QUI_01_ Physico-chemical assays of solids
AF Unit 2: Basic physico-chemical experiments

2.1Determining the rate of dissolution

Source: en.wikipedia.org
Source: commons.wikimedia.org

In this unit you’ll work in the laboratory and you’ll see how solids behave when it comes to the dissolution process. But before that, you should know that there are smartphone applications that can help you during the entire formative activity. In addition to applications to help you with your English, there are tones of apps about chemistry that might be helpful as well. Go to google play (Android smartphones) or the App Store (IOs smartphones) and look for chemistry apps. The following apps are recommended:

  • “PSE HD”: Great interactive periodic table, always in the palm of your hand. Tap on an element to get all the information you need.
  • “Complete chemistry”: The best part of this app is the “dictionary”; it contains lots of chemical concepts explained.

Before going to the lab, let’s learn more about dissolutions. Sometimes, concepts like “solubility” and “rate of dissolution” lead to confusion. Although they are related to the dissolution process, they refer to different things. Read carefully the following definitions and ask your teacher if you don’t understand them:

HIGHLIGHTS
Solubility is the amount of solute that can be solved in 100 g of a solvent at a given temperature. Therefore, it is measured in “g of solute / 100 g of solvent”.
Rate of dissolution quantifies the speed of a dissolution process. That’s why we will measure it in “g of solute solved / second”.

Let’s focus now on the rate of dissolution. Imagine that you try to solve 3 grams of a certain solid in a solvent at 20 ºC, and it takes 10 seconds for the dissolution process to be completed. Then, imagine that you repeat the same experiment at 30 ºC and it takes only 5 seconds for the substance to be dissolved. Then, we could calculate the speed of dissolution:

Why is the speed different? The reason is we have changed the temperature. The factors that affect the rate of dissolution are:

  • Nature of the solvent and solute.
  • Temperature (and to a small degree, preassure).
  • Degree of undersaturation.
  • Presence of mixing.
  • Interfacial surface area.

Task 1: Let’s learn some words you are going to need to carry out your experiment. If you already know their meaning, it’s a good chance to learn how to pronounce them correctly. Take the following quizlet.

Use the different parts of the quizlet: “flashcards”, “learn”, “speller”, “test”, “scatter” and “race”.

/ Quizlet to learnimportantvocabulary for your first experiment.

Task 2: Connect each of the following examples to the factor affecting the rate of dissolution:

  • Sugar is easier to solve in hot milk rather than in cold milk. ______
  • Sugar dissolves faster in alcohol than in water. ______
  • Pulverized salt dissolves faster than non pulverized salt. ______

Task 3: Let’s see if you are able to understand laboratory procedures in English. Work in pairs and try to order the following sequence of operations to get the saponification procedure:

Task 4. Now you are ready to perform your experiment. You are going to work in pairs in the lab. It’s important that you follow the instructions given in the procedure. After the experiment, you are going to share your results with the rest of the class using a google drive document. In this way, all class will have access easily to all the results, and this will be a good source of information for your conclusions.

Procedure:

Attention! To be able to compare the dissolution processes, you have to use the same magnetic stirrer and the same flea (stir bar).

  • Experiment A: Take two beakers of 100 mL. Weight 1 g of non-pulverized CuSO4 in the first beaker. Pulverize a sample of CuSO4 in a mortar and weight 1 g of the pulverized CuSO4 in the second beaker. Add 50 mL of distilled water and measure the time that takes in each beaker for the sample to dissolve (use a stopwatch). Use the same stirring speed in both samples.
  • Experiment B: Weight 1 g of non-pulverized CuSO4 in a beaker of 100 mL. Add 50 mL of distilled water and stir. Measure the time that takes for the sample to dissolve. Then, repeat the experiment again with half of the stir speed and measure the time needed for the sample to dissolve.
  • Experiment C: Weight 1 g of non-pulverized CuSO4 in a beaker of 100 mL. Add 50 mL of distilled water at a temperature of 30 ºC and measure the time that takes for the sample to dissolve completely. Then, repeat the experiment with water at a temperature of 80 ºC and measure the time needed for the sample to dissolve completely.

Calculations: Calculate the rate of dissolution for each experiment. The units of this magnitude should be “g/hour”.

