Chemistry Friends/Solutions

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Classifying Solutions[edit]

A solution is when two or more things mix together homogeneously. These things are called the solvent and the solute. Solutes can be solid, liquid or gas. One of the most popular solvent is water.[1]

Solutions of Electrolytes and Non-Electrolytes[edit]

An electrolyte is an aqueous solution that conducts electricity. Pure water isn't an electrolyte, but if you dissolve something in water, it can conduct electricity. Acids and bases are electrolytes. Molecular compounds usually aren't.[1]

Acidic, Basic, and Neutral Solutions[edit]

Solutions can be acidic, basic, or neutral. The stronger the acid, the more elecricly conductive it is.[1]

Understanding Solutions[edit]

When something is dissolved into a solution, the compound dissociates. This means each molecule and ion goes off in its own with water. If you put NaCl in water, in would dissociate and become Na+ and Cl-. The sodium chloride wouldn't be separated, the ions would just get farther apart. Dissociation makes solutions electrolytes. The Na is positive and the Cl is negative, so if they are dissociated in water, the charges help electricity flow through the solution. This works with acids and bases too. If an acid or base dissolves, it dissociates. Then one of the ions is negative and one is positive.[1]

Substances in Water[edit]

Some things dissolve in water better than others. Ions' solubility is very predictable. There are tables to show how soluble different ions are. Molecules are trickier. There is no simple was to predict how soluble different molecules are. You just have to remember.[1]

Reactions in Solutions[edit]

Chemicals are put into solutions to make it easier to transport, load and store them. It also helps with reactions. If something is in a solution, it can have a faster, more complete reaction and can change the results. Reactions are faster and more complete for this reason: a dissolved chemical has more surface area than if it were on its own. [1]

Net Ionic Equations[edit]

Let's say you have sodium oxide and chlorine and you mix it into a solution of sugar water. The equation might look like this.

Na2O(aq)+C6H12O6(aq)+ Cl(aq)= NaCl + C6H12O6(aq) + O(aq)

Now we put the theory of dissociation into effect.

Na2(aq)+O(aq)+ C6(aq) +H12(aq) +O6(aq) +Cl(aq) =NaCl + C6H12O6(aq) + O(aq)

The thing is, this equation seems pretty cluttered and some of the ions don't even seem to change. Let's cancel them out!

Na2(aq)+O(aq)+ C6(aq) +H12(aq) +O6(aq) +Cl(aq) =NaCl + C6H12O6(aq) + O(aq)

This leaves us with the equation

Na(aq)+ Cl(aq)= NaCl(aq)

That makes more sense![1]

Qualitative Chemical Analysis[edit]

There are two kinds of measurements: qualitative and quantitative. Qualitative is looking at litmus paper to see if it is blue or red, noticing bubbles in a solution and seeing rust is forming. You can see results but there are no units or numbers you can put in. That's what quantitative measurements are. A pH of three, twenty seven grams of HCl and two moles of oxygen are all quantitative. They are more exact than qualitative measurements.[1]

Qualitative Analysis by Colour[edit]

Some substances can be identified by colour. The colour of the solution might change, there might be a coloured gas or a flame. Litmus paper changes colour based on if the solution it is placed in is acidic or basic. When burned, copper(II) has a green flame.[1]

Concentration of a Solution[edit]

If a solution has very little solute in it, it is a dilute. If a substance has lots of solute in it, it is a concentrated solution.[1]

Communicating Concentration Ratios[edit]

Use this ratio to find the concentration of a solution.

concentration=quantity of solute /quantity of solution

If there are 2 mL for every 100mL of solution, the ratio would be:

2mL/100mL and the answer would be 2% v/v.

The v/v thing means volume to volume. Some times you need to calculate w/v which is weight to volume. This would be like if there were 4 grams of sugar in a 100mL sugar water solution. There are ways to express very small units. Parts per million is one. If there was three grams of salt in a million mL lake, the concentration of salt would be 3ppm. There is a ratio to find the molar concentration. It is the amount of the solute in moles when dissolved in a liter of water.

Molar concentration= amount of solute (in moles)/ volume of solution (in liters)

Molar concentration is measured in (mol/L)

Calculations Involving Concentration[edit]

In chemistry, it is nice to know the quantity, volume and concentration of a solution. If you only know two of these things, you can use a formula to figure the thrid one out.[1]

mass/volume=concentration

Preparation of Standard Solution from a Solid[edit]

Standard solutions are solutions with a precise concentration.[1]

Preparation of Standard Solution by Dilution[edit]

