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Chapter 5 Solutions. Science January 17, 2011. What would happen if you put sand in a test tube. The sand would fall to the bottom of the test tube and never dissolve in the water. What would happen if you put salt into a test tube of water?.
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Chapter 5 Solutions Science January 17, 2011
What would happen if you put sand in a test tube • The sand would fall to the bottom of the test tube and never dissolve in the water.
What would happen if you put salt into a test tube of water? • The salt would fall to the bottom of the test tube but unlike the sand the salt would dissolve and you would have a solution of water
The salt is being dissolved into a solution sand does not dissolve in water. Salt and water together is a type of solution: a mixture in which the particles of one substance are evenly mixed with the particles of another substance.
In salt water, sodium ions and chlorine ions are evenly mixed with the molecules of water. There are different types of solutions. • Liquid Solutions: formed when solids, liquids, and gases dissolve in a liquid
Examples: • Salt water: salt is a solid and water is a liquid • Soda: Carbon Dioxide, which is a gas is dissolved in the soda, which is a liquid. • Antifreeze: Antifreeze is a liquid that is dissolved in water which is also a liquid.
Other Solutions are those where different substances are dissolved into solids and gases • Examples: Air is an example of a gas dissolved in a gas. Oxygen and other gases are dissolved into Nitrogen (most abundant). Brass is an example of a solid dissolved in a solid; It is zinc and copper.
Did you know that tap water is a solution? • Tap water sometimes has dissolved iron in it or even compounds made with calcium. It can also have chlorine in it.
Did you know the air you breathe is a solution? • Air is full of different gases. There is nitrogen (which makes up most of the air that you breathe), oxygen and other gases all mixed together.
A solution is made when: particles of one substance are evenly mixed with the molecules of another substance. How is salt water made? • You take water and put it into a beaker. • You then add salt to the water • You can stir the mixture to make the salt dissolve faster. • Let’s make a mixture of salt water!!
The part of the solution that dissolves is called the solute. The part of the solution that the solute dissolves in is called the solvent. The solvent is usually the part of the solution that is present in a greater amount.
Examples: Salt water: salt is the solute and the water is the solvent. In air, nitrogen would be the solvent because there is more of it in the mixture. Oxygen and the other gases are considered the solutes. • In most solutions the solute usually disappears. When some solutes dissolve in a solvent they can also create a colored solution.
Examples: • Solutes that disappear: salt and sugar • Solute that produces a color: if you place a crystal of a compound called potassium permanganate into water and let it sit, the water will eventually turn to a pink color after it sits for a few minutes.
Some substances are more soluble, able to dissolve, than other substances. • Solubility: the maximum amount of a substance that will dissolve in a given quantity of a solvent at a given temperature.
Example: • 38g of table salt (sodium chloride) will dissolve in 100g of water at 20°C. If you add more than this, the extra salt you added will not dissolve. It will sink to the bottom.
There are also many substances that will not dissolve in water. Examples: Marbles, Aluminum foil, plastic, and many others. • They are called insoluble. • Example: When you pour water into a glass, does the class dissolve….no!!
Some substance will be soluble in some solvents but are insoluble in other solvents. • Example: Sugar will dissolve in water but not oil. Oil does not dissolve in water either. • It all depends on the chemical makeup of the solvent that will determine if a substance dissolves in it or not.
As you may have learned before, water molecules are polar. A polar molecule is a molecule in which one end has a positive charge and the other end has a negative charge. The charge is so small that you cannot call the bond an ionic bond, it is covalent.
What is water made up of? • One oxygen atom and two hydrogen atoms. • The atoms are covalently bonded together. • Oxygen has a slight negative charge. • The hydrogen atoms have a slight positive charge.
Water is sometimes called the universal solvent. It is called this because many types of substances dissolve in it. The electric charges that water has help dissolve different substances. The charged ends of the water molecules help separate particles of a solute and spread them out throughout the water.
Example: • Sugar water: When we put the sugar cube in the water, the sugar began to dissolve. What was happening was the water molecules were pulling the sugar molecules to them because water has a slight charge. This separated the molecules of sugar so that the sugar molecules were evenly distributed through the water.
A molecular solution is formed when there are forces of attraction between the solvent and the solute. The attraction between the two is greater than the force of attraction between the atoms that make up a molecule. • Example: The force between the water and a sugar molecule is greater than the force between the sugar molecules.
