Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.
Do you have a mysterious solution sitting around? Learn to identify it using one of the following methods: through precipitation, using conduction, using a flame test, using an acid-base indicator and observing the color of the solution.
What do Sherlock Holmes, Perry Mason, Nancy Drew and chemists all have in common? They are all detectives. You might agree on the first three, but chemists? Yep, it takes some sleuthing to figure out chemistry sometimes.
For example, you might get two test tubes mixed up, or forget the reactant you added to your beaker. By the way, reactants are the substances that are about to undergo a chemical reaction, whereas products are the result from the chemical reaction.
Don't worry, you are about to undergo some serious chemistry detective training, and by the end of this lesson, you will be an expert on several test methods, including flame testing, conduction and many more! But we don't have all day, so let's get started!
Before you have your first chemistry detective lesson, let's get a few more words under our belt. We'll be using the word solution a lot, so it's worth noting a solution contains a solute, which is the lesser ingredient, and a solvent, which dissolves the solute. For example, if you wanted to make a solution of salt water, you would add some solute, or salt, to some water, or the solvent. For this lesson, when a substance can dissolve in water it's called soluble, and when it can't it's called insoluble. When a chemical is dissolved in water, it's called an aqueous solution. Okay, I think you're ready.
Your first lesson involves precipitation, and I don't mean rain! This type of reaction occurs when you have an aqueous solution where positively and negatively charged ions combine to form an insoluble product, called a precipitate. You can actually see precipitates as a solid in the solution. And in case you're wondering, ions are just atoms that have a charge.
So, your first lesson is to help Sarah, a chemistry student. While in lab, she got her test tubes mixed up. She knows there's a transition metal in one and a halide in the other, but can't figure out which is which.
Okay, I think you're ready. You can identify some chemicals that are in an aqueous solution by adding additional chemicals and observing what precipitate forms. For transition metals, we often use sodium hydroxide.
So, go ahead and take some samples from your mysterious test tubes and then add sodium hydroxide to see which one has a transition metal. You notice that in one of the test tubes, a white precipitate forms, but what does that mean? Let's take a look at this table.
Color of Precipitate
*The numbers after iron just denote the charge of the ion
Unfortunately, the only way to learn this stuff is to memorize it. Now would be a great time to pause the video and copy down this table so you can use it to study later. You can do this for other tables in this lesson too!
Since only one of the test samples had the white precipitate, you know which sample contained the transition metal. You were even able to figure out the name of the transition metal: zinc!
Great job, but you're not done yet. Let's use a precipitation reaction to figure out which halogen is in the halide. This time you add silver nitrate to the sample and you see that the precipitate that forms is yellow. But which halide does the yellow precipitate belong to? Let's take a look at this table.
Color of Precipitate
So, it looks like you have iodine. Fantastic!
You're ready for your second lesson. This one is on conduction, which means electricity is transmitted through a solution to see if it can carry an electrical current. This time, Adam wants to see if there are any ions in his drinking water. Before we begin, there are some things you need to know about how atoms attach, or bond together, because this influences whether or not they can carry an electrical current.
An ionic compound is a substance that contains atoms that are bonded together through an ionic bond, where electrons are transferred from one atom to another. Conversely, a covalent bond is when electrons are shared between atoms. And one last word before we go on: some substances can produce electrolytes when they are dissolved in water. This means the substance can conduct an electrical current.
You will use a conductivity probe to see if your water contains electrolytes, which will help Adam get some answers. Go ahead and place the probe in the water. Okay, it looks like you are getting a high value, so that means your water contains electrolytes, but what does that tell you about Adam's water? Don't worry, here are some trends regarding electrolytes and conduction:
Ionic compounds that are soluble are typically electrolytes
Strong acids and strong bases are electrolytes
Covalently-bonded compounds are usually bad electrolytes, meaning they can't conduct an electrical current
As are weak acids and weak bases
So, based on your test, it would appear the water might contain ionic compounds, strong acids or strong bases. Why don't you hold on to Adam's water, you'll need it again for the acid/base test.
Acid and Base Test
So, we were able to narrow down what was in Adam's water, but now we need to see if it contains a strong acid or a strong base. We don't want Adam to burn his mouth!
We mentioned acids and bases earlier in the lesson, but let's take a closer look. An acid is a substance that releases hydrogen ions, or H+, when dissolved in water. Whereas, a base is a substance that releases hydroxide ions, or OH-, when dissolved in water.
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For this test, you will use a pH strip. If you dip a pH strip into your solution, it will change colors, indicating if your substance is an acid, base or neutral. The pH scale runs from 0 to 14.
A pH of less than 7 indicates you have an acid
A pH of 7 indicates your substance is neutral
A pH greater than 7 indicates you have a base
Let's go ahead and test Adam's water. It looks like it is neutral! So, we know that it contained ions from our conduction test and that it is neutral from our acid/base test.
A pH of 7 means the solution is neutral
In this lesson, you will help Bobby identify his mysterious solution. He knows it contains lead, lithium or potassium, but he can't remember which one. For this, you will conduct a flame test, which means you will light the sample on fire and then observe what color the flame turns. Different ions will emit different colors when they are burned. Since you have an aqueous solution, you need to take a sample out, let the water evaporate and then use a wire loop to gather the dried powder.
Now you can place it into the flame and then observe the flame color. It appears your sample is emitting a red glow, so let's look at some possible colors with the ions:
So, what is your mysterious sample? Yeah, I agree: lithium.
It seems like seconds ago you were a brand-new chemistry detective, and look at this, you're onto your final lesson. We are back to help Sarah in chemistry class. After we left, she got to talking to a friend and wasn't paying attention. Unfortunately, she has several colored solutions containing different transition metals, but she doesn't know which metal is in which cup.
You don't have to do a whole lot in this colored solution test. You just need to observe what color the solutions are. Easy enough, right? Certain transition metal ions have specific colors, so this can help you identify them by looking at the solution. Although some of the colors overlap, it can help you at least narrow down your ion choices. Here are some clues to help you identify these ions when they are in solution.
Green, blue or greenish-blue
Purplish-green, green or purple
Light green, yellow or purple
Well, we can identify some of Sarah's unknown substances. It looks like she has a solution that contains cobalt, one that contains copper and one that contains iron.
You are a full-fledged chemistry detective. You have experience with several methods that allow you to narrow down and, in some cases, identify mysterious chemicals in solution. Let's take a moment to review these methods.
In precipitation, you can add a chemical to a solution and observe what precipitate forms.
In conduction, you can see if electrolytes are present. Strong and weak electrolytes will give you clues about the identity of the chemicals
Acid/base tests will tell you the pH of the substance and whether it is an acid, base or neutral substance.
Flame tests are used to identify ions based on the color the flames emit.
Some solutions are a predictable color, so by looking at colored solutions, you can try to figure out some of the chemicals in the solution.
And now you are ready to graduate. Congratulations, you have earned your C.D.D., or Chemistry Detective Degree!
Now that you have finished this lesson, you should be able to:
Identify the five tests that you can use to determine what type of solution you have
Discuss how to test precipitation
Explain how to interpret a conductivity probe
Recite the pH of acidic, base and neutral solutions
Recall the meaning behind the outcomes of the flame and color tests
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