Error in Measurement• Some error or uncertainty always exists in

any measurement.

– skill of the measurer

– limitations of measuring instruments

How To Take Measurements

• For digital equipment (the type that shows you the numbers on a little screen), just write down whatever it says.

– For example, if the numbers say “1.20 grams”, you should record “1.20 grams” as your data.

• For analog equipment (it’s got some sort of scale with lines on it that you have to figure out for yourself), write down the answer you directly get from the equipment and add one digit that you estimate.

Recording Measurements

• Record the volume of liquid in the graduated cylinder to the correct number of digits.

Graphing RelationshipsGraphing Relationships• Many times there is a relationship that exists

between two quantities.

• Graphing allows us to better see the connection.

• Graphing involves two variables:

– Independent variable – variable you control.– Dependent variable – variable affected by the

independent variable

• Example: Pressure and Volume

– Independent variable – pressure– Dependent variable – volume

• When graphing, the independent variable is always placed on the x-axis and the dependent variable is placed on the y-axis.

Volume

Pressure

Relationship Between QuantitiesMany times changing one quantity can have an affect on another quantity.

There are two types of relationships:

Direct and Inverse

Direct – When one factor changes by x, the other factor changes by x. (If one factor doubles, the other factor doubles)

Example: Temperature and Volume of a gas

Graph of a direct relationship is a straight line with a positive slope.

Inverse – When one factor changes by x the other factor changes by 1/x. (If one factor doubles, the other factor decreases by 1/2)

Example: Pressure and volume of a gas.

The graph of an inverse is a curve.

Graphing Stuff• When making graphs in science, always use a line

graph. There are other kinds of graphs out there, but we don’t use them much in chemistry.

• To make a good line graph, you need to follow these rules:– Give it a title. A good title is “The dependence of

[the dependent variable] on [the independent

variable.]

– The x-axis is always the independent variable and the y-axis is always the dependent variable.

– Make sure you label both axes with units so people know what they’re looking at.

– Don’t connect the dots. It’s always good to make either a best fit line or a smooth curve when graphing data.

– Practice: Graph the following data of an unknown substance

Mass (g) Volume (mL)

50 50

200 200

350 350

425 425

500 500

What is your substance?

• Common Densities:

• Hydrogen- 0.00009 g/mL

• Carbon- 2.3 g/mL

• Water- 1.00 g/mL

• Sucrose- 1.6 g/mL

One Minute Paper• You have one minute to answer these two

questions concerning graphing and relationships between quantities.

– What was the most important thing you learned?

– What is still muddy?