Now, all the groups will share their results. To do so, create a google document and share it with all your partners. Then, all groups should write their results in the same document. It is highly recommended that you show your results clearly in a table like this one:

Group 1 / Group 2 / Group 3
Experiment A (pulverized). Rate:
Experiment A (non-pulverized). Rate:
Experiment B (high speed). Rate:
Experiment B (low speed). Rate:
Experiment C (high temp.). Rate:
Experiment C (low temp.). Rate:
/ Google Drive

2.2Determining the melting point of a solid substance

Source: en.wikipedia.org

In your second experiment, you are going to determine the melting point of two solid substances.

To do so, you’ll be using a laboratory device called Thiele tube.

HIGHLIGHTS
Crystalline and pure solids have a characteristic melting point, which is the same as the freezing point.

Because the melting point of a solid is a characteristic property of the solid, sometimes it can be useful to identify a substance or to determine whether the substance is pure or not.

TO KNOW MORE
There is plenty of information on the Internet about melting and boiling points of substances. If you read the following website, you’ll discover there are plenty of things you didn’t know about theses physico-chemical processes.
/ Melting point and freezing point.
TO KNOW MORE
Check these cool videos about a process named “supercooling”. Some liquids can sometimes go lower than its freezing temperatures and not become solids.
/ Supercooled water– Explained!.
/ Instant Ice – Waterbending in real Life!

Task 5: In your next experiment, you’ll learn how to determine the melting point of a solid substance. In this way, you’ll be able to identify a compound. It’s also a good method to determine whether a substance is pure or not. But first, complete the following quizlet to learn some important vocabulary about the melting point determination. In case you already know the vocabulary, it’s a good time to learn its pronunciation:

Use the different parts of the quizlet: “flashcards”, “learn”, “speller”, “test”, “scatter” and “race”.

/ Quizlet to learn important vocabularyabout melting point determination

Task 6: Complete the following sentences related to the melting point determination. Check your answers with your partner:

  • Melting point: the t______at which a given s______will melt
  • A solid sample is usually pulverized in a m______before analysis.
  • A ______is a tube that has a diameter of hairlike thinness.
  • If we bring the end of a c______close to a flame, we can s______the end.
  • The m______p______of a s______can be useful to identify the substance.
  • We repeat the experiment two times to calculate the a______of the tworesults.

Task 7: Before going to the lab, try to identify each part of the Thiele tube assembly. Look closely at the image to learn the correct position of the heating fluid, the thermometer, the capillary tube and the flame.

WORDS TO PLACE IN THE BOXES: Capillary tube, thermometer, rubber band, sample, heating fluid, flame, Bunsen burner, thiele tube, stopper.

Image author: Javier Marín Cerdá

Task 8: Work in pairs and follow the lab procedure provided here. Then, share your results with your partners using a google drive document just as you did in task 4.

Aim of the experiment: Determine the exact melting point of two solid samples. Then, identify both substances using the Handbook of Chemistry and Physics.

Procedure:

  • Bring one end of a capillary close to the flame to seal one of its ends.
  • Pulverize the solid sample with a mortar.
  • Press the open end of the capillary against the sample. In this way, the sample will get into the capillary.
  • Hit the sealed end of the capillary tube against the table several times. In this way, the sample will go down to the sealed end of the capillary.
  • Tie the capillary to the thermometer using tape. Make sure the part of the capillary that contains the sample is at the same level of the thermometer bulb.
  • Put together the Thiele. To do so, ask your teacher.
  • Introduce the polyethylene glycol into the Thiele tube. The polyethylene glycol level has to be above the arm of the tube, as shown in the image.
  • Make an approximate determination by heating the fluid fastly. In this way, you’ll know the approximate melting point of the substance.
  • In the second determination, heat the fluid fastly until you are 15 ºC under the approximate melting point of the substance. Then, heat the fluid very slowly until you reach the exact melting point.
  • Determine the melting point of the substance again. Then, calculate the average of the two results.

Now, all the groups will share their results. To do so, create a google document and share it with all your partners. Then, all groups should write their results in the same document. It is highly recommended that you show your results clearly in a table.

/ Google Drive

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QUI_01_ Physico-chemical assays of solids. Materials / Unit 2