3 Solubility in Water
Organisms are made up of mostly water. The human body, for example, may be up to 70% water. This is because water is used for numerous functions such as digestion, absorption, transport, insulation, temperature buffer and medium for metabolic reactions. However, it will not be able to perform these tasks unless molecules dissolve in it. Hence, most of the molecules that are inside the body of organisms are water soluble (can dissolve in water).
These water soluble molecules mix well with water because they are alike; they have a charge and are said to be polar. The charges on polar molecules like water and the molecules of the soluble substances attract each other, making these soluble substances hydrophilic (waterloving). In contrast, the lipids of the cell membrane are nonpolar or hydrophobic (water-hating) and serve as a barrier to the passage of polar molecules across the membrane.
Lipid substances such as vegetable oil and steroids are nonpolar. They are soluble in other lipids but not in water. Such nonpolar molecules are, therefore, able to diffuse into the cells by passing through the lipid layers of the membrane.
Detergents have a special property. They can literally act as a bridge between the polar and nonpolar worlds. One end of the detergent molecule is polar and dissolves in water; the other end is nonpolar and can dissolve in fats and oils. Molecules such as detergent that have both polar and nonpolar regions are called amphipathic.
Hypotheses and predictions
· If a substance is polar, then it will dissolve in polar substances such as water.
· If a substance is nonpolar, then it will not dissolve in water, but instead will dissolve in nonpolar substances such as oil.
· To demonstrate solubility of different substances in water.
· To determine the effect of adding detergent to a mixture of water and a nonpolar substance.
Materials for DIY (Do-it-yourself) experiment
· Oil (ie. vegetable or olive oil)
· Food coloring (liquid or powder such as Kool-Aide or Jello mix)
· Dish soap or detergent
· Jar with lid
· Spoon to stir
Procedure for DIY experiment
(*if your group does not have above materials, please see alternative assignment at end of lab)
1. Half-fill a jar with water.
2. Add a few drops of food coloring into water & stir using spoon. Record your observations in the table. Take a picture.
3. Add oil into the jar with water & food coloring until it is about ¾ full, cover with lid tightly & shake. Wait about 1 min & record your observations in the table. Take a picture.
4. Add dish soap or detergent into the same jar & cover tightly again. Shake the jar gently and immediately record your observations in the table. Take a picture.
5. Wash out the jar & return all materials to their proper place.
Table 1. Results of combining various substances with water
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