Identification of an Unknown Metal



Identification of an Unknown Metal

Purpose: In this lab you will use two physical properties, density and specific heat capacity, to try to identify an unknown metal.

Introduction (Background Information): Like density, specific heat capacity is an intrinsic or intensive property that can be used to characterize or even identify a substance. Specific heat capacity, often simply called specific heat, is the amount of heat energy needed to raise the temperature of one gram of a substance by 1 (C. In the metric system, its units are J/g((C and is calculated according to the formula, q = mc(T, where q is the heat in Joules, m is the mass of the sample being heated in grams, and (T represents the temperature change in degrees Celsius. The specific heat capacity of water is 4.184 J/g((C. In this lab you will be measuring the specific heat of an unknown metal by measuring the amount of heat it transfers to a known amount of water. To do this you will use a calorimeter – in this lab, that’s a Styrofoam cup.

Safety Precautions:

Goggles must be worn. Please do not stir things with the thermometers. Use a stirring rod.

Known values for reference

|Metal |Specific Heat Capacity (J/g·ºC) |Density |

| | |(g/mL) |

|Lead |0.1276 |11.34 |

|Tin |0.2274 |7.31 |

|Silver |0.23502 |10.50 |

|Copper |0.38452 |8.94 |

|Nickel |0.4442 |8.902 |

|Titanium |0.5226 |4.94 |

|Aluminum |0.9025 |2.70 |

|Magnesium |1.024 |1.738 |

Part I: Density

Materials: Unknown metal, graduated cylinder, balance, tap water

We have done a lab before where we found the density of a metal using water displacement so I will let you figure out this procedure. Remember to record your data with units and to the appropriate number of decimal places. This means read your equipment to the greatest accuracy possible.

Table I. Data Table for Density Determination.

|Mass of dry metal (g) |Initial volume of water |Final volume of water (mL) |Volume of metal (mL) |Density (g/mL) |

| |(mL) | | | |

| | | | | |

1. Calculate the density of your metal. Show your work (with units). Record your result in Table I.

2. What unknown metal do you think you have? Briefly discuss how certain you are.

Part II: Specific Heat

Materials

3-5 pieces of unknown metal (be sure to dry it off if it is wet)

Balance

Hot Plate

Beaker (250 mL or larger)

Styrofoam Cup

Thermometer

Tongs

Tap water

Procedure:

Heating the metal:

Find and record the mass of the metal pieces.

Place the metal pieces in a beaker and add approximately 100 mL of tap water. Heat on a hot plate (set the hot plate to about 5). When the water is boiling, measure and record the temperature of the water. Do not let the thermometer rest on the bottom of the beaker, and don’t assume that the boiling temperature is exactly 100.0(C! Remove the thermometer and let it cool.

Keep the metal in the boiling water for at least 3 more minutes.

Preparing the calorimeter:

While the water is heating, zero the balance with the empty Styrofoam cup. Then add tap water to the Styrofoam cup so that the water level looks about 1 inch deep. Record the mass of the water.

Measure and record the temperature of the tap water.

Using the calorimeter – measuring specific heat:

After the metal piece has been in the boiling water for at least 3 minutes, remove the hot metal using tongs and quickly drop the metal into the Styrofoam cup. Immediately put the thermometer back in to the cup to monitor the water temperature. The temperature will rise quickly and then level off. Record the water’s highest temperature.

|Table II. Specific heat data | |

|Mass of metal | |

|Temperature of boiling water | |

|Initial temperature of hot metal | |

|Initial temperature of water in cup | |

|Final temperature of water + metal | |

|Mass of tap water in cup | |

|Specific Heat Capacity of water |4.18 J/g·ºC |

|Change in tap water temperature | |

|(use the absolute value of the difference) | |

|Change in metal temperature | |

|(use the absolute value of the difference) | |

3. Using the formula q=mc∆T, calculate the heat (q) absorbed by the water. Show your work!

(Think….should you use the mass of the water here or the mass of the metal? Which change in temperature should you use? The specific heat of water is given above.)

4. How much heat was lost by the metal? Explain how you know.

5. Using your rearranged formula, calculate the specific heat of your unknown metal. Show your work! (Think….. should you use the mass of the water here or the mass of the metal? Which change in temperature should you use?)

Analysis Questions

6. What do you believe is the identity of your unknown metal? Discuss your conclusion and how certain you feel. Use full sentences and compare your experimental values with the theoretical, “accepted values” listed on the first page.

7. Calculate the percent error for your density measurement and your specific heat measurement based on what metal you think you have. Please show your work!

% error = [pic][pic] x 100

Density Specific Heat

8. No experiment is perfect. What might be a source of error in this experiment and how would it have affected your results? (In others word explain why the source of error would make your experimental results larger or smaller in magnitude than the theoretical answer.)

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