7th Science Chapter 5 Short Answer final #3

A water molecule (H2O) is polar. This means that it has a slight negative charge near the oxygen atom and a slight positive charge near the hydrogen atoms. Using what you know about covalent bonding in the water molecule, explain why a water molecule is polar.

In the covalent bond between oxygen and hydrogen in a water molecule, electrons are shared between the oxygen atom and the hydrogen atom. But the electrons are not shared equally. They are attracted a bit more to the oxygen so the oxygen part of the molecule is slightly negative. Since the electrons are pulled away from the hydrogen atoms a bit more, the hydrogen parts of the molecule are slightly positive.

Water molecules attaching to a substance releases energy, causing the temperature to go _________.

up

Water molecules pulling a substance apart absorbs energy, causing the temperature to go _________.

down

Your teacher added liquid food coloring to water. The food coloring mixed and dissolved thoroughly in the water. Would you guess that food coloring molecules are polar or non-polar? Explain.

I think they are probably polar. Since they mix so well with the water, maybe they have polar areas that the water molecules are attracted to that helps them mix thoroughly together.

Is it better to compare mass or volume if you want to measure equal amounts of two different solid substances?

If you are trying to measure an equal amount of each substance, it is better to measure the mass of the two substances. If the pieces of the two substances are different size and shape, they will fill a scoop or spoon or other volume measure differently and you will have different amounts.

What does it mean to say that something is a "polar molecule"?o

A polar molecule is a molecule that has a slight positive charge in one part of the molecule and a slight negative charge in another. This happens in certain molecules, like water, because the sharing of electrons in their covalent bonds is not equal.

Why does water dissolve salt more effectively than isopropyl alcohol does?

Alcohol molecules only have one area that is polar and they have a large area which is not polar. These molecules cannot surround and attract the ions in salt as well as water molecules can. Therefore, alcohol does not dissolve salt as well as water does.

You saw that water is better than alcohol at dissolving salt (sodium chloride). Using what you know about water molecules, alcohol molecules, and the positive and negative ions in sodium chloride, explain why water is better than alcohol at dissolving salt.

Alcohol molecules only have one area that is polar and they have a large area which is not polar. These molecules cannot surround and attract the ions in salt as well as water molecules can. Therefore, alcohol does not dissolve salt as well as water does.

Solubility is a characteristic property of a substance. This means that the amount of a substance that dissolves in water is unique for that particular substance. Use what you know about the process of dissolving to explain why a substance would have its own characteristic solubility.

Dissolving depends on the attractions and interaction between the liquid (solvent) and the substance being dissolved (solute). These attractions depend on the atoms and molecules that make up the solvent and the solute. Since each substance is made up of its own unique type, number, and arrangement of atoms and molecules, each substance should have its own unique solubility.

To see if temperature affects dissolving, you put the same color M&M in cold, room temperature, and hot water. Explain why it was important to use the same color M&M in each temperature.

It's possible that different M&M colors dissolve at different rates. So you want to use the same color to see if temperature affects dissolving. If you used different colors, you wouldn't know if it was the temperature or the color that might be causing your results.

Why does sugar not dissolve in mineral oil?

Oil is made of carbon atoms bonded to hydrogen atoms. These covalent bonds are not very polar. Sugar is made of sucrose molecules which do have polar areas. When sugar is placed in oil, the oil molecules are not attracted to the sucrose molecules. The sucrose molecules are more attracted to each other than to the oil so the oil does not dissolve the sucrose.

When you put an M&M in oil, none of the sugar (sucrose) coating dissolved in the oil. Use what you know about the polarity of oil molecules and sucrose molecules to explain why oil does not dissolve the sucrose.

Oil is made of carbon atoms bonded to hydrogen atoms. These covalent bonds are not very polar. When an M&M is placed in oil, the oil molecules are not attracted to the sucrose molecules. The sucrose molecules are more attracted to each other than to the oil so the oil does not dissolve the sucrose.

You may have heard the phrase "oil and water don't mix". Using what you know about atoms, molecules, and dissolving, explain why you think this might be.

Oil is made up of carbon atoms bonded to hydrogen atoms. These C-H bonds are not very polar. When oil is mixed in water, the polar water molecules are more attracted to each other than to the oil so the oil and water do not mix.

You mixed alcohol, oil, and corn syrup into water to see which of these liquids dissolve in water. The alcohol and corn syrup dissolved in the water but the oil did not. Explain, on the molecular level, why the oil did not dissolve in the water.

Oil is made up of carbon atoms bonded to hydrogen atoms. These C-H bonds are not very polar. When oil is mixed in water, the polar water molecules are more attracted to each other than to the oil so the oil and water do not mix.

