Teacher Notes

Percent Water in a Hydrate

Student Laboratory Kit

Materials Included In Kit

Aluminum potassium sulfate dodecahydrate, AlK(SO4)2•12H2O, 15 g*
Manganese(II) chloride tetrahydrate, MnCl2•4H2O, 15 g*
Zinc sulfate heptahydrate, ZnSO4•7H2O, 15 g*
Test tubes, borosilicate, 16 x 150 mm, 15
Weighing dishes, 15
*Crystalline hydrates, randomly distributed in vials A–C, 2 g per vial.

Additional Materials Required

(for each lab group)
Balances, centigram (0.01-g) precision, 3 (may be shared)
Bunsen burner
Clamp, uncoated, plain-jaw
Spatula
Support stand
Test tube clamp
Wire gauze or heat-resistant ceramic pad

Safety Precautions

Review the general safety rules for working with a Bunsen burner. Keep long hair tied back and make sure that hair, clothing and hands are a safe distance from the flame at all times. Never reach over the top of a burner flame. Instruct students how to light the burner and not to leave a lit burner unattended. Always turn the burner off when not in use. Use only heat-resistant, borosilicate glassware (e.g., Pyrex®). Heated materials remain hot for a very long time, and hot glass looks exactly the same as cold glass. Place the test tube on a wire gauze or on a heat-resistant pad to cool. The crystalline hydrates used in this lab are slightly toxic by ingestion and may be irritating to skin and eyes. Avoid contact of all chemicals with eyes and skin. Wear chemical-splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information. Remind students to wash their hands thoroughly with soap and water before leaving the lab.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The anhydrous residues may be disposed of in the solid waste according to Flinn Suggested Disposal Method #26a.

Lab Hints

  • The laboratory work for this experiment can be completed in a typical 50-minute class period. Please review the safety precautions for working with a Bunsen burner and hot glassware. Additional class time may be needed if students are not familiar with the safe use of a Bunsen burner. Set up a model apparatus before class to demonstrate the setup and the techniques for lighting a Bunsen burner and for handling a hot test tube.
  • Randomly distribute the unknown hydrates in labeled vials A–C, about 2 g per vial.
  • We tested a number of different hydrates for this experiment. The hydrates listed in the Materials section were selected because they gave excellent results and showed a range of color changes. Although the percent water is very similar for copper(II) sulfate pentahydrate and manganese chloride tetrahydrate, these hydrates may be easily distinguished based on their color. Copper(II) sulfate pentahydrate is the classic choice for the hydrate lab, but it is also the example that is featured in most textbooks. Please see the Supplementary Information in the Further Extensions section for the recommended heating times for each hydrate tested.
  • If you wish to use additional hydrates as unknowns, try magnesium sulfate heptahydrate (51% water) copper(II) sulfate pentahydrate (36% water) and barium chloride dihydrate (14.8% water). Using barium will require dedicated heavy metal waste disposal. Nickel (known carcinogen) and cobalt (possible carcinogen) chloride hydrates are colorful but are not recommended because of their toxicity. Do not use carbonate or nitrate salts. Carbonates decompose upon heating, releasing carbon dioxide and nitrates are strong oxidizers and will decompose and burn at high temperatures. Magnesium chloride hexahydrate (53.2% water) is highly deliquescent—it rapidly absorbs moisture from the air and does not give accurate results.

Teacher Tips

  • Is a hydrate a mixture or a pure substance? This is a good discussion question for a review of the properties of matter. How is heating a hydrate to remove water different from heating a salt solution to evaporate the water? There are distinct hydrate “stages” (e.g., monohydrate, hexahydrate) for many ionic compounds, where each stage represents a vapor pressure plateau. This suggests that hydrates are compounds (pure substances). It is possible, however, to stop the dehydration (heating) process at intermediate stages that do not represent distinct hydrates. This implies that the composition of the hydrates is variable (not constant), which is characteristic of a mixture, not a pure substance. Many hydrates (e.g., sodium thiosulfate pentahydrate, sodium acetate trihydrate) are ACS reagent chemicals (see Reagent Chemicals, 8th Edition, published by the American Chemical Society) and should therefore be considered pure substances. Solids corresponding to partial loss of water may be described as mixtures of a specific (higher) hydrate and the anhydrous salt.

