QUI_01_ Physico-chemical assays of solids
AF Unit 2: Basic physico-chemical experiments
2.1 Determining the rate of dissolution
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”.
Source: en.wikipedia.org
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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:
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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.
Key words: magnetic stirrer, stir bar, stopwatch, dissolution process, g/h, CuSO4, solute,
solvent and surface
Use the different parts of the quizlet: “flashcards”, “learn”, “speller”, “test”, “scatter” and “race”.
Quizlet to learn important vocabulary for your first experiment.
Task 2: Connect each of the following examples to the factor affecting the rate of
dissolution:
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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. ______________________
QUI_01_ Physico-chemical assays of solids. Materials / Unit 2
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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).
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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.
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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.
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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.
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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
2.2 Determining the melting point of a solid substance
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.
Source: en.wikipedia.org
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.
QUI_01_ Physico-chemical assays of solids. Materials / Unit 2
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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:
Key words: Thiele tube, melting point, capillary tube, sample, thermometer bulb, average,
seal, pulverize
Use the different parts of the quizlet: “flashcards”, “learn”, “speller”, “test”, “scatter” and “race”.
Quizlet to learn important vocabulary about melting point determination
Task 6: Complete the following sentences related to the melting point determination.
Check your answers with your partner:
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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 two results.
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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:
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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.
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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
QUI_01_ Physico-chemical assays of solids. Materials / Unit 2
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