Chemistry Review: Water
- Have you ever noticed the way color comes off an M&M when it gets wet? The moisture seems to dissolve the colored coating. Why does this happen?
- Why do substances dissolve in water?
What is water made of?
- To answer the question “why do substances dissolve in water”, let’s look at water on the molecular level.
- Notice the water (H2O) molecule in the animation to the right. The two hydrogen atoms are shown in white and the oxygen atom is shown in red.
- Each hydrogen atom has one proton (positive charge) and one electron (negative charge). The oxygen atom has 8 protons and 8 electrons.
- Because the water molecule has the same number of protons as electrons, a water molecule is neutral. It has no overall or “net” charge.
How are the atoms in water bonded together?
- The atoms in a water molecule are bonded together in a special way that helps to make water a good dissolver.
- The oxygen and hydrogen atoms in a water molecule are held together by a covalent bond.
- This means that the electrons from the oxygen and hydrogens go around all three atoms instead of just the atom they started with. To form a covalent bond, the oxygen and hydrogen atoms share several electrons.
- But this sharing is not equal.
Water's covalent bond
- In the animation, the gray clouds represent the area where electrons are in the oxygen and hydrogen atoms.
- Once the water molecule is formed, the cloud around the oxygen is darker than the clouds around the hydrogens.
- The difference in darkness indicates that the electrons spend more time near the oxygen than near the hydrogens. This is because oxygen has a stronger attraction for electrons than the hydrogens do.
- This makes the area near the oxygen slightly negative and the area near the hydrogens slightly positive.
- Water is a polar molecule. A polar molecule has no overall charge, but has an area of positive charge separated from an area of negative charge.
Different water molecules
- You may see water represented in several different ways.
- This is the ball and stick model with an electron “cloud” around it. The different shading of the cloud represents the area around the molecule where the electrons are most likely to be.
- This is the space-filling model. Notice the overall shape incorporates the electron cloud. It shows the space occupied by the electrons shared between the oxygen and the hydrogen atoms.
- This is a charge density model with a colored cloud. The red area near the oxygen is slightly negative and the blue area around the hydrogen is slightly positive.
Water molecules attract each other
- The negative area of one water molecule (near the red shading) is attracted to the positive area of another (near the blue shading), and vice versa.
Water molecules in liquid water
- The space-filling model of liquid water shows water molecules moving around, sliding past, and bumping into each other.
- Attractions between the water molecules are constantly breaking and forming with other water molecules.
Why do M&Ms dissolve in water?
- An M&M coating has coloring on the outside, but is mostly made from sugar. The type of sugar in M&M’s is called sucrose.
- Sucrose is made up of carbon, hydrogen and oxygen atoms (C12H22O11).
- Like water, sucrose has hydrogen atoms (light gray) bonded to oxygen atoms (red). Also like water, the areas near the oxygens are slightly negative and the areas near the hydrogens are slightly positive.
What holds sugar together?
- In this animated molecule of sucrose, the positive areas (near the hydrogens) are colored blue and the negative areas (near the oxygen) are colored red.
- The oppositely charged areas are what holds the sucrose molecules together within a grain of sugar.
So, why do M&Ms dissolve in water?
- Since sucrose has positive and negative areas and water has positive and negative areas, water molecules are attracted to the oppositely charged areas of sucrose molecules.
- When the attraction that water molecules have for sucrose molecules overcomes the attraction the sucrose molecules have for each other, the water separates the sucrose molecules from each other and they dissolve.
- A water molecule has no overall or “net” charge. It is neutral.
- Because of the way oxygen and hydrogen are bonded together, water has an area of positive charge and an area of negative charge. This makes the water a polar molecule.
- Because of water’s polarity, water molecules are attracted to each other.
- Sugar (sucrose) also has oxygens and hydrogens bonded to each other. This gives sucrose many areas of positive and negative charge.
- Water and sucrose are attracted to one another based on the attractions of opposite charges.
- When the attraction that water molecules have for sucrose overcomes the attraction sucrose molecules have for each other, the sucrose dissolves.
- The polar nature of water is what makes water so good at dissolving many substances.
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.