🧪AP Chemistry
8 min read•Last Updated on June 18, 2024
Dalia Savy
Jeremy Kiggundu
Dalia Savy
Jeremy Kiggundu
If you recall from the last section, matter can be categorized according to state or composition. The three states of matter will be elaborated upon in unit three of this AP Chemistry course, but we will delve deeper into categorizing matter by composition in this guide.
Remember that the first division in the classification of matter by composition is between a pure substance and a mixture. A pure substance is composed of a single type of atom or molecule, whereas a mixture is a composition of two or more elements or compounds.
As discussed, mixtures are materials made up of two or more substances that are physically combined.
To better clarify the difference between pure substances and mixtures, let's discuss them in terms of formula units. As a review, a formula unit represents the lowest whole number ratio of atoms that can be used to describe a compound.
While pure substances contain atoms or formula units of a single type, mixtures contain atoms or formula units of two or more types (and their proportions vary).
Just like there are different types of pure substances, there are different types of mixtures!
💭 Remember that a pure substance can be either an element or a compound. Both are composed of a single type of atom or molecule, but compounds can be chemically decomposed into smaller parts (elements).
Mixtures are separated into two categories: homogeneous and heterogeneous.
Chemists often want to separate a mixture into its components in chemical experiments. There are several ways to separate mixtures, but the most common ways in chemistry are distillation and filtration. These techniques use different physical or chemical properties of the components in a mixture in order to separate them.
Distillation separates components in liquid mixtures by evaporating the most volatile substance first, usually the substance with the lowest boiling temperature. The most important thing to know is that it separates based on differences in boiling points.
Think about a mixture of water and alcohol. Since the boiling point of alcohol is lower than that of water, the alcohol is going to evaporate first. Therefore, the vapor collected will contain more alcohol than water.
Filtration separates mixtures by using a mesh or filter to separate solids from liquids. However, this only works with heterogeneous mixtures.
Imagine you are filtering a solution of salt, water, and sand. Can you guess what will get caught up in the filter paper? Only the sand will. This is because the salt will dissolve in the water since it is soluble.
Only insoluble substances will be filtered with filtration.
The filtrate would then be both salt and water, or salt water. Therefore, filtration isn't effective if you want to fully separate all of these elements. After filtering the sand out, you would have to evaporate the water out of the salt water solution to separate the salt from the water.
Chromatography is a method used to identify and compare mixtures based on their attraction to solids and liquids or differences in polarity (polarity is further explained in future units).
In the first step, someone drew a dot with a marker. Most of the ink in markers are a mixture of a number of substances and thin-layer chromatography can separate these substances.
In step two, the solvent climbed up the filter paper📄 and began to separate the sample spot by its substances.
Polarity is one way to compare solvents or substances. A substance may be polar or nonpolar.
You may hear the phrase, "Like dissolves like" a lot. This means polar substances dissolve in polar solvents and nonpolar substances dissolve in nonpolar solvents.
Do you know how oil and water never mix? Water is a polar substance and oil is a nonpolar substance. Since they aren't the same polarity, they never mix.
Most TLC plates are made up of polar silica, which you can think of as a very small powder. This silica is considered the stationary phase, while the solvent in the TLC chamber is considered the mobile phase. A general rule of thumb is that polar compounds are more strongly attracted to the stationary phase, and will move less, while nonpolar compounds are more readily eluted with the solvent.
☝️Let's refer back to the image above. If the solvent is nonpolar, could you guess which component is more nonpolar? If Compound A traveled farther up the TLC plate, that means it was more readily eluted with the solvent. With this being said, Compound A is less polar than Compound B.
AP NOTE: There are other types of chromatography as well but this one is the most commonly tested on the AP Chemistry Exam.
This question is adapted from the 2018 AP Chemistry Exam posted on the College Board website.
There are questions like this one on several past AP exams. This is only part a, but it's good to recognize what they are asking here. They basically want you to draw the reactant mixture given the information you are provided with.
The two things you want to note in this question are:
Step 2) If you count how many atoms of each element are present, you would get 8 N atoms and 12 O atoms.
Since you have fewer Nitrogen atoms, it'd be good to draw the NO reactant first. Draw as many as you can until you run out of nitrogen atoms. This would get us to have 8 NO molecules. Now, we have 4 O atoms remaining. Since each O2 compound contains two atoms of oxygen, we could only draw 2 O2 molecules with the remaining oxygen.
