasing temperature. IV. Conversion Factors -A conversion factor is a ratio derived from the equality between two different units that can be used to convert from one unit to the other. -Because conversion factors are equal to 1, they can be multiplied by other factors in equations without changing the validity of the equations. You can use conversion factors to solve problems through dimensional analysis -Dimensional analysis is a mathematical technique that allows you to use units to solve problems involving measurements: quantity sought = quantity given × conversion factor A. Deriving Conversion Factors -You can derive conversion factors if you know the relationship between the unit you have and the unit you want. Section 3: I. Accuracy and Precision -Accuracy refers to the closeness of measurements to the correct or accepted value of the quantity measured -Precision refers to the closeness of a set of measurements of the same quantity made in the same way -measured values that are accurate are close to the accepted value. measured values that are precise are close to one another but not necessarily close to the accepted value. A. Percentage Error -Percentage error is calculated by subtracting the accepted value from the experimental value, dividing the difference by the accepted value, and then multiplying by 100. % error= value (experimental) - value (accepted) / value (accepted) X 100 %error=value(experimental)-value(accepted)/(value (accepted) )*100 B. Error in Measurement -Some error or uncertainty always exists in any measurement. -When you use a properly calibrated measuring device, you can be almost certain of a particular number of digits in a reading. II. Significant Figures -Significant figures in a measurement consist of all the digits known with certainty plus one final digit, which is somewhat uncertain or is estimated. -the term significant does not mean certain. A. Determining the Numer of Significant Figures -The significance of zeros in a number depends on their location