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Chemistry Review: Dissolving

Defining Dissolving

A glass of carbonated soda
  • One way to make a soda pop is to add a sweetened drink mix to carbonated water. The drink mix dissolves well in water but may not dissolve as well in other liquids.
  • Why do certain solids, liquids, and gases dissolve in water but not other liquids?

Water is a great dissolver

Water molecules, with areas of positive and negative charge.  The negative parts of the water molecule are attacted to positive parts of other water molecules.
  • To understand why certain solids, liquids, and gases dissolve in water, we need to look at water on the molecular level.
  • Because of the way the oxygen atom and the hydrogen atoms are bonded together in the water molecule, there is a slight negative charge near the oxygen and a slight positive charge near the hydrogens.
  • Notice that the positive part of one water molecule is attracted to the negative part of another.

A look at the solute, drink mix

A sodium and chloride ion, shown with their corresponding positive and negative charges

Sucrose. In this space filling model, red represents oxygen, light gray represents hydrogen, and dark gray represents carbon.

  • Drink mix (the solute) is a solid made up of many molecules of a sugar called sucrose.
  • Sucrose is composed of carbon, hydrogen and oxygen atoms.
  • Like water, sucrose has oxygen atoms bonded to hydrogen atoms. The area near the oxygens are slightly negative and the area near the hydrogens are slightly positive.
  • Sucrose molecules are attracted to each other based on these opposite charges.
  • The coloring in the drink mix is made of molecules that also have positive and negative charge. These molecules are also attracted to the sucrose molecules resulting in colored sugar.

Solute and solvent mix

This animation depicts the process of dissolving sugar at the molecular level. Because sugar is polar (having areas of positive and negative charge) and water is also polar, water is attracted to sugar molecules. Eventually, when enough water molecules surround a sugar molecule, a sugar molecule becomes more attracted to the water than it is to other sugar molecules. This is dissolving.

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  • When the drink mix is added to the water, the water molecules (the solvent) and the sucrose molecules (which comprise the solute) are attracted to each other.
  • When the attraction between the water and the sucrose molecules overcomes the attraction that the sucrose molecules have for each other, the sugar dissolves.
  • The color also dissolves because there is enough mutual attraction between the color molecules and the water molecules to overcome the attraction that the color molecules have for the sucrose molecules

Drink mix and vegetable oil

A vegetable oil molecule composed of only carbon and hydrogen atoms.

Vegetable Oil. In this space filling model, gray represents hydrogen, and dark gray represents carbon.

  • Just because a substance dissolves in water doesn’t mean it will dissolve in all liquids.
  • Vegetable oil is mostly made of molecules of carbon and hydrogen atoms bonded together in long carbon-hydrogen (C—H) chains. Unlike water and sucrose, with their O—H bonds and charged areas, the C—H bonds of oil have very little positive and negative charges.
  • These C—H chains have very little attraction for the sucrose or the food coloring.
  • Because there is little attraction, neither the sucrose nor the coloring dissolves in vegetable oil.

Liquids dissolve in liquids

This animation depicts the process of dissolving sugar at the molecular level. Because sugar is polar (having areas of positive and negative charge) and water is also polar, water is attracted to sugar molecules. Eventually, when enough water molecules surround a sugar molecule, a sugar molecule becomes more attracted to the water than it is to other sugar molecules. This is dissolving.

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  • Just like solids, liquids can also dissolve in liquids.
  • To understand how liquids dissolve in liquids, let’s look at alcohol, corn syrup and vegetable oil dissolving in water.
  • In isopropyl alcohol, an oxygen atom is bonded to a hydrogen atom. Because of the characteristics of the O—H bond, the oxygen is slightly negative and the hydrogen is slightly positive.
  • The mutual attractions between the water molecules and the alcohol molecules cause the alcohol to dissolve in water.

Corn syrup and water

This video shows how corn syrup dissolves in water.

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  • Corn syrup is made up of glucose and fructose molecules. These two molecules also have O—H bonds and areas of positive and negative charge.
  • The mutual attraction between the water molecules and the corn syrup molecules cause the corn syrup to dissolve in water.

Vegetable oil in water I

This video shows that vegetable oil does not dissolve in water.

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  • Vegetable oil does not dissolve in water.
  • There is little attraction between the molecules of the oil and water, so the water does not dissolve the vegetable oil.

Vegetable oil in water II

This animation shows that molecules of vegetable oil do not dissolve in water because water is composed of polar molecules, while vegetable oil is composed of non-polar molecules.

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  • Vegetable oil is made of long chains of carbon and hydrogen atoms bonded together. These C—H bonds produce only a very small amount of positive and negative charge.
  • There is little attraction between the molecules of the oil and water, so the water does not dissolve the vegetable oil. This video appears courtesy of Roy Tasker.
    © Roy Tasker, VisChem Project.

Carbon dioxide dissolves in water

A space filling model of carbon dioxide with area of positive and negative charge

A space filling model of carbon dioxide.

A charge density model of carbon dioxide

A charge density model of carbon dioxide.

Water molecules dissolving carbon dioxide.

Water dissolves carbon dioxide.

  • Gases, like solids and liquids, are also able to dissolve in water. The dissolving of a gas like CO2, shown here, depends on the interaction between water molecules and the gas molecules.
  • The positive and negative areas on water molecules are attracted to the oppositely charged areas of the carbon dioxide molecules.
  • This results in carbon dioxide gas (CO2) dissolving in water (H2O) to make carbonated water.

Summary

  • Solids, liquids, and gases can all dissolve.
  • Dissolving depends on the molecules of the substance doing the dissolving, called the solvent, and the molecules of the substance being dissolved, called the solute.
  • Dissolving is the process in which these molecules interact and attract each other to form a solution.
  • The extent to which a substance dissolves is a characteristic property of that substance called its solubility.
  • Water is a good dissolver because of its areas of positive and negative charge.
  • The mutual attraction between water molecules and other substances with positive and negative charges causes these substances to dissolve.

Molecular Animation Downloads

The animations featured in the slideshow above are available for download by clicking on the "Download this animation" link below each file.

Each video is offered for download in both Quicktime Movie (.mov) and Windows Media Video (.wmv) format.

To play these videos, you will need either Quicktime or Windows Media Player.