A sample response looks like the following:
Make sure you use the key they gave you when drawing the molecules! You should always look out for requirements and directions on how to draw diagrams. The orientation at which you draw the molecules doesn't matter, as long as you draw them correctly.
This question is adapted from the 2017 AP Chemistry Exam posted on the College Board website.
This is an AP free-response question testing your knowledge about thin layer chromatography! In this question, they are giving you several pieces of information:
Part (a) is asking you which of the known dyes (A, B, or C) is the least polar and to justify your response. Since they explicitly asked, your justification must involve the interactions between the dyes and the solvent or the dyes and the paper.
You should automatically be able to eliminate A as the least polar dye, since it travels a distance between C and B. You can see that dye C travels the farthest in a nonpolar solvent, while dye B has traveled the least. Remember "like dissolves like," so the least polar dye, or the most nonpolar dye, will travel the farthest in a nonpolar solvent!
Therefore, your answer to part a should be similar to one of the following:
"Dye C is the least polar dye because it moved the farthest on the TLC plate. This is because nonpolar dyes are more attracted to the nonpolar solvent."
"Dye C is the least polar dye because it moved the farthest on the TLC plate. This is because nonpolar dyes are less strongly attracted by the polar TLC plates."
Both of these responses would earn you the full two points on this part of #4.
Part (b) asks you to figure out which dye is present in the unknown sample and justify. This question can easily check many students if they aren't paying close attention to the difference in the TLC plates.
The solvent in the developed TLC plates with the dyes has traveled much farther than the one with the unknown. This is important to note because in order to accurately tell which dye is in the unknown, the solvent fronts must be in the same location. Therefore, we have to use proportions to answer this question.
The unknown dye seems to have traveled half the distance that the solvent did, so we have to find which dye did the same on the other TLC plate. It looks like dye A followed the same behavior, so it must be in the unknown.
According to the scoring guidelines, either of the following responses would earn you a full two points:
"Dye A is present in the unknown sample. The unknown sample moves to a position that is midway between the origin and the solvent front, and so does dye A"
Dye A has a retention factor (Rf) that is close to 0.50 on the chromatogram with the three dyes, and the unknown also has a retention factor close to 0.50."
We haven't discussed Rf yet, but it is basically a measure of the distance traveled by the dye relative to the distance traveled by the solvent. This number is always going to be between 0 to 1.
Rf = (distance traveled by the component) / (distance traveled by the solvent)
Distillation is a process used to separate components based on their different boiling points. It involves heating a liquid until it vaporizes, then cooling the vapor back into liquid form and collecting it separately.
Term 1 of 14
Distillation is a process used to separate components based on their different boiling points. It involves heating a liquid until it vaporizes, then cooling the vapor back into liquid form and collecting it separately.
Term 1 of 14
Distillation is a process used to separate components based on their different boiling points. It involves heating a liquid until it vaporizes, then cooling the vapor back into liquid form and collecting it separately.
Term 1 of 14
A pure substance is a form of matter that has a constant composition and properties that are consistent throughout the sample. It cannot be separated into different types of matter by physical means.
Element: A type of pure substance which cannot be broken down into simpler substances by ordinary chemical methods.
Compound: Another type of pure substance composed of two or more elements in fixed proportions.
Homogeneous Mixture: A mixture (not a pure substance) that has uniform composition and properties throughout.
A mixture is a combination of two or more substances where each retains its own chemical identity. The components can be separated from one another by physical means without changing their individual properties.
Heterogeneous Mixture: A type of mixture where the components are not uniformly distributed and can usually be visually distinguished.
Solution: This is a homogeneous mixture where one substance (the solute) is dissolved in another (the solvent).
Suspension: This is a heterogeneous mixture in which solid particles are dispersed throughout a liquid or gas.
Formula units refer to the simplest collection of atoms from which an ionic compound’s formula can be established. It represents the ratio between cations (positively charged ions) and anions (negatively charged ions) in an ionic compound.
Molecular Formula: This shows the types and numbers of atoms combined in a single molecule of a molecular compound.
Empirical Formula: This is the simplest positive integer ratio of atoms present in a compound.
Ionic Compound: A chemical compound composed of ions held together by electrostatic forces termed ionic bonding. The compound is neutral overall, but consists of positively charged ions called cations and negatively charged ions called anions.
