Chapter 15: Solutions

What does “dissolve” mean? When a compound dissolves, the atoms in the compound break apart and spread out.

Substance 1 breaks apart the crystal lattice of substance 2 by pulling on it: Substance 1 Substance 2

Substance 1 is called the solvent. It does the breaking. Substance 2 is called the solute. It gets broken apart. Substance 1 Solvent Substance 2 Solute

Dissolving is NOT a chemical reaction. The atoms are NOT rearranged into new substances, they are just separated temporarily. If you remove the solvent, the solute will go back to its crystal lattice.

Dissolving: Dissolving is when two substances mix and one compound is broken apart by the other. The solute is the substance that breaks apart (usually a solid) The solvent is the substance that does the breaking (usually a liquid) Dissolving is NOT a chemical reaction, so there are no reactants or products.

Think! A teaspoon of dry coffee crystals dissolves when mixed in a cup of hot water. The original crystals are classified as a solute. solvent. reactant. product.

What is a solution? When a substance dissolves in another substance, the whole thing is called a solution. Ocean water is a solution of salt dissolved in water Air is a solution of oxygen dissolved in nitrogen Steel is a solution of carbon dissolved in iron

Types of Mixtures May exist in all states of matter (gas, liquid, solid) Examples: Oxygen and Nitrogen gas (gas/gas) Carbon Dioxide in water (gas/liquid) Water in air (Liquid/gas) Alcohol in water (Liquid/Liquid) Mercury in Silver (Liquid/Solid) Sugar in water (solid/liquid) Copper in Nickel (Solid/Solid)

Solution (homogeneous mixture) Solutions: Solution – a homogeneous mixture of a solute and a solvent Homogeneous means that it looks the same throughout. Ex: Sugar dissolves in water to make sugar water Solute Solvent Solution (homogeneous mixture)

Solutions are homogeneous mixtures of a solute and a solvent. Looks the same throughout

Think! Label the picture using the vocabulary words: homogeneous, heterogeneous Heterogeneous Homogeneous

Think! Label the picture using the vocabulary words: solution, solvent, solute Solute Solution Solvent

Why do substances dissolve? Remember: Particles are in constant, random motion Gases move quickly and freely Liquids flow Solids vibrate in place So all particles bump into each other (whether they are solid, liquid, or gas)

Even though they bump into each other, compounds hold together because of: – Intramolecular forces (within compounds) – Intermolecular forces (between compounds)

But what if they bump into a different substance that they are more attracted to? H+ O–2 H+ Attraction! Na+ Cl–

If the intermolecular attraction (between compounds) is stronger than the intramolecular attraction (within the compound), the compound will dissolve (break apart) H+ O–2 H+ Cl– Stronger Intermolecular Attraction! Weaker Intramolecular Attraction Na+

Polar dissolves polar

Why do substances dissolve? Particles are in constant, random motion. Particles are bumping into each other. If a particle bumps into another particle that it is more attracted to, it can break apart from its crystal lattice and dissolve.

Which particles will be most attracted to each other? Some compounds do not share their electrons evenly. They are called polar molecules.

Other molecules share their electrons evenly. They are called nonpolar molecules.

Dissolving Rule: Like dissolves like Nonpolar solvents will dissolve nonpolar solutes Polar solvents will dissolve polar solutes Ex: water is polar and NaCl is polar, so NaCl will dissolve in water Ex. chalk is nonpolar and ethanol is nonpolar, so chalk will dissolve in ethanol

Polar dissolves polar

Water and NaCl

Think! Label each solute as polar or nonpolar: NH4Cl – C10H8 C2H5OH CO(NH2)2 Polar Nonpolar Half is Polar & half is nonpolar Polar

How fast will something dissolve? Four things affect how quickly a substance will dissolve: Agitation (stirring, shaking, swirling) Temperature Pressure Surface Area

Agitation (stirring, shaking, swirling): When a solid dissolves in a liquid, it takes time for the liquid molecules to grab pieces of the solid and float away. If the liquid is stirred, it speeds up the floating away processes, so more liquid can grab more solid in the same amount of time. Thus stirring or shaking will help a solid dissolve faster in a liquid. The opposite is true for gases dissolved in liquids. When a gas dissolves in a liquid, it is basically trapped by the liquid molecules. If the liquid is stirred, it removes the liquid molecules that are trapping the gas, and the gas will form bubbles and eventually leave the liquid. This is why shaking a carbonated beverage makes so many bubbles.

Which dissolves faster? Sugar in ice water Sugar in hot water Heat makes solid solutes dissolve faster!

