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How to Identify Chemicals in Solution: Test Methods & Materials

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  • 0:01 Precipitation
  • 3:44 Conduction
  • 5:25 Acid and Base Test
  • 6:31 Flame Test
  • 7:28 Colored Solutions
  • 8:38 Lesson Summary
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Lesson Transcript
Instructor: Julie Zundel

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.

Precipitation

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 Transition Metal
Blue Copper
White Zinc
Green Iron (II)*
Orange-Brown Iron (III)*
*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 Halide
No precipitate Fluorine
White Chlorine
Cream Bromine
Yellow Iodine

So, it looks like you have iodine. Fantastic!

Conduction

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.

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
pH strip test

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