Unit 1: Matter, Measurement, and unit conversions ≈

Big Idea Chemistry, being the science of matter, requires that we have systems for classifying, describing and measuring quantities of matter.

Topic 3: Experiments and Data Learning Goal: You will be able to describe the terms and concepts important to carrying out experiments and reporting data. Success Criteria: You will know you have met the learning goal when you can truthfully say: 1. I can differentiate between accuracy and precision. 2. I can describe the types of variables in an experiment. 3. I can present and interpret data in graphical form. Materials: rocks (inch wide-ish) and meter sticks (1 per table), magnetic darts (12), printed targets (4)

Success Criteria 1: I can differentiate between accuracy and precision We often use the terms accurate and precise interchangeably, but in science they mean very different things. Accurate refers to how close to the true value our measurements or our calculations are. Precise refers to how close our measurements or calculations are to each other. The best measurements are accurate and precise.

Success Criteria 1: I can differentiate between accuracy and precision How would you describe the archers who shot the arrows at the four targets on the right? Accurate and precise Accurate, but not precise Not accurate, but precise Neither accurate nor precise 2 3 4

Success Criteria 1: I can differentiate between accuracy and precision Task 1.3.1: (Class activity) Four people take three magnetic darts each and try to hit targets on the whiteboard. Sketch where they land on the provided targets, then explain who is the least precise, most precise, least accurate, and most accurate.

Success Criteria 1: I can differentiate between accuracy and precision Task 1.3.2: Using a rock and a meter stick (and a timer on your phone or computer), drop the rock from 1.0 m off the ground and time how long it takes to hit the ground. Perform five good trials and record your results in the table. Trial 1 Trial 2 Trial 3 Trial 4 Trial 5 Determine the average time by adding up all of your times and dividing by five. Determine how close your value is to the true value by subtracting your average time from the true value of 0.46 seconds. Determine your range by subtracting your lowest time from your highest time. Average Proximity to true value Range

Success Criteria 1: I can differentiate between accuracy and precision Task 1.3.2 (cont): Put your proximity to the true value and range on the whiteboard. What does the proximity to the true value tell you? Which group was the most accurate? What does the range tell you? Which group was the most precise?

Success Criteria 2: I can describe the types of variables in an experiment Imagine you are conducting an experiment to determine how temperature affects how fast gas is produced in a chemical reaction. You conduct several trials where the reaction takes place at different temperatures. The variable that you directly alter in the experiment (the temperature) is called the manipulated variable or the independent variable. The variable that you observe, the one that is affected by the changes you make (how fast gas is produced) is the responding variable or the dependent variable.

Success Criteria 2: I can describe the types of variables in an experiment. Trial 1 Trial 2 Trial 3 Trial 4 Temp. °C 20 50 80 100 Seconds taken to produce 5.0 mL of gas 13.2 9.3 7.5 4.0 You raised the temperature of the reaction vessel (manipulated variable), which resulted in gas being produced faster (responding variable).

Success Criteria 2: I can describe the types of variables in an experiment. For the results of your experiment to be trustworthy, everything but the manipulated and responding variables must be kept the same. This might include the length of time the reaction takes place, the equipment used to measure the volume of gas, and the equipment used to heat the reaction vessel. These are all controlled variables or constants. If you were to change them, then you couldn’t reliably conclude that the changes you made to the manipulated variable were indeed what caused the changes to the responding variable.

Success Criteria 2: I can describe the types of variables in an experiment. Imagine you want to test out a new diet to see if it can help you lose weight. However, instead of just going on the diet, you also start exercising three times a week and you start taking a diet pill every day. Imagine you lose 15 lbs over two months. Can you conclude that the diet caused your weight loss? No!!! You didn’t properly control the other variables (exercise and the diet pills) that could reasonably be expected to affect your weight, so any conclusion you make about the efficacy of the diet is unreliable.

Success Criteria 2: I can describe the types of variables in an experiment. Task 1.3.3 a) What type of variable doesn’t change between experimental trials, such as the equipment used to take measurements? b) What type of variable changes due to the intentional manipulation of another variable, such as how long a chemical reaction takes to run to completion? c) What type of variable does the experiment intentionally alter in order to observe the effects on another variable, such as the amount of time a sample is exposed to a heat source?

Success Criteria 2: I can describe the types of variables in an experiment. Task 1.3.3 (cont) d) List the variables from the experiment in the correct columns below.   Rebecca knows that heating up a glass of water will help table salt dissolve faster, but she wants to know how much faster. She does 12 trials of an experiment: three with 200 mL of water at 10°C, three with 200 mL of water at 30°C, three with 200 mL of water at 60°C, and three with 200 mL of water at 95°C. In each trial, she dissolves 5.0 grams of salt (NaCl) and stirs it vigorously for 3 seconds. She finds that on average, it takes the salt 13.2 seconds in the 10°C water, 9.2 seconds in the 30°C water, 7.5 seconds in the 60°C water, and 4.3 seconds in the 95°C water. Manipulated variable Responding variable Controlled variables (three examples)  

Success Criteria 3: I can present and interpret data in graphical form Data has to be analyzed before we know what it is telling us. One common tool used to analyzed data is to find a regression line (a line of best fit). This helps to reduce the uncertainty that is high among individual pieces of data and find the trend present in the data. Sometimes it is described as determining the “signal” amidst the “noise” of the data. Manipulated variable usually on x axis.

Success Criteria 3: I can present and interpret data in graphical form. It is from the regression line that conclusions about the data are generally reached. For example, in this graph, at x = 2, the data point corresponds to a y value of about 0.3, but the regression line gives a value of about 0.8. Since the regression line represents an average using many data points, 0.8 would be the y value we would use at x = 2. Manipulated variable usually on x axis.

Success Criteria 3: I can present and interpret data in graphical form. Task 1.3.4: Graph this data from an experiment on HCl concentration (manipulated variable on x axis), and battery current (responding variable on y axis). Remember to consider your range of data when deciding how to label your axes. Then draw a regression line and answer the questions. Drops of HCl added to battery solution Current produced by battery (mA) 1 3 2 4 8 12 5 14 6 19 7 22 Manipulated variable usually on x axis.

Success Criteria 3: I can present and interpret data in graphical form. What is the current produced with 2 drops of HCl? What is the current produced with 6 drops of HCl. Why do you think the regression line does not match the data points exactly. Why do we have more confidence in the value given by the regression line that we do for individual data points? Manipulated variable usually on x axis.

Wrap-up Task 1.3.5: Write in complete sentences eight things that you learned in this topic.