Temperature Heat makes things dissolve faster Heat makes particles move faster Then they bump into each other more This makes them dissolve faster

Pressure Higher pressure makes things dissolve faster When pressure increases, particles are forced closer together Then they bump into each other more This makes them dissolve faster

When you want something to dissolve faster, what do you do? Drop it in water and leave it alone Drop it in water and crush it up More surface area makes things dissolve faster

Surface Area More surface area makes things dissolve faster If particles have more surface area, they will have more places to get bumped More surface to bump into means more chances to dissolve

Dissolving happens as the solvent touches the solute and starts to pull it into pieces. This happens only on the outside particles of the solute.

Solid dissolved in liquid Gas dissolved in liquid Factors that SPEED UP the rate of solution Method Solid dissolved in liquid Gas dissolved in liquid Temperature Increase temperature Decrease temperature Particle Size (surface area) Decrease particle size No effect Pressure Increase pressure Agitation (stirring, swirling, shaking) Agitate Do not agitate

3/30/2011 Concentration The concentration of a solution is how much solute is dissolved in a specific amount of solvent. Levels of concentration: Dilute (not much solute dissolved) Concentrated (lots of solute dissolved) Saturated (the maximum amount of solute dissolved)

Which cup of tea has the most tea dissolved in it?

Think! Label each beaker as dilute, concentrated, or saturated.

Is there a limit to how much solute will dissolve? Will 1 teaspoon of salt dissolve in a cup of hot water? Will 1 tablespoon of salt dissolve in a cup of hot water? Will 1 cup of salt dissolve in a cup of hot water? Will 1 container of salt dissolve in a cup of hot water? YES YES Maybe No

Yes, there is a limit! The limit is called the solubility It is a number with a unit It is different for every solute It depends on the temperature and pressure Example: The most sugar that will dissolve in 100 grams of water at 25°C is 67.9 grams. The solubility of sugar in water at 25°C is 67.9 g/100 g H2O.

Solubility The solubility is the maximum amount of solute that can be dissolved in a given amount of solvent. Example: The most salt that will dissolve in 100 grams of water at 25°C is 36.2 grams. The solubility of NaCl in water at 25°C is 36.2 g/100 g H2O.

Think! If the solubility of NaCl at 25°C is 36.2 g/100 g H2O, what mass of NaCl can be dissolved in 50.0 g of H2O? Solute Solvent Temperature 36.2 g NaCl 100 g H2O 25°C 18.1 g NaCl 50 g H2O ½ as much solute ½ as much solvent

Temperature is the independent variable (the one you control). This is shown on the bottom of the graph. You can warm up the solution with a stove or cool it down in a freezer.

The amount (in grams) of how much will dissolve at a particular temperature is the dependent variable (it depends on the temperature). This is shown on the left side of the graph. Notice this is for all the solutes in 100 grams of water.

If the temperature and amount of dissolved solute is on the line shown for that solute, then the solution is saturated. For example, 70 grams of NH3 in 100 grams of water will be a saturated solution at 10 °C.

If the temperature and amount of dissolved solute is above the line, then the solution is supersaturated. If the saturated solution of 70 grams of NH3 is very carefully heated, it will still have 70 grams of NH3 at 20 °C. As this amount is above the line, this would be a supersaturated solution.

If the temperature and amount of dissolved solute is below the line, then the solution is unsaturated. An unsaturated solution could still dissolve more solute if it is added.

If a solute dissolved in solutions ionizes (breaks apart into positive and negative parts) these parts will allow electricity to travel through the solution. For example: pure water will not conduct electricity, but if table salt is added, then the sodium and chlorine that break apart will conduct electricity. NaCl → Na+ + Cl- Electrolytes

Solutions that conduct electricity are called electrolytes Solutions that conduct electricity are called electrolytes. The more of an ionizing solute that is dissolved, the better the conduction of electricity will be.

On the other hand, solutes that don’t ionize (like sugar) will not conduct electricity in water. So sugar water will not conduct electricity. Solutions that don’t conduct electricity are called nonelectrolytes.

Concentration Calculations In general, if there is a lot of solute in the solvent, the solution is called concentrated, and if there is only a little bit of solute in the solvent it is called dilute. But chemists need a more exact way to know how concentrated or dilute the solution is… Percent by mass Percent by volume Molarity Parts per million Dilutions (preparing a solution) *We only learn the green concentration calculations!!

Molarity (M) = moles of solute liters of solution Example: How many moles of HNO3 are needed to prepare 5.0 L of a 2.0 M solution of HNO3?

Molarity (M) = moles of solute liters of solution 2.0 M = moles of solute 5.0 L (2.0 M) × (5.0 L) = moles of solute 10 moles of solute (HNO3)