The Properties of Water Worksheet

Transcription

1 Name Date The Properties of Water Worksheet 1. Water is an unique molecule that is essential for life on Earth. The molecular formula for water is O. Water is made of two hydrogen atoms and one oxygen atom. The hydrogen H 2 atoms are connected to the oxygen atom through covalent bonds. What is a covalent bond? 2. The electrons of each hydrogen atom are shared with the oxygen atom to create a polar covalent bond. The water molecule is polar. One side of the water molecule carries a slight positive charge and the other side of the molecule carries a slight negative charge because the electrons are not evenly shared between the atoms. Predict the polarity of the water molecule by placing the minus sign over the negative region and the plus sign over the positive region. Take a photo. 3. The hydrogen side has a slight positive charge and the oxygen side has a slight negative charge. Each hydrogen atom has one electron and the oxygen atom needs two electrons. Even though the electrons are shared, oxygen, being bigger and stronger, more electronegative, pulls on the electrons more. The electrons spend a bit more time orbiting the oxygen and a little less time orbiting the hydrogens, thus there is an overall slight difference in the charge on the two sides of the molecule. 1

2 4. It is because of this polarity that water molecules are attracted to each other. Using your background knowledge on charge and attraction, arrange the water molecules to demonstrate how they would arrange themselves because they are attracted one another. Take a photo. 5. In between two different water molecules hydrogen bonds form. One hydrogen atom from one molecule forms a weak bond with an oxygen atom on another molecule. Water molecules are attracted to other water molecules and resist separating from each other. This is known as cohesion. Open your Backpack and add three new models that demonstrate cohesion. Take a photo. 6. The cohesive properties of water result in surface tension, the strength of the water s surface due to the water molecules forming hydrogen bonds and sticking together. This gives water an invisible film that allows some organisms to run or walk on water without breaking the surface! You may also have experienced water tension first hand if you have ever belly flopped into a pool. Ouch! 2

3 7. Water also has a high specific heat because it is a polar molecule that likes to stick to itself. Specific heat is the amount of heat needed to raise the temperature of 1 gram of a substance 1 degree celsius. It takes 4140 Joules or 1 Cal of energy to raise the temperature of 1 gram of water. In comparison, it takes 1005 Joules or 0.24 Cal to raise the temperature of 1 gram of air. 71% of the Earth s surface is covered by water. Living cells are made mostly of water. Your body is 73% water. Why is a high specific heat capacity for water advantageous for life? 8. As water evaporates, the hydrogen bonds between different water molecules break and the water molecules are free to move around as a vapor. As water vapor cools and becomes more dense, the hydrogen bonds between different water molecules constantly break and reform, giving water fluidity. What do you think happens to the hydrogen bonds as water molecules continue to cool down and freeze? Hint: Remember that the oxygen atom of one water molecule will be attracted to an hydrogen atom of a different water molecule, but also be repelled by the oxygen atom of that different water molecule at the same time! 9. The oxygen atom of one water molecule is attracted to an hydrogen atom of a different water molecule and repelled by the oxygen atom of that different water molecule at the same time! This push between oxygen atoms and pull between oxygen and hydrogen atoms causes water to form a crystalline or rigid lattice structure. Each molecule is a fixed distance from neighboring molecules. How does this impact the density of frozen water compared to liquid water? 10. Because the molecules in ice are farther apart, ice is less dense than liquid water. This is why ice floats! Why is it important that ice floats and does not sink in aquatic environments such as lakes? 3

4 11. As you learned earlier, water is polar and forms hydrogen bonds with other water molecules. These two properties make water an universal solvent. A solvent is a substance that combines with other substances. A great example of one of these other substances is salt, or NaCl. Salt is a polar molecule that dissolves quickly in water because salt is hydrophilic, water loving. Sodium, or Na+, has a slight positive charge and chloride, or Cl-, has a slight negative charge. Sodium is attracted to the oxygen atom in water and chloride is attracted to the hydrogen atom. Salt dissolves in water to form a solution or homogeneous mixture because both are polar molecules. Dissect the salt model to demonstrate how salt dissolves in water. Take a photo. 12. Knowing that water is an excellent solvent, provide at least one reason why your body is 73% water. Hint: What processes can happen in your body because you contain so much water? 13. Fat molecules, like olive oil, are nonpolar, meaning they are not charged like a water molecule. Nonpolar molecules are said to be hydrophobic or water fearing. Predict what you think happens at a molecular level when oil is added to water. 4

5 14. Cell membranes are made of fats. The cell cytoplasm is mostly made of water. Explain how the interaction between fats and water, nonpolar and polar molecules, allows for the structure of cells. 15. Summarize the properties of water in 3-4 sentences. 5