Dilution is a way of changing the concentration of a solution by adding more of the solvent or some other liquid. If your lemonade is too strong, you might try adding water. This would be diluting the lemonade. Dilution is a pretty easy was to alter the concentration of a solution. Diluted solutions are used a lot today. If a chemical reaction happens too fast, a chemist can dilute the chemical and slow down the reaction. If a doctor needs to prescribe a thousandth of a mL of medicine to you, he might instruct you to take a mL of the medicine, mix it with a L of water and take a mL of the solution. It is easier to measure in milliliters and liters than it is to measure in thousandths of a milliliter. There are many tools to make exact measurements. Volumetric flasks like graduated cylinders and beakers and scales are very common tools. Another tool is called a volumetric. It is a long tube that looks like an eye dropper except the measurements on the side have 0 on the top and the biggest number on the bottom. To use, you fills the tube all the way to the zero and moves the volumetric over the container you want the measured solution in. Since the solution reaches the 0 mark, no solution has dropped into the container. Let's say you need three units of solution. Jiggle the nob on the top of the tube. This lets little bits of air into the tube so the solution drips out the bottom. Carry on until the solution line drops to the three. Put the volumetric away and you have 3 units of solution![1]

Calculating Dilution[edit]

It isn't too hard to calculate a concentration of a solution after it has been diluted. If the amount of solvent doubles, the concentration halves. There is a simple formula to figure it out.

vici=vfcf

The vi is the volume before you diluted the solution. vf is the volume after the solution was diluted. c is for the concentration. i for initial and f for final. As long as you know three of the pieces of information, you can use the equation to find out the fourth one.[1]

Solubility[edit]

Every solution has a limit to how much of a solute can be added. If more than the limit is added, the access won't dissolve. It'll just sit there.[1]

Solubility Rules and Examples[edit]

When mixing substances, there are rules the solutions follow. They are nice to know so you can dissolve things better.

  • If you heat water to a higher temperature, more solids solute can dissolve into it.
  • If you want to get more gas to dissolve in the water, cool it down. Gas always dissolves better in cold water.
  • Gases also dissolve better at high pressure.
  • Some oils won't ever dissolve in water because they are immiscible. It's pretty much imposable so don't bother trying.
  • The opposite to some thing that is immiscible is something that is miscible! This means there is no limit to how much of the substance you can dissolve. Well, the limit is how much of the substance you can get your hands on.
  • Most elements don't dissolve well in water, but halogens and oxygen do alright. Also, metals from group I and II can dissolve pretty well.

You can see evidence of solutions all around. If you let the water evaporate from the solution, any solid solutes left behind form crystals. Stalagmites and stalactites are good examples of left behind minerals. You might also notice that when you open a can if pop, some air escapes. What that is is carbon dioxide that use to be dissolved in the drink when it was cold, but undissolved as the pop warmed up. Fish survive because of the oxygen dissolved in the water.[1]

Concentration of Ions[edit]

Moles of Ions and Ionic Compounds[edit]

In chemistry, it can be very useful to know the molar concentration of ions. First, let's remember that a mole is a number, like a dozen. It isn't an amount, it is a number. A complicated number, but a number none the less. So if we have one mole of an ion like CaCl2, There is one mole of CaCl2, one mole of Ca, and 2 moles of Cl. Just like if there were a dozen cats, there would be a dozen tails, two dozen eyes and four dozen paws. (Unless, of coarse, there were any mutant cats with three eyes or unfortunate cats who lost an eye.)[1]

Molar Concentration[edit]

So anyway, another thing we need to know is the symbol for molar concentration. It is square brackets. So [NaCl] is the molar concentration of sodium chloride.[1]

Calculating Molar Concentration[edit]

When we calculate molar concentration, we dissociate the ions and calculate them separately. So let's say we are looking for the molar concentration of CaCl2 in a 0.123 mol/L solution. First thing we do is dissociate the ions.

CaCl2(aq) → Ca(aq) + 2Cl(aq)

Let's find the molar concentration of Ca first! We multiply the concentration by how many moles there are. In this case, there is one mole.

[Ca(aq)] = 0.123 mol/L × 1 = 0.123 mol/L

So, There are 0.123 mol/L of carbon in the solution. Now for the chlorine. Remember, there are 2 moles of chlorine.

[Cl(aq)] = 0.123 mol/L × 2 = 0.246 mol/L

There are 0.246 mol/L of chlorine![1]

Molar Concentration of Hydrogen[edit]

It is very important to know the molar concentration of hydrogen ions because hydrogen ions make acids. The higher the hydrogen concentration, the more acidic the solution. Pure water is neutral. It has a pH of 7. What this means is the hydrogen concentration of water is 1×10-7 mol/L. Something more acidic, like vinegar, has a pH of about 2 and therefore a hydrogen concentration of 1×10-2. Something basic with a pH of 11 would have a hydrogen concentration of 1×10-11.[1]

References[edit]

  1. a b c d e f g h i j k l m n o p q r s t Jenkins, F., van Kessel, H., Tompkins, D., Lantz, O., (1996). Nelson Chemistry, British Columbia, Nelson Canada.