Briefly explain, on the molecular level, how water dissolves salt.

Salt (NaCl)is made of positive sodium ions (Na+) and negative chloride ions (Cl-). The positive hydrogen end of water molecules are attracted to the negative chloride ion. And the negative oxygen end of water molecules are attracted to the positive sodium ion. When enough water molecules attract these ions, they can pull them apart, resulting in dissolving.

Here are pictures of a molecule of carbon dioxide (CO2) and carbon dioxide dissolved in water. The carbon dioxide molecule has a slight negative charge near the oxygen atoms and a slight positive charge near the carbon atom. Using what you know about water molecules, and information from the drawings, explain why carbon dioxide molecules dissolve in water.

Since carbon dioxide has areas of slight positive and negative charge, and water has areas of slight positive and negative charge, they are attracted to each other. They are attracted enough so that carbon dioxide gas will dissolve in water.

When you let the water evaporate from the solutions you made, the recrystallized substances all looked different. Explain on the molecular level why the substances look different when they recrystallize.

Since each substance has its own unique type, number, and arrangement of atoms and molecules, each substance would have a unique way of coming together and forming a crystal after the water evaporates.

You put a drop of water and a drop of alcohol on a paper towel and saw that the alcohol evaporated faster than the water. Alcohol molecules are not as polar as water molecules. Use the difference in polarity between water and alcohol molecules to explain why alcohol evaporates faster than water.

Since water is more polar than alcohol, water molecules are more attracted to each other than alcohol molecules are to each other. This makes it harder for water molecules to come apart from one another to evaporate.

Sodium chloride (NaCl) is made up of sodium ions (Na+) and Chloride ions (Cl-). Use what you know about polar water molecules to explain why water is able to dissolve salt

Since water is polar, it has an area of slight negative charge at one end of the molecule and slight positive charge at the other. The positive end of the water molecules attracts the negative chloride ions and the negative end of the water molecule attracts the positive sodium ions. Enough water molecules attracting the ions with enough energy can pull the ions apart and cause the salt to dissolve.

What is surface tension and why does water have a strong surface tension?

Surface tension is how strong the surface of a liquid is. Water has a strong surface tension because water molecules are very attracted to each other. The ones at the surface are attracted down and in and form a strong surface.

How many variables is it necessary to control in order to conduct a fair test?

To conduct a fair test, you need to control all the variables except the one you are trying to learn about.

You placed an M&M in water and saw the sugar (sucrose) coating dissolve in the water. Use what you know about the polarity of water molecules and sucrose molecules to explain the process of water dissolving sucrose.

Water is polar because of the covalent bond between oxygen and hydrogen. There is a slight positive charge near the hydrogen and a slight negative charge near the oxygen. Sucrose also has a lot of covalent bonds between oxygen and hydrogen so it also has positive and negative areas. The polar water molecules attract the oppositely charged area of the sucrose and pull one sucrose molecule away from another causing the sucrose to dissolve.

When you placed an M&M into cold, room temperature, and hot water, you saw that more of the sugar coating dissolved in the hot water than in the room temperature or cold water. Explain, on the molecular level, why you think the hot water caused more of the sugar coating to dissolve.

Water molecules are moving faster in hot water. Also, the sucrose molecules are vibrating faster. The fast-moving water molecules that attract the sucrose molecules can pull on them with more force, causing more molecules to break away from other sucrose molecules and dissolve.

Why does increasing the temperature cause more solute to dissolve in a solvent?

Water molecules are moving faster in hot water. Also, the sucrose molecules are vibrating faster. The fast-moving water molecules that attract the sucrose molecules can pull on them with more force, causing more molecules to break away from other sucrose molecules and dissolve.

The surface of water bends but doesn't break under the weight of a paper clip or water strider. What is it about water molecules and the way they interact that gives water this strong surface tension?

Water molecules are polar which means that they are slightly positive at one end and slightly negative at the other. They are attracted to one another based on these positive and negative areas. The water molecules at the surface of water are pulled together and don't have any molecules above them to attract them in another direction so the attractions at the surface give it a strong surface tension.

Explain on a molecular level why water molecules have an attraction for each other?

Water molecules have an attraction for each other because they are polar molecules. They have a slight negative charge near the oxygen atom and a slight positive charge near the hydrogen atoms. The positive part of one molecule attracts the negative part of another.

In terms of making and breaking bonds, explain why the temperature sometimes changes when solids are dissolved in liquids.

When water bonds to the solute, energy is released. But it takes energy to break the bonds of the solute and pull it apart. So if more energy is released in the bonding than it takes to break the bonds, the temperature goes up (exothermic). If it takes more energy to break the bonds of the solute than is released when water bonds to it, the temperature goes down (endothermic).