Further Extensions

Accuracy and Precision—Percent Water in Hydrates

{13533_Extensions_Table_7}

Correlation to Next Generation Science Standards (NGSS)†

Science & Engineering Practices

Planning and carrying out investigations
Analyzing and interpreting data
Using mathematics and computational thinking
Constructing explanations and designing solutions

Disciplinary Core Ideas

MS-PS1.A: Structure and Properties of Matter
MS-PS1.B: Chemical Reactions
HS-PS1.A: Structure and Properties of Matter

Crosscutting Concepts

Scale, proportion, and quantity
Stability and change

Answers to Prelab Questions

  1. Briefly describe three general safety rules for working with a Bunsen burner.

    Keep long hair tied back, do not “reach over” a lit Bunsen burner flame to move or retrieve something, and never leave a lit burner unattended.

  2. The following data were obtained when a sample of barium chloride hydrate was analyzed as described in the Procedure section. Calculate (a) the mass of the hydrate, (b) the mass of water lost during heating and (c) the percent water in the hydrate.

    Mass of empty test tube: 18.42 g
    Mass of test tube and hydrate (before heating): 20.75 g
    Mass of test tube and anhydrous salt (after heating): 20.41 g
    Mass of hydrate (before heating) = 20.75 g – 18.42 g = 2.33 g
    Mass of water lost during heating = 20.75 g – 20.41 g = 0.34 g

    {13533_PreLabAnswers_Equation_2}
  3. The general formula of barium chloride hydrate is BaCl2•nH2O, where n is the number of water molecules. Calculate the theoretical percent water for each possible value of n—divide the sum of the atomic masses due to the water molecules by the sum of all the atomic masses in the hydrate, and multiply the result by 100. Complete the table.
    {13533_PreLabAnswers_Table_3}
  4. Compare the percent water in the hydrate (see Question 2) with the theoretical values calculated for different values of n. What is the most likely formula for barium chloride hydrate?

    The compound is barium chloride dihydrate, BaCl2•2H2O. The percent water in the hydrate is in excellent agreement with the theoretical value (within experimental error, which is ±0.01 g in each mass measurement). The percent water is too high for a monohydrate, and too low for a trihydrate (assuming the solid has been heated to constant final mass).

Sample Data

{13533_Data_Table_4}
See Further Extensions section for typical classroom data on all three unknown.

Answers to Questions

  1. Calculate (a) the original mass of the hydrate, (b) the mass of water lost upon heating and (c) the percent water in the hydrate.
    1. Mass of hydrate = 23.73 g – 21.74 g = 1.99 g
    2. Mass of water lost upon heating = 23.73 g – 22.86 g = 0.87 g
    3. Percent water in hydrate = (0.87 g/1.99 g) x 100% = 44%
  2. The unknown hydrate is one of the following substances. Complete the table to calculate the theoretical percent water for each hydrate (see Question 3 in the Prelab Questions).
    {13533_Answers_Table_5}
  3. What is the probable identity of the unknown crystalline hydrate? Explain your reasoning.

    The unknown is most likely zinc sulfate heptahydrate, ZnSO4•7H2O. The percent water in the hydrate is between the values for the other unknowns.

  4. Assume you have correctly identified the unknown. Calculate the percent error in the percent water analysis.
    {13533_Answers_Equation_3}
    {13533_Answers_Equation_4}
  5. Compare your results for the percent water in the hydrate with other groups that analyzed the same unknown. How precise are the results?

    See the Supplementary Information in the Further Extensions section for a summary of typical class results for all three hydrates. The range for the percent water in each hydrate is generally ±0.5%.

  6. Describe any changes in the color and appearance of the solid before and after heating.

    Zinc sulfate hydrate is white and does not change color when heated. The anhydrous salt is powdery, not crystalline. See the Further Extensions section for the color changes observed for the other possible unknowns.

  7. Is the unknown hydrate a mixture or a pure substance? Explain your reasoning.

    Based on the precision of the results obtained by different student groups for the same unknown, the composition of the hydrate appears to be constant. The hydrate is a pure substance.

