Why is sugar dissolving and salt different?

Introduction
Have you ever added a spoon of sugar to your tea and wondered why it disappeared? Where did it go? The sugar did not actually disappear—it changed from its solid form into a dissolved form in a process called chemical dissolution. The result is a tea–sugar solution in which individual sugar molecules become uniformly distributed in the tea. But what happens if you increase the amount of sugar that you add to your tea? Does it still dissolve? In this activity you will find out how much of a compound is too much to dissolve.

Background
Chemistry is the study of matter and how it behaves and interacts with other kinds of matter. Everything around us is made of matter, and you can explore its properties using common chemicals around your home. The way it behaves is called a property of matter. One important property is called solubility. We think about solubility when we dissolve something in water or another liquid. If a chemical is soluble in water, then the chemical will dissolve or appear to vanish when you add it to water. If it is not soluble, or insoluble, then it will not dissolve and you will still see it floating around in the liquid or at the bottom of the container.

When you dissolve a soluble chemical in water, you are making a solution. In a solution the chemical you add is called the solute and the liquid that it dissolves into is called the solvent. Whether a compound is soluble or not depends on its physical and chemical properties. To be able to dissolve, the chemical has to have the capability to interact with the solvent. During the process of chemical dissolution, the bonds that hold the solute together need to be broken and new bonds between the solute and solvent have to be formed. When adding sugar to water, for example, the water (solvent) molecules are attracted to the sugar (solute) molecules. Once the attraction becomes large enough the water is able to pull individual sugar molecules from the bulk sugar crystals into the solution. Usually the amount of energy it takes to break and form these bonds determines if a compound is soluble or not.

Generally, the amount of a chemical you can dissolve in a specific solvent is limited. At some point the solution becomes saturated. This means that if you add more of the compound, it will not dissolve anymore and will remain solid instead. This amount is dependent on molecular interactions between the solute and the solvent. In this activity you will investigate how much of various compounds you can dissolve in water. How do you think sugar and salt compare?

Materials

• Distilled water
• Measuring cup that measures milliliters
• Eight glasses or cups that each hold eight ounces
• Four spoons
• Measuring spoon
• Epsom salts (150 grams)
• Table salt (50 grams)
• Table sugar (cane sugar, 250 grams)
• Baking soda (20 grams)
• Scale that measures grams
• Marker
• Masking tape
• Paper
• Pen
• Thermometer (optional)

Preparation

• Using the marker and masking tape label two cups for each compound: “table salt,” “table sugar,” “baking soda” and “Epsom salts.”
• Into one table salt cup measure 50 grams of salt.
• Into one table sugar cup measure 250 grams of sugar.
• Into one baking soda cup measure 20 grams of baking soda.
• Into one Epsom salts cup measure 150 grams of Epsom salts.
• For each cup weigh it and write down the mass (weight).
• Add 100 milliliters of distilled water into each cup. Use the measuring cup to make sure each cup has the same amount of water. The water should be at room temperature and the same for all cups. You can use a thermometer to verify that.

Procedure

• Take both of the cups you labeled with table salt. With the measuring spoon carefully add one teaspoon of table salt to the 100 milliliters of distilled water.
• Stir with a clean spoon until all the salt has dissolved. What do you notice when you add the salt to the water?
• Keep adding one teaspoon of salt to the water and stirring each time, until the salt does not dissolve anymore. What happens when the salt does not dissolve anymore?
• Repeat these steps with both cups labeled Epsom salts. At what point does the Epsom salts solution become saturated?
• Repeat the steps with the baking soda. How many teaspoons of baking soda can you dissolve in the water?
• Repeat the steps with the sugar. Did you add more or less sugar compared with the other compounds?
• Put each of the cups containing the remaining solids onto the scale and write down the mass (weight) of each one. How much of each substance did you use?
• Subtract the measured mass from your initial mass (see Preparation) for each compound. What does the difference in mass tell you about the solubilities of each of the compounds? Which compound is the most or least soluble in distilled water?
• Extra: Does the solubility change if you use a different solvent? Repeat the test, but instead of using distilled water use rubbing alcohol, vegetable oil or nail polish remover as solvent. How does this change your results?
• Extra: Can you find other substances or chemicals that you can dissolve in distilled water? How do their solubilities compare with the compounds you have tested?
• Extra: Solubility of compounds is also highly dependent on the temperature of the solvent. Do you think you can dissolve more salt or sugar in hot or cold water? Test it to find out!

Observations and results
Did all of your tested compounds dissolve in distilled water? They should have—but to different extents. Water in general is a very good solvent and is able to dissolve lots of different compounds. This is because it can interact with a lot of different molecules. You should have noticed sugar had the highest solubility of all your tested compounds (about 200 grams per 100 milliliters of water) followed by Epsom salts (about 115 grams/100 milliliters) table salt (about 35 grams/100 milliliters) and baking soda (almost 10 grams/100 milliliters).

This is because each of these compounds has different chemical and physical properties based on their different molecular structures. They are all made of different chemical elements and have been formed by different types of bonds. Depending on this structure it is more or less difficult for the water molecules to break these bonds and form new ones with the solute molecules in order to dissolve them into a solution.

