Air and steel are solutions?! Yes! Episode 8 starts by defining solutions (1:30).
Air and steel are solutions?! Yes! Episode 8 starts by defining solutions (1:30). Using sugar water and salt water it distinguishes between electrolytes and non-electrolytes (2:30). The episode also focuses on factors affecting the solubility like polarity (4:04) and temperature (5:10). Last, but not least, concentration of solutions and how to prepare them is being discussed (6:12).
Question: Is milk a solution? (8:42)
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Hi and welcome to the APsolute Recap: Chemistry Edition. Today’s episode will recap solution chemistry. So maybe this is more appropriately the apSOLUTE recap today.
Let’s zoom out:
Imagine listening to this podcast while sitting on the beach. It is a warm summer day, you are in the shade. You can hear the waves crashing on the shore, you smell the sunscreen on your arms and you feel the warmth of the sun on your skin… sigh… At this moment, solutions are all around you! And no, I am not talking about the answers to everything (which we all know is 42), but chemical solutions: air, saltwater, the steel making up your beach chair and umbrella. Solutions are an integral part of our everyday life and of chemistry.
Let’s zoom in:
Back to our examples: saltwater, air and steel. Your first thought might have been: What? - Air and steel are solutions? Usually, when we mentioned solutions, we think of aqueous solutions, like saltwater. In general, however, solutions are defined as homogenous mixtures of two or more substances. One of the two components is in greater abundance - that is the solvent, the other one is the minor component and called solute. The author of our chemistry podcast, Sarah, hates these two terms and always mixes them up. Here’s her tip: “water is the uniVersal solVent”, both with a V. In salt water the water is the solvent and the salt is the solute. As long as you have a homogenous mixture, where the solute is uniformly mixed with the solvent, without any visible boundaries, you have a solution - no matter the state of matter. Air, therefore, is a gas-gas solution, with nitrogen gas being the solvent and oxygen, carbon dioxide and other gases being the solutes. Steel is an example for a solid-solid solution: it has iron as solvent and carbon as solute.
In your chemistry course, however, you will mostly encounter aqueous solutions. So let’s take a closer look at our saltwater and compare it to sugar water. They both look the same, so how can you tell the difference? There are a variety of tests you could do - for example taste them - which we NEVER do in chemistry! - or measuring the electrical current. As discussed in episode 4, aqueous solutions of ionic compounds conduct electrical current, aqueous solutions of molecular compounds don’t. Why is that?
Let’s take a look at what happens when these two compounds are in water: ionic compounds dissociate, molecular compounds dissolve. Isn’t that the same thing? No! During dissociation, the water molecules separate the crystal lattice of the ionic structure. You now have positive cations and negatively charged anions surrounded by water molecules floating in the solution. The presence of these charged particles is a prerequisite for conducting electrical current. Solutions that conduct electrical current are called electrolytes. This term might sound familiar: you can usually find it on sports drink labels, indicating the presence of ions. What about dissolution? Molecular compounds experience intermolecular forces between the individual molecules - in our example of sugar water, sucrose, C12H22O11. These intermolecular forces are broken in the presence of water and the molecules are surrounded by water molecules. In comparison to ionic compounds, however, the sugar molecules do not have a charge and therefore do not conduct electrical current. These solutions are called nonelectrolytes.
The solubility of a solute in a solvent depends on a variety of factors, for example - polarity of the molecules and temperature.
Water molecules are polar - we’ve briefly mentioned electronegativity in episode 4. The difference in electronegativity between oxygen and the two hydrogen is not enough to form ions, but oxygen is the stronger player at the tug of war - the shared electrons are closer to oxygen than they are to hydrogen. Together with the bent shape of the molecule, this gives oxygen a slightly negative partial charge and the hydrogen a partially positive charge, making the molecule polar. Oil, on the other hand, consists of long chains of carbon and hydrogen. The difference in electronegativity between carbon and hydrogen is not large enough to have an unequal distribution of shared electrons. Therefore, there is no partial charge. Oil is nonpolar. When you mix polar water and nonpolar oil, they behave like oil and water, literally: they do not mix. In chemistry, the term “like dissolves like” can help us: polar substances like water form solutions with other polar substances and non-polar substances form solutions with other non-polar substances.
Another factor that influences solubility is temperature. Generally speaking, an increase in temperature increases the solubility in a solution. When increasing the temperature of a solvent, we are increasing the kinetic energy, which makes it easier to overcome the intra- and intermolecular forces holding chemical compounds together. In chemistry, we often use solubility curves to show how many grams of solute can be dissolved in 100g of solvent at a certain temperature. If you have less solute than you could dissolve at that temperature, our solution is unsaturated. If you have exactly the amount you can dissolve at that temperature, the solution is saturated. You can, under certain circumstances, have a solution that contains more solute than it can hold under normal conditions. These solutions are called supersaturated. One way is to heat it to a higher temperature, create a saturated solution and then let it cool off. If disturbed, the excess will precipitate and form crystals - for example, when making rock candy.
It is very important to know the concentration of the solution. Most chemical processes call for specific concentration - if your solution is too weak, then the reaction won’t happen. It is also something we experience in everyday life: The weak coffee in the morning, the overly sweet lemonade for lunch or the vinegar in your salad dressing. In chemistry we use molarity as a measurement of concentration. It is defined as the number of moles of solute per liter of solution. One mole of solute is 6.022 x 1023 particles. That means a 1 molar sugar solution is 6.022 x 1023 sugar molecules in a 1 liter solution. Using the molar mass of sucrose, we can convert this into grams, in our case 342.3 grams, and then mix up our solution!
Oftentimes in the chemistry lab we have stock solutions - these are solutions of higher concentration that we can dilute to get the concentration that we need. Let’s say an experiment calls for 0.1 L of 1 molar hydrochloric acid. The stock solution is a 6 molar hydrochloric acid. We can now dilute the 6 molar hydrochloric acid with distilled water. Concentration and volume are inversely proportional - meaning if I increase the volume, the concentration decreases. Chemists use this equation to figure out how much water has to be added for their dilution: Molarity of the stock solution x Volume of the stock solution equal to Molarity of the diluted solution x Volume of the diluted solution. In our case that means: Volume of the stock solution is: 1 x 0.1 divided by 6, which is: 0.0167 L - 16.67 mL. That means you are filling a volumetric flask with some distilled water, you add 16.67 mL of 6M hydrochloric acid and then fill it to the 1L mark. And voila - you 1 M hydrochloric acid solution!
To recap……
A solution is a homogenous mixture with two components: solvent and solute. Ionic compounds dissociate in solution while molecular compounds dissolve. Solution formation is influenced by temperature and polarity of the components. Concentration is measured as molarity and we can dilute stock solutions.
Coming up next on the Apsolute RecAP Chemistry Edition: OUR AP Series! AP Overview and Exam Structure
Today’s Question of the day is about solution chemistry.
Question: Is milk a solution?