As you know ionic compounds are made up of charged particles. These particles are positive and negative. • How are they held together? • By a force of attraction between opposite charges.
Example of an ionic solution: • Salt water: Sodium ions and the chloride ions are attracted to the charged ends of the water molecules which is polar. • Q: Why doesn’t sodium explode in water? • A: it’s an ion not an atom!
So how can you make a solute dissolve faster in a solvent? One way to make the solute dissolve faster would be to stir the solution. Stirring is going to help the crystals of a certain substance to come apart quicker. • Another factor that can speed up the rate that a solid solute dissolves in a liquid solvent is temperature. • Example: If the temperature of water is higher then solid solutes will dissolve faster.
When you have a liquid solvent and a gaseous solute, the opposite happens. The higher the temperature the slower the gas will dissolve in the liquid. • Example: When soda is made, it is made at a low temperature because otherwise the gases will not dissolve.
The surface area of the solute will also determine how long it will take to dissolve. The smaller the surface area the faster it will dissolve. The larger the surface area, the slower it will dissolve. • Example: a solid sugar cube will dissolve slower than a crushed sugar cube. This is because the surface area of the solid sugar cube is greater than the crushed one.
When it comes to gases, the amount of pressure that is present will affect the rate of solubility. If the pressure increases, the faster the gas will dissolve. • Example: When you have a full can of soda you cannot squeeze it because it’s pressurized. When the soda cap makes that noise when you open the can the gas is trying to escape. When you open the can the gas comes out of the solution because the pressure is lost.
Polar Solvents: • Water • Dissolve compounds that are polar and ionic compounds but will not dissolve solutes that are non polar such as oil and fat.
There are different concentrations that a solution might have. You may have hear the terms dilute and concentrated before. Well if you haven’t lets go over what they mean. Both represent how much of a solute and solvent are in a solution. • A dilute solution: a solution containing a small amount of solute compared with the amount of solvent present. These solutions are weak.
A concentrated solution: a solution containing a large amount of solute compared with the amount of solvent present. These solutions are strong. • Example: 100mL of water and one sugar crystal is a dilute solution and is weak • Example: Frozen Concentrate Orange Juice has a lot of orange and sugar and little water and is a concentrated strong solution.
There are also solutions in which we call unsaturated and saturated solutions. An unsaturated solution is a solution containing less solute than it can hold at a given temperature. • Example: Having one gram of salt dissolved in 100mL of water at 20 ° C when you know that 38 grams can be dissolved at this temperature
A saturated solution is a solution containing all the solute it can hold at a given temperature. • Example: 38 grams of salt dissolved in 100mL of water at 20°C.
There is also another kind of solution called a supersaturated solution. A supersaturated solution is a solution containing more solute than it can normally hold at a given temperature. • Example: Had a saturated solution of salt water. Raise the temperature up to 50°C. You can now add more salt because it is at a higher temperature. Let the solution cool and now the water will be holding more salt than it can normally hold.
How do solutes affect freezing point? • Freezing point: temperature at which a liquid changes to a solid. • Freezing point depression: decrease in the freezing point of a liquid solvent because of the addition of a solute. • Note: The freezing point of pure water is 0°C – when water freezes its molecules form a crystal pattern called a lattice.
Salt water however does not freeze at 0°C because the salt particles dissolved in the water interfere with the change from a liquid to a solid, therefore its freezing point is lower. Freezing point depression: • Adding a solute to the solvent will lower its freezing point ex. Water is 0°C and salt water is lower than 0°C.
Example in our everyday life: if you have ever sprinkled rock salt or calcium chloride pellets on an icy sidewalk, you have lowered a freezing point. These ice melters lower the freezing point of water. When the ice starts to melt, the chemical compounds dissolve in the water forming solutions that freeze at a lower temperature than water.
How do solutes affect boiling point? • Boiling point: temperature at which a liquid changes to a gas • Boiling point elevation: increase in the boiling point of a liquid solvent because of the addition of a solute
Boiling point of water: - boiling point of pure water at sea level is 100°C - water -> steam Note: adding heat to boiling water dosen’t raise its temperature - salt water however doesn't boil at 100°C - because salt water contains dissolved salt particles that interfere with the change from a liquid to a gas - therefore the boiling point temperature of salt water is higher than 100°C
Boiling point elevation: as the amount of solute in the solvent increases, the boiling point increases.
