Solubility indicates the ability of a compound to dissolve completely in water. An insoluble compound forms a precipitate in the solution; however, it may also be partially insoluble, but when chemistry problems occur in school, it is considered insoluble. Memorizing the rules of this phenomenon greatly simplifies the calculations of chemical equations. By dedicating a little time, effort and a few memory tricks to studying, you can learn these rules by heart in no time.
Steps
Method 1 of 3: Learn the Solubility Rules
Step 1. Remember that the salts that contain the elements of group 1A are soluble
The periodic table is organized in rows and columns, which are called periods and groups, respectively. The first column contains the elements of group 1A which are the alkali metals: Li, Na, K, Cs and Rb.
For example: KCl and LiOH are water soluble
Step 2. Know that salts that contain nitrates, chlorates and acetates are water-soluble
When a nitrate (NO3-), a chlorate (ClO3-) or an acetate (CH3COO-) form a salt, the latter dissolving in water.
For example, KNO3, NaClO3 and CH3COONa are all soluble.
Step 3. Understand that all ammonium salts are soluble
The ammonium ion (NH4+) causes the compound to dissociate completely in the presence of water and there are no exceptions to this rule.
NH4OH is soluble even though it contains a hydroxide.
Step 4. Remember that most hydroxides are insoluble
Some of them are slightly soluble, such as those formed with the elements of group 2 (Ca, Sr and Ba). The exception to this rule is the hydroxy salts formed with the elements of group 1, since those that fall into group 1A are always soluble.
For example: Fe (OH)3, Al (OH)3 and Co (OH)2 they are insoluble, but not LiOH and NaOH.
Step 5. Recognize that salts containing Group 17 non-metals generally dissolve in water
These are chlorine (Cl-), bromine (Br-) and iodine (I-). The exceptions to this rule are silver, lead and mercury; compounds made with these non-metals and ions are not soluble.
- For example, AgCl and Hg2Cl2 they do not dissolve in water.
- Note that PbCl2, PbBr2 and PbI2 they are soluble in water very hot.
Step 6. Be aware that most carbonates, chromates and phosphates are insoluble
The chemical formulas for these compounds are: CO3 (carbonates), CrO4 (chromates) and PO4 (phosphates). Metals of group 1A and compounds with NH are exceptions to this rule4+ which are soluble.
For example, compounds such as CaCO3, PbCrO4 and Ag3BIT4 they do not dissolve in water, while compounds such as Na3BIT4 and (NH4)2CO3 they are soluble.
Step 7. Remember that most sulfate salts are soluble
The salts containing the SO group4 dissolve in water, except for ions: Ca+2, Ba+2, Pb+2, Ag+, Sr+2 and Hg+2. The sulphate salts that contain them do not dissolve in water.
For example: Na2SO4 it is completely water-soluble, but CaSO4 and BaSO4 they are not.
Step 8. Know that most sulfides are insoluble
As with the other rules described above, there are exceptions regarding barium, calcium, magnesium, sodium, potassium and ammonium; the only sulphides that dissolve in water are those composed of these elements.
- For example: CdS, FeS and ZnS are all insoluble.
- Transition metal sulphides are highly insoluble.
Method 2 of 3: Using Memory Techniques
Step 1. Use the acronym NAG SAG
This is an easy way to remember soluble compounds and exceptions to the rules. Write the acronym and memorize the meaning of each letter; although it does not include all the rules of water solubility, it does allow you to remember several of them. Each letter represents soluble molecules.
- N: nitrates (NO3-);
- A: acetates (CH3COO-);
- G: alkali metals of ggroup 1 (Li+, Na+ and so on);
- S: sulphides (SO4-2);
- A: ions tommonium (NH4+);
- G: non-metals del ggroup 17 (F-, Cl-, Br-, I.- and so on).
Step 2. Write the acronym PMA for the first exception
The letter "P" stands for Pb+2 (lead); "M" stands for mercury (Hg2+2) and "A" for silver (Ag+). These three ions are never soluble in water when they form a compound with a sulfate group or a non-metal of group 17.
When writing these acronyms that help you memorize, put an asterisk next to PMA and a similar one next to the SAG letters "S" and "G" to remind you that they are exceptions
Step 3. Consider the Castro Bar words which represent the second exception
As funny as they may seem to you, they help to memorize the three ions: calcium (Ca+2), strontium (Sr+2) and barium (Ba+2), which are never soluble with sulphates.
Again, put a cross next to the words Castro Bar and another near the "S" of SAG to remember that they are the exceptions to the solubility of sulphates
Method 3 of 3: Memorize the Basics
Step 1. Review the study material often
Remembering information by heart takes time and effort. The more often you re-read chemistry topics, the more likely you are to remember them later; re-read the rules often and challenge yourself every day.
- Ask friends and family to ask questions about solubility during lunch or dinner.
- Keep a hard copy of the rules handy in case you have any memory lapses.
Step 2. Prepare and use flashcards
They represent a very useful study tool that allows you to quickly review and memorize several topics. Make tiles with the rules of solubility and some examples of compounds; use them and review them until you learn which are the soluble and insoluble compounds.
- Take them with you everywhere and use them when you are sitting in the car or waiting for friends.
- Whenever you don't remember a detail, it's a good time to review the subject with flashcards.
Step 3. Take advantage of mnemonics
It is a series of "tricks" that allow you to quickly memorize information in a simple way; when you use these techniques, it is worth writing them several times until you have internalized them. Acronyms and various methods are only useful if you know what they mean!
- Practice often writing acronyms or phrases that allow you to memorize the rules, also indicating the meaning of each letter.
- When taking an exam or class assignment, first write down these acronyms so you can refer to them for the rest of the time.
