chemistry lab report

Lab Report

Scientific experiments are designed to answer a research question. A procedure is developed and used to collect data that can then be analyzed to seek an answer to the research question. In this lab activity, a possible research question that can be investigated using the data collected is: Does heating a substance result in a physical or a chemical change?

In order to answer this question, the data collected during lab must be analyzed. Data are usually first described in terms of its composition (how many samples, what type of measurements, etc.) and then calculations using the data may be performed, graphs constructed, and any patterns in the data described.

Data Analysis

In this activity, you collected data over three steps in which different solid ionic compounds were described, heated using a Bunsen burner, and interacted with water. You measured mass, temperature and solubility.

First, you’ll examine the Class Data file and describe the data collected by several students. This file contains three worksheets of data for each step of the lab activity, labeled as “Step 1”, “Step 2”, and “Step 3”. Each worksheet can be accessed by selecting the ‘tab’ located at the bottom of the window.

In the following table determine the number of data sets (i.e., rows of data) that were collected during each step for each type of substance. You should sort the class data file using the same methods as covered in the Interim Assignment. Here, you should sort each worksheet by the substance name. An online tutorial on how to sort a spreadsheet can be found at the URL http://youtu.be/1-QKOolDfg8 (copy and paste the URL into a web browser).

Table 1. Number of Data Sets Collected for each Substance

You may choose to "hand count" the number of rows of data, but it is easier to use existing functions in Excel to complete this task. By selecting the range of cells that you wish to count, Excel will provide summary statistics in the lower right hand corner of the spreadsheet. These statistics usually include the functions COUNT, AVERAGE, and SUM. An online tutorial on how to count rows of data in a spreadsheet can be found at the URL:

http://youtu.be/rQbjWcxiD10 (copy and paste the URL into a web browser).

Step 2 Data Analysis

In step 2, samples of each compound were heated using a Bunsen burner. The data collected in lab included the Mass of Substance and the Mass of Residue after the last heating cycle. A new variable, Change in Mass upon Heating, can be calculated using the formula:

Change in Mass on Heating = Mass of Substance - Mass of Residue

For the substance potassium chloride, identify the largest data value for “Mass of Substance before Heating” and its corresponding value for “Mass of Residue after Last Heat” using the data from Step 2. Calculate the change in mass upon heating using these two data values.

Table 2. Potassium Chloride Data from Step 2

The values calculated for change in mass upon heating cannot be directly compared across different samples because they each started with a different initial mass of substance before heating. Using a percent scale is one way to compare values by imposing a common scale of parts per 100.

Percent Change in Mass on Heating = [(Mass of Substance – Mass of Residue) / (Mass of Substance) ] x 100

Calculate the percent change in mass upon heating for the same sample of potassium chloride used above.

Table 3. Change in Mass of Potassium Chloride from Step 2

You will need to analyze the entire set of data for KCl and KClO3. In the Class Data file on the Step 2 worksheet, add column headers to hold the calculated results for the two variables “Change in Mass upon Heating” and "Percent Change in Mass of Substance". While you may perform these calculations by hand, it is easier and there will be fewer calculation errors, if formulas are written in the spreadsheet.

In the table below, example data from Step 2 for potassium chloride are provided. In column D, a formula can be entered into the cells to calculate the change in mass upon heating. To enter a formula, start with the equal sign and then use cell references to point to the data values that are part of the calculation. For example, in cell D2 the change in mass is calculated by subtracting cell C2 from cell B2.

Instead of writing a formula several times down a column, you can write the formula in one cell and copy it to other cells. When you select a cell, it is highlighted and usually contains a small square in the lower right corner of the cell. To copy the formula in that cell, use the cursor to select the small square in the lower right corner and move the cursor down to other cells. The cell references in the formula will automatically change for each new row.

An online tutorial on how to copy a formula in an Excel spreadsheet can be viewed at the URL:

http://youtu.be/UTXzIKvTdGw (copy and paste the URL into a web browser).

Step 3 Data Analysis

In step 3, samples of the substance and the residue from Step 2 were interacted with water. The data collected in lab included Mass of Substance, Initial Temperature of Water, Final Temperature of Solution, Temperature Change of Solution, and Solubility in Water.

Data entry errors can occur and need to be corrected before any analysis. If possible, a formula should be used to calculate the values of new variables. For example, the Temperature Change of the Solution in the data file could be calculated using a spreadsheet formula rather than entered by hand.

Use cell references to write a formula to calculate the value for “Temp Change of Solution” located in cell F2 in the table above. The formula you write requires the correct cell references for the data in row 2 of the table.

In the Class Data file for Step 3, you can now copy the formula from cell F2 to other cells in that same column. An online tutorial on how to copy a formula in an Excel spreadsheet can be viewed at the URL:

http://youtu.be/UTXzIKvTdGw (copy and paste the URL into a web browser).

In addition to temperature data, the solubility in water of the substance and the residue was also measured in Step 3 and recorded using a scale that had the levels of soluble, nearly soluble, somewhat soluble and insoluble.