  8. What is the difference between a physical change and a chemical change? Is the conversion of the hydrate to its anhydrous salt an example of a physical change or a chemical change? Explain.

    A physical change does not change the composition of a pure substance. A chemical change results in a change in the composition and the properties of a substance. The composition of the hydrate changed when it was converted to its anhydrous salt. Assuming that the hydrate is a pure substance, then heating it to remove water is a chemical change.

  9. Consider the following potential sources of error in this experiment. Predict whether the experimental percent water in the hydrate will be high (H), low (L) or unchanged (NC) as a result of each error.
    {13533_Answers_Table_6}

References

This laboratory has been adapted from Flinn ChemTopic™ Labs, Volume 2, Elements, Compounds and Mixtures, Cesa, I., Flinn Scientific, Batavia, IL. 2005.

Recommended Products

Item No. Description
AP7032 Percent Water in a Hydrate—Student Laboratory Kit
OB2142 Flinn Scientific Electronic Balance, 410 x 0.01-g
AP8354 Single Buret Clamp, Plain Jaw
AP8336 Spatula - Science Lab
AP8217 Test Tube Clamp with Finger Grips- Chemistry
AP1189 Wire Gauze Squares, Steel, with Ceramic Centers, 5" x 5"

Student Pages

Percent Water in a Hydrate

Introduction

When an ionic compound is crystallized from aqueous solution, the solid crystals may appear to be perfectly dry. When the crystals are heated, however, the mass of the solid may decrease as water is released from the crystal structure. The form or appearance of the crystals may change and, in some cases, the color of the crystals may also change. Compounds that contain water molecules as part of their crystal structure are called hydrates. Are hydrates pure substances, or are they simply “wet salts” (i.e., mixtures containing variable amounts of water)?

Concepts

  • Compounds vs. mixtures
  • Law of definite proportions
  • Hydrates
  • Chemical formula

Background

A compound is a pure substance—it has a fixed (constant) composition. The composition of a pure substance is the same throughout and does not vary from one sample to another. According to the law of definite proportions, a compound always contains the same elements in the same proportions by mass, regardless of the amount of the sample, where it was found or how it was prepared. A mixture, on the other hand, may contain variable amounts of different substances. The composition of a mixture is not constant.

A hydrate is a pure substance that contains water molecules embedded in its crystal structure. Heating a hydrate “drives off” the water molecules, and the solid that remains behind is called anhydrous, meaning “without water.” The chemical formula of a hydrate specifies the relative number of each kind of atom in a formula unit of the compound, as well as the number of water molecules bound to each formula unit. Calcium chloride dihydrate, which is commonly used as road salt, is an example of a hydrate. The chemical formula for calcium chloride dihydrate is CaCl2•2H2O. The “dot” in the chemical formula indicates that two water molecules (H2O) are attached or bound to the ions in solid calcium chloride (CaCl2) by weak chemical bonds. The water molecules in calcium chloride dihydrate can be removed by heating the hydrate (Equation 1).

{13533_Background_Equation_1}
The number of water molecules in a typical hydrate is characteristic of the particular salt and is usually a small whole number from 1 to 10. The chemical formula of a hydrate can be determined by analyzing the percent water in the hydrate—the ratio of the mass of water lost upon heating divided by the mass of the original hydrate. The formulas of some common hydrates and their anhydrous salts are summarized in Table 1.

Table 1. Formulas of Hydrates and Their Anhydrous Salts
{13533_Background_Table_1}

Experiment Overview

The purpose of this experiment is to analyze the percent water in a crystalline hydrate and to identify the hydrate from a list of possible unknowns. The solid hydrate will be heated to remove the water, and the percent water will be determined by measuring the mass of the solid before and after heating. The hydrate will be identified by comparing the percent water in the hydrate with the percent water calculated for a series of possible unknowns.

Materials

Crystalline hydrate “unknown,” 2 g
Balance, centigram (0.01-g precision)
Bunsen burner
Clamp, uncoated, plain-jaw
Matches or striker
Spatula
Support stand
Test tube, borosilicate, medium
Test tube clamp
Wire gauze or heat-resistant ceramic pad

Prelab Questions

  1. Briefly describe three general safety rules for working with a Bunsen burner.
  2. The following data were obtained when a sample of barium chloride hydrate was analyzed as described in the Procedure section. Calculate (a) the mass of the hydrate, (b) the mass of water lost during heating and (c) the percent water in the hydrate.