A homogeneous mixture is a type of mixture in which the components are uniformly distributed throughout. This means you can't distinguish one component from another.
Solution: A type of homogeneous mixture where one substance (the solute) is dissolved in another (the solvent).
Alloy: A solid solution that's typically made up of two or more metals.
Colloid: A type of homogeneous mixture where tiny particles are dispersed but not fully dissolved in a medium.
Heterogeneous mixtures are those where the components aren't uniformly distributed and can be visually distinguished.
Suspension: A type of heterogeneous mixture where solid particles settle out over time if left undisturbed.
Emulsion: A suspension of tiny droplets of one liquid in a second liquid with which the first will not mix, such as oil and water.
Aggregate Material: Materials composed of various distinct parts that don't blend together, like concrete.
Distillation is a process used to separate components based on their different boiling points. It involves heating a liquid until it vaporizes, then cooling the vapor back into liquid form and collecting it separately.
Fractional Distillation: A type of distillation used to separate a mixture of liquids with close boiling points.
Simple Distillation: A process used to separate components of a liquid mixture that have significantly different boiling points.
Steam Distillation: A special type of distillation used for temperature sensitive materials like natural aromatic compounds.
Filtration is a physical method used to separate solids from liquids by passing the mixture through a medium that only allows the liquid to pass through.
Sedimentation: This is the process where particles settle to the bottom of a solution. It's often used before filtration.
Decanting: This involves gently pouring off a liquid and leaving behind any solid residue. It's another method that can be used alongside or instead of filtration.
Centrifugation: This technique uses centrifugal force to separate mixtures based on their densities. It's like an advanced form of filtration.
Polarity refers to the distribution of electric charges in a molecule, leading to regions of positive and negative charge. It determines how molecules interact with each other.
Nonpolar Molecules: These are molecules where electrons are shared equally by the atoms, resulting in no charge separation.
Dipole Moment: This is a measure of the overall polarity of a molecule.
Hydrogen Bonding: A type of intermolecular force that occurs when a hydrogen atom bonded to an electronegative atom interacts with another electronegative atom. It's stronger due to polarity.
A solvent is a substance that dissolves a solute (a chemically different liquid, solid or gas), resulting in a solution.
Solute: This is what gets dissolved by the solvent - like our party guests.
Solution: The result when a solute is fully dissolved in a solvent - the successful party.
Concentration: How much solute is dissolved in the solvent - akin to how many guests are at our party.
A polar substance is a type of chemical substance where electrons are not shared equally between atoms in a molecule. This unequal sharing results in regions with slight electrical charges.
Dipole Moment: This term refers to the measure of polarity in a chemical species.
Hydrophilic: These are substances that can mix with or dissolve in water. Polar substances tend to be hydrophilic because they can form hydrogen bonds with water.
Electronegativity: This term describes an atom's ability to attract shared electrons towards itself. The difference in electronegativity values determines whether a bond will be nonpolar or polar.
A nonpolar substance is a type of chemical substance where the electrons are shared equally between atoms in a molecule, resulting in no electrical poles or charges.
Covalent Bond: This is a type of chemical bond where two atoms share electrons.
Symmetry: In chemistry, this refers to how identical parts are arranged around a point or axis.
Hydrophobic: This term describes substances that do not mix with water. Nonpolar substances are typically hydrophobic because "like dissolves like."
In chromatography, the stationary phase is the substance which stays fixed inside the column. It's a medium on or in which the sample components are separated.
Adsorption: This is when molecules stick onto a surface - just like how some songs 'stick' with our concert-goers more than others.
Partition Coefficient: This measures how much a compound prefers to be in one phase over another - similar to how some fans might prefer slower ballads over fast-paced rock tunes.
Retention Time: The time it takes for a particular compound to pass through the system under set conditions - akin to how long each song plays during the concert.
The mobile phase is the liquid or gas that moves through the stationary phase, carrying along with it different components of a mixture.
Eluent/Elution: The process by which analytes are washed out from a column by flowing solvent(s), similar to how passengers can be moved along by conveyor belts.
Flow Rate: How quickly your mobile phase moves through your column, akin to speed settings on an airport's conveyor belt.
Gradient Elution: A technique where concentration of solvent changes during separation process. It’s like changing speed of conveyor belt to separate people based on their walking speed.