Scientific Measurement

Quantitative vs. Qualitative Observations Qualitative – observations made with adjectives “The water is clear and cool.” Quantitative – observations that include a measurement or other numeric data “There are 40mL of water.”

Precision and Accuracy Accuracy refers to the agreement of a particular value with the true value. (how close) Precision refers to the degree of agreement among several elements of the same quantity. (how repeatable)

Target (a) shows neither accuracy or precision. Target (b) shows precision, but not accuracy. Target (c) shows both accuracy and precision.

Uncertainty in Measurement A digit that must be estimated is called uncertain. The last digit in a measurement always shows uncertainty.

Liquids can form a meniscus in a graduated cylinder. Each individual line on the cylinder represents 0.1 mL accurately. This means you can record a measurement to the .0_, hundredth position. That position will include uncertainty, but gives a value that is more accurate than to the tenths. This measurement is 20.96 mL to 20.98 mL.

Two parts of measurements Quantity – indicates size or magnitude (how much?) Unit – tells us what is to be measured and compares it to a previously defined size (of what?) Measurements must have both a quantity and a unit to be valid.

Calculating Changes in Measurement Δ, greek letter for delta, means change in when used in equations. Final value – initial value = Δx

International System of Units Length – meter Mass – kilogram Temperature – Kelvin Energy – joule Amount of a substance – mole Electric current - ampere Volume – m3 Density – g/cm3 Weight - Newton SI units are defined by a system of objects or natural phenomena that are of constant value and are easy to reproduce used as a standard of measurement.

Derived Units Derived units are formed from a combination of other units. Examples include: m/s & km/hr (speed), cm3 & dm3(volume), J/g·°C (specific heat), g/mol (molar mass), g/cm3 & kg/m3 (density)

Commonly Used Prefixes in the Metric System Meaning Exponent mega (M) 1 000 000 106 kilo (k) 1000 103 hecto (h) 100 102 deka (da) 10 101 deci (d) 1/10 10-1 centi (c) 1/100 10-2 milli (m) 1/1000 10-3 micro (µ) 1/1 000 000 10-6 nano (n) 1/1 000 000 000 10-9 pico (p) 1/1 000 000 000 000 10-12

Measurement Tools Distance = Meter Sticks & Metric Tapes Volume = Graduated Cylinder Time = Stopwatch Mass = Balance Weight = Spring Scale

Mass vs. Weight Mass is the amount of matter in an object; weight is the effect of gravity on a mass. Mass is measured on a balance; weight is measured with a scale. Mass remains constant at all locations; weight varies with change in gravitational pull.

Volume Never measure in a beaker. They are for estimation only! 2. Place the graduated cylinder on a level surface and read the bottom of the meniscus. 3. Check the scale of the graduated cylinder. Different scales for different sizes! Use displacement to find the volume of irregular solids.

Mass Make sure the balance is on a level surface. Use the same balance in the same place for all parts of a procedure. 3. DO NOT MOVE A BALANCE ONCE IT IS ZEROED!

Length Rulers & meter sticks wear on the ends – start at a point other than zero. Choose the unit most reasonable for the item you are measuring – make sure you convert your number accordingly.

Conversion Factors Conversion factors are equalities written in ratio form: 1 km = 1000m 1km = 1000 m 1000 m 1 km Choose the format that allows you to cancel the original units and leave the new units. Ex. 2.5 km = ________ m You would choose 1000 m km

Conversion Factors Which of these equalities are correct? Make sure that you have a valid equality before writing your conversion factor. Which of these equalities are correct? 1 m = 1 x 10-6 µm 1 m = 1 x 106 µm 1 x 10-6 m = 1µm