    Mass of empty test tube 18.42 g
    Mass of test tube and hydrate (before heating) 20.75 g
    Mass of test tube and anhydrous salt (after heating) 20.41 g

  3. The general formula of barium chloride hydrate is BaCl2•nH2O, where n is the number of water molecules. Calculate the theoretical percent water for each possible value of n—divide the sum of the atomic masses due to the water molecules by the sum of all the atomic masses in the hydrate, and multiply the result by 100. Complete the table.
    {13533_PreLab_Table_2}
  4. Compare the percent water in the hydrate (see Question 2) with the theoretical values calculated for different values of n. What is the most likely formula for barium chloride hydrate? Explain.

Safety Precautions

Review the general safety rules for working with a Bunsen burner. Keep long hair tied back and make sure that hair, clothing and hands are a safe distance from the flame at all times. Never reach over the top of a burner flame. Light the burner only as instructed by the teacher and do not leave a lit burner unattended. Always turn the burner off when not in use. Use only heat-resistant, borosilicate glassware (e.g., Pyrex®). Heated materials remain hot for a very long time, and hot glass looks exactly the same as cold glass. Place the test tube on a wire gauze or on a heat-resistant pad to cool. The crystalline hydrates used in this lab are slightly toxic by ingestion and may be irritating to skin and eyes. Avoid contact of all chemicals with eyes and skin. Wear chemical-splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

  1. Observe the color and appearance of the unknown hydrate and record the observations, along with the label code, in the data table.
  2. Set up a support stand with an uncoated, plain-jaw buret clamp.
  3. Adjust the height of the buret clamp so that a Bunsen burner can be moved freely under the test tube during heating (step 8) (see Figure 1).
    {13533_Procedure_Figure_1}
  4. Measure the mass of a clean, dry test tube to the nearest 0.01 g. Record the mass in the data table.
  5. Use a spatula to transfer about 2 g of crystalline hydrate to the test tube. Measure the combined mass of the test tube and the hydrate and record the mass in the data table.
  6. Pick up the test tube with a test tube clamp. Hold the test tube almost horizontally and gently tap the test tube until the hydrate is spread out over the bottom one-third portion of the test tube (see Figure 1). Note: Some of the hydrates will spatter when they are heated.
  7. Slide the test tube into the buret clamp so that the test tube is suspended at about a 60° angle, with the open end slightly elevated (see Figure 1). Note: Always point the test tube away from you and other students.
  8. Light the Bunsen burner and gently “sweep” the bottom of the test tube with the top of the burner flame. Continue to sweep the flame along the portion of the test tube containing the hydrate for about 4–5 minutes.
  9. Periodically brush the flame over the top of the test tube as well to evaporate any water droplets that collect there.
  10. Turn off the burner flame when there is no more water vapor escaping from the opening of the test tube and no further changes are observed in the appearance of the test tube contents. Note: For most hydrates, 5 minutes of heating (steps 8 and 9) will be sufficient. However, for a few hydrates, up to 15 minutes heating will be required.
  11. Allow the test tube to cool in the buret clamp for a few minutes. When the test tube is cool enough to handle, remove the test tube using a test tube clamp and set the test tube on the wire gauze.
  12. Allow the test tube to cool completely for 8–10 minutes. Note: The test tube is cool enough to weigh if you do not feel any radiant heat when placing your hand above the test tube.
  13. Measure and record the combined mass of the test tube and the anhydrous residue in the data table.
  14. Reheat test tube and contents an additional 2–3 minutes, allow to cool, and measure the combined mass again. If there is no change in mass between steps 13 and 14, all of the water has been removed from the hydrate.
  15. Observe the color and appearance of the anhydrous residue and record the observations in the data table.
  16. Dispose of the contents of the test tube as directed by the instructor. Clean the test tube.

Student Worksheet PDF

13533_Student1.pdf

†Next Generation Science Standards and NGSS are registered trademarks of Achieve. Neither Achieve nor the lead states and partners that developed the Next Generation Science Standards were involved in the production of this product, and do not endorse it.