Chemistry: An Introduction Chapter 1

1-2 What is chemistry? Because chemistry is the study of matter and how it interacts……..It is important to have a good definition of matter when studying chemistry Matter is anything that has mass and volume Mass – quantity of matter in an object (expressed in grams or kilograms and use a balance to measure it) Volume – amount of space an object occupies (expressed in L, mL or dm3 and use a graduated cylinder to measure liquids) What is the difference between mass and weight?

1-2 What is Chemistry? Chemical is any substance with a definite composition. Can you come up with any examples? Misconception: That chemicals differ from substances and materials in nature.

Have you ever wondered… “How does soap clean?” “Where does plastic come from?” “Why do we need sunscreens?”

Have you ever wondered… “Why do we get hot when we work out?” “Why does fire transform wood to ashes, but ashes can’t become wood again?”

The universe obeys a set of unwritten rules… … called natural laws.

Science is about discovering… … what those natural laws are.

How can we approach these questions? Evidence Theory

Does sugar dissolve faster in hot water? Can you make an educated guess?

“I think sugar dissolves faster in hot water.” Does sugar dissolve faster in hot water? “I think sugar dissolves faster in hot water.”

“I think sugar dissolves faster in hot water.” Does sugar dissolve faster in hot water? “I think sugar dissolves faster in hot water.” a hypothesis! hypothesis: a tentative explanation for something, or a tentative answer to a question.

Testing a hypothesis requires scientific evidence from experiments. experiment: a situation specially set up to observe how something happens or to test a hypothesis.

Objective observation: One experimental setup: Objective observation: The time it takes the sugar to dissolve changes in each trial.

“I think sugar dissolves faster in hot water.” One experimental setup: Does this support our hypothesis? YES NO MAYBE “I think sugar dissolves faster in hot water.”

“I think sugar dissolves faster in hot water.” One experimental setup: Does this support our hypothesis? YES NO MAYBE “I think sugar dissolves faster in hot water.”

Variables One experimental setup: Water temperature Amount of sugar What could affect dissolving? Water temperature Amount of sugar Amount of water Variables

variable: a quantity that is measured or changed in an experiment or observation. We don’t know if our hypothesis is correct because more than one variable changed at the same time! What could affect dissolving? Water temperature Amount of sugar Amount of water Variables

Objective observation: A second experimental setup: Objective observation: The time it takes the sugar to dissolve changes in each trial.

A second experimental setup: What could affect dissolving? Water temperature Amount of sugar Amount of water Control variables (do not change)

A second experimental setup: What could affect dissolving? Water temperature Amount of sugar Amount of water Experimental variable (changes)

“I think sugar dissolves faster in hot water.” A second experimental setup: Does this support our hypothesis? YES NO MAYBE “I think sugar dissolves faster in hot water.”

“I think sugar dissolves faster in hot water.” A second experimental setup: Does this support our hypothesis? YES NO MAYBE “I think sugar dissolves faster in hot water.”

variable: a quantity that is measured or changed in an experiment or observation. experimental variable: the single variable that is changed to test its effect. control variable: variables that are kept constant.

These numbers are different! The same experiment is repeated 3 times. The measurements are different each time. Is the investigator making a mistake?

Error and uncertainty Measurements always contain some uncertainty or error. In this case, error is not a mistake. error: the unavoidable difference between a real measurement and the unknown true value of the quantity being measured.

Accuracy

Accuracy Why take the average? To improve accuracy Comparing the actual measurement with the average gives you an estimate of the error

Assume the average is the true value. Accuracy How can we know the error if we don’t know the true value? Assume the average is the true value.

Does sugar dissolve slower in warmer water?

Does sugar dissolve slower in warmer water? Estimated error: +/– 4 seconds The largest difference between the data and the average is: 53 s – 48.7 s = 4.3 s The estimated error is about +/- 4 seconds Estimated error: +/– 4 seconds

Does sugar dissolve slower in warmer water? Estimated error: +/– 4 seconds 0.6 seconds Estimated error: +/– 4 seconds

Does sugar dissolve slower in warmer water? No, the difference in time (0.6 s) is not significant, because it is smaller than the estimated error (4 s). Estimated error: +/– 4 seconds 0.6 seconds Estimated error: +/– 4 seconds

The scientific method

Objectivity Stating only what happened Variables Experimental Control Repeatability Uncertainty Precision/accuracy