Cleanup
You can dispose of each of your solutions in the sink. Keep the water running for a while afterward to flush your sink properly. Dispose of all remaining solids in the regular trash. Wash your hands with water and soap.

More to explore
Saturated Solutions: Measuring Solubility, from Science Buddies
Salty Science: How to Separate Soluble Solutions, from Scientific American
Solubility Science: How to Grow the Best Crystals, from Scientific American
Science Activity for All Ages!, from Science Buddies

This activity brought to you in partnership with Science Buddies

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Solubility

Why Do Some Solids Dissolve in Water?

The sugar we use to sweeten coffee or tea is a molecular solid, in which the individual molecules are held together by relatively weak intermolecular forces. When sugar dissolves in water, the weak bonds between the individual sucrose molecules are broken, and these C12H22O11 molecules are released into solution.

It takes energy to break the bonds between the C12H22O11 molecules in sucrose. It also takes energy to break the hydrogen bonds in water that must be disrupted to insert one of these sucrose molecules into solution. Sugar dissolves in water because energy is given off when the slightly polar sucrose molecules form intermolecular bonds with the polar water molecules. The weak bonds that form between the solute and the solvent compensate for the energy needed to disrupt the structure of both the pure solute and the solvent. In the case of sugar and water, this process works so well that up to 1800 grams of sucrose can dissolve in a liter of water.

Ionic solids (or salts) contain positive and negative ions, which are held together by the strong force of attraction between particles with opposite charges. When one of these solids dissolves in water, the ions that form the solid are released into solution, where they become associated with the polar solvent molecules.

We can generally assume that salts dissociate into their ions when they dissolve in water. Ionic compounds dissolve in water if the energy given off when the ions interact with water molecules compensates for the energy needed to break the ionic bonds in the solid and the energy required to separate the water molecules so that the ions can be inserted into solution.

Solubility Equilibria

Discussions of solubility equilibria are based on the following assumption: When solids dissolve in water, they dissociate to give the elementary particles from which they are formed. Thus, molecular solids dissociate to give individual molecules

and ionic solids dissociate to give solutions of the positive and negative ions they contain.

When the salt is first added, it dissolves and dissociates rapidly. The conductivity of the solution therefore increases rapidly at first.

The concentrations of these ions soon become large enough that the reverse reaction starts to compete with the forward reaction, which leads to a decrease in the rate at which Na+ and Cl- ions enter the solution.

Eventually, the Na+ and Cl- ion concentrations become large enough that the rate at which precipitation occurs exactly balances the rate at which NaCl dissolves. Once that happens, there is no change in the concentration of these ions with time and the reaction is at equilibrium. When this system reaches equilibrium it is called a saturated solution, because it contains the maximum concentration of ions that can exist in equilibrium with the solid salt. The amount of salt that must be added to a given volume of solvent to form a saturated solution is called the solubility of the salt.

Solubility Rules

There are a number of patterns in the data obtained from measuring the solubility of different salts. These patterns form the basis for the rules outlined in the table below, which can guide predictions of whether a given salt will dissolve in water. These rules are based on the following definitions of the terms soluble, insoluble, and slightly soluble.

• A salt is soluble if it dissolves in water to give a solution with a concentration of at least 0.1 moles per liter at room temperature.
• A salt is insoluble if the concentration of an aqueous solution is less than 0.001 M at room temperature.
• Slightly soluble salts give solutions that fall between these extremes.

Solubility Rules for Ionic Compounds in Water

Soluble Salts

1. The Na+, K+, and NH4+ ions form soluble salts. Thus, NaCl, KNO3, (NH4)2SO4, Na2S, and (NH4)2CO3 are soluble.

2. The nitrate (NO3-) ion forms soluble salts. Thus, Cu(NO3)2 and Fe(NO3)3 are soluble.

3. The chloride (Cl-), bromide (Br-), and iodide (I-) ions generally form soluble salts. Exceptions to this rule include salts of the Pb2+, Hg22+, Ag+, and Cu+ ions. ZnCl2 is soluble, but CuBr is not.

4. The sulfate (SO42-) ion generally forms soluble salts. Exceptions include BaSO4, SrSO4, and PbSO4, which are insoluble, and Ag2SO4, CaSO4, and Hg2SO4, which are slightly soluble.

Insoluble Salts

1. Sulfides (S2-) are usually insoluble. Exceptions include Na2S, K2S, (NH4)2S, MgS, CaS, SrS, and BaS.

2. Oxides (O2-) are usually insoluble. Exceptions include Na2O, K2O, SrO, and BaO, which are soluble, and CaO, which is slightly soluble.

3. Hydroxides (OH-) are usually insoluble. Exceptions include NaOH, KOH, Sr(OH)2, and Ba(OH)2, which are soluble, and Ca(OH)2, which is slightly soluble.

4. Chromates (CrO42-) are usually insoluble. Exceptions include Na2CrO4, K2CrO4, (NH4)2CrO4, and MgCrO4.

5. Phosphates (PO43-) and carbonates (CO32-) are usually insoluble. Exceptions include salts of the Na+, K+, and NH4+ ions.