Determine the frequency (i.e., number of data rows) that occurred for each solubility level for both the original substance and residue that resulted after heating. Complete the following table for the data collected in Step 3 for potassium chlorate and potassium chloride. Again, you should sort the data so that you can more easily examine data values for “Solubility in Water”.

Use existing functions in Excel to complete this task by selecting the range of cells that you wish to count. Summary statistics, including COUNT, will be displayed in the lower right hand corner of the spreadsheet. An online tutorial on how to count rows of data in a spreadsheet can be found at the URL:

http://youtu.be/rQbjWcxiD10 (copy and paste the URL into a web browser).

Table 4. Solubility Frequency for Potassium Chloride, KCl

Table 5. Solubility Frequency for Potassium Chlorate, KClO3

Overall Analysis of the Data

A research question that can be answered using the data collected in this lab activity is:

Does heating a substance result in a physical or a chemical change?

A chemical change is one in which the chemical identity (i.e., the chemical formula) of the substance changes during the process. During a physical change, the chemical identity of the substance does not change.

In order to answer this research question, you need to identify overall patterns in the data for each substance. For example, in Step 2 you heated the substance to produce a residue. You then calculated the percent change in mass for each sample in the Class Data file. Use these data to determine the average percent change in mass over all samples for each substance.

Use existing functions in Excel to complete this task by selecting the range of cells that you wish to analyze. Summary statistics, including AVERAGE, will be displayed in the lower right hand corner of the spreadsheet. An online tutorial on how to view summary statistics in a spreadsheet can be found at the URL:

http://youtu.be/rQbjWcxiD10 (copy and paste the URL into a web browser).

Table 6. Average Percent Change in Mass upon Heating

In Step 3, you dissolved similar masses of the original substance and the residue in water and measured changes in temperature and solubility. It is expected that the data values for temperature change and solubility will vary with the data values for mass. That is, the magnitude of the temperature change increases as the mass increases. In order to compare the temperature and solubility data of the original substance and the residue, we need to select data that have similar masses.

In the Class Data file for Step 3, identify three sets of data in which the mass of the original substance and the mass of the residue agree to the hundredths place. For example, a data value of 0.1434 g for mass of original substance and 0.1490 g for mass of residue have the same value up to the hundredths place (0.14_ _ g).

In order to complete this task, you will need to sort the Class Data file in Step 3. In the following, select the criteria you used to sort the Step 3 data. Also indicate the order in which the criteria were used (1st, 2nd, 3rd) to sort the data. If no criterion was used, select “None”.

For the three sets of data you identified from the Class Data file for Step 3 for the original substance and the residue with similar masses, provide the corresponding data for Temperature Change of Solution and for Solubility in Water.

Table 7. Comparison between Original Substance and Residue after Heating of Temperature changes and Solubility

Now that the data from Steps 2 and 3 have been analyzed, it may be possible to make statements about overall patterns in the data for each substance.

For the substance potassium chloride, KCl, select the option (greater than, less than, equal to) that best completes each statement of an overall pattern in the data.

For the substance potassium chlorate, KClO3, select the option (greater than, less than, equal to) that best completes each statement of an overall pattern in the data.

In this lab activity, data were collected and then analyzed in order to answer a research question.

Research Question: Does heating a substance result in a physical or a chemical change?

In order to determine whether heating a substance results in a physical or chemical change, we need to compare the data collected on the original substance to the data collected on the heated residue.

Answer the research question for each substance by making a claim of whether heating resulted in a physical or a chemical change.

Claims are supported by an explanation. A scientific explanation has the following attributes: evidence (data), logic, and clarity. Your explanation should address these attributes.

You can use all of the data across steps 1, 2, and 3 as evidence for your choice. Your explanation should specifically cite data from the class data file that compares the original solid to its residue after heating.

Explain why you selected the type of change that occurs upon heating potassium chloride, KCl.

Explain why you selected the type of change that occurs upon heating potassium chlorate, KClO3.

Scientific Models

Scientists often use models to represent important properties of an object or a process. Models are drawn to illustrate the primary features, or components, of the object being modeled.

All models have a purpose in terms of what phenomena they aim to illustrate. For example, the purpose of the two models below is to illustrate a solid ionic compound at the particulate level.

Is Model #1 different from Model #2? Select one answer. Yes No

Briefly explain why Model #1 and Model #2 are the same or different.

Models are often altered to represent changes that occur during an experiment. For example, in Step 2 each of the solid ionic compounds was heated to produce a residue.

Suppose Model #2 was used to illustrate solid potassium chloride, KCl, at the particulate level as potassium ions K+ and chloride ions Cl-.

Explain and describe the changes to the model, if applicable.

Suppose Model #2 was used to illustrate solid potassium chlorate, KClO3, at the particulate level at potassium ions K+ and chlorate ions ClO3-.

Explain and describe the changes to the model, if applicable.