Forming Solutions 15.1: Pgs

Objectives To understand and describe the process of dissolving To understand and describe the process of dissolving To learn why certain substances dissolve in water and others won’t To learn why certain substances dissolve in water and others won’t To understand factors that affect the rate at which a solid dissolves To understand factors that affect the rate at which a solid dissolves

Important Solution Vocab: Solution - a homogenous mixture. Solution - a homogenous mixture. – Solvent - the substance present in the largest amount. – Solutes - the other substance or substances. Aqueous solutions - solutions with water as the solvent. solvent solute solution

Solubility of Ionic Substances Solubility of Ionic Substances What is occurring when something dissolves? What is occurring when something dissolves? The ionic forces that hold the salt together are overcome by the attractions between the ions and the polar water molecules. The ionic forces that hold the salt together are overcome by the attractions between the ions and the polar water molecules.

Ionic Compounds The resulting solution contains ions that can move about independently! The resulting solution contains ions that can move about independently! –Ex: NaCl (s)  Na + (aq) + Cl - (aq)

Step 1 of Lab 1. Obtain a CLEAN test tube!!! 2. Place 2.2 grams of Na 2 SO 4 into a test tube and add 10mL of water. 3. Dissolve the solid: Heat the test tube in a bunsen burner until you hear a “pop”. Remove the test tube from the bunsen burner and stir the contents with a stirring rod. Continue this process until the solid dissolves. 4. Place the test tube into the test tube rack when the solid is dissolved. 5. Place 2 ice cubes into a beaker and fill the beaker with water (You will use this later) 6. Sit back in your seat.

Factors Affecting the Rate of Dissolving Surface area Surface area –The larger the surface area, the faster the dissolving will occur Stirring will increase the rate of Stirring will increase the rate ofdissolution Temperature Temperature –Dissolving occurs faster at higher temperatures

Solubility of Polar Substances Water can also dissolve nonionic substances Water can also dissolve nonionic substances –Examples: sugar, ethanol These have polar O-H groups in their structure that are attracted to the water These have polar O-H groups in their structure that are attracted to the water Polar OH groups that allow sugar to dissolve in water!

Substances Insoluble in Water Not all substances are soluble in water Not all substances are soluble in water –Ex: Petroleum  What happens when it leaks from an oil tanker in the ocean? Nonpolar substances are not compatible with polar water molecules Nonpolar substances are not compatible with polar water molecules –Ex: Oil and water don’t mix!

An oil layer floating on water.

How Substances Dissolve Follow the saying “like dissolves like” Follow the saying “like dissolves like” –A solvent usually dissolves solutes that have polarities similar to its own. Ex: Water and Sugar: Ex: Water and Sugar: –Since water and sugar are both polar, the solute – solvent interactions formed are similar to the water – water interactions present in the pure solvent

As we add more salt to the solution, eventually there won’t be enough water to surround each ion. What will happen then?

Step 2 of the Lab 1 person from your team must place your test tube into the beaker of ice water and then return to your seat. 1 person from your team must place your test tube into the beaker of ice water and then return to your seat.

Solution Composition Saturated – When a solution contains as much solute as will normally dissolve at that temperature. Saturated – When a solution contains as much solute as will normally dissolve at that temperature. Unsaturated – When a solution contains less solute than can normally dissolve Unsaturated – When a solution contains less solute than can normally dissolve Supersaturated – it contains more solute than a solution will normally hold at that temperature Supersaturated – it contains more solute than a solution will normally hold at that temperature

Solution Composition Supersaturated solutions are very unstable! Supersaturated solutions are very unstable! –A saturated solution can be forced to precipitate by adding one crystal of the solid. Precipitation will continue until the solution reaches the saturation point Precipitation will continue until the solution reaches the saturation point

Last step of the lab GENTLY remove the test tube from the cold water. Add a small amount of Na 2 SO 4 and observe. Clean up the lab: All solution/compounds can be rinsed down the drain. Sit back in your seat.

What just happened??? How would you classify the solution BEFORE the extra solid was added to the test tube? How would you classify the solution BEFORE the extra solid was added to the test tube? –What does this mean? What did the addition of the extra solid do to your solution? What did the addition of the extra solid do to your solution? How would you classify your solution now??? How would you classify your solution now???

Yesterday we covered… 4 factors that will increase the rate of dissolution 4 factors that will increase the rate of dissolution Today we will cover… How temperature not only increases the rate of dissolution, temperature also affects the amount that can dissolve.

Solubility Solubility: the amount of a solute that can dissolve in a given amount of solvent at a specific temperature Solubility: the amount of a solute that can dissolve in a given amount of solvent at a specific temperature The solubility of a substance depends on TWO things: The solubility of a substance depends on TWO things: –The nature of the solute –The temperature of the solvent

Factors Affecting Solubility Solubility is VERY temperature dependent! Solubility is VERY temperature dependent! –Ex: 36.2g of NaCl dissolves in 100g H 2 O at 25 o C, HOWEVER 39.2g NaCl can dissolve in 100g H 2 O at 100 o C! For most substances, solubility increases as temperature of the solvent increases For most substances, solubility increases as temperature of the solvent increases

Expressing Solubility Solubility is expressed as follows: Solubility is expressed as follows: Can use solubility like to set-up proportion problems. Can use solubility like to set-up proportion problems.

Problem: I dissolved 5.2g of a substance into 16g of water. What is the solubility of this substance in units of g / 100g of water? I dissolved 5.2g of a substance into 16g of water. What is the solubility of this substance in units of g / 100g of water?

Solubility Curves What substance has the highest solubility at 50 o C? (exclude KI) What substance has the highest solubility at 50 o C? (exclude KI) What has the lowest at 50 o C? What has the lowest at 50 o C?

Solubility Curve Lab Not all substances are stable enough to create a supersaturated solution. Most substances can only form a saturated solution. Not all substances are stable enough to create a supersaturated solution. Most substances can only form a saturated solution. We will complete this as a class. Each pair of students will be assigned a specific amount of salt to use and we will collaborate our data to create a graph. We will complete this as a class. Each pair of students will be assigned a specific amount of salt to use and we will collaborate our data to create a graph.

Directions Heat the test tube in hot water on a hot plate. Make certain all the solid has dissolved. Heat the test tube in hot water on a hot plate. Make certain all the solid has dissolved. Remove the test tube from the hot water and place a thermometer into the test tube. Remove the test tube from the hot water and place a thermometer into the test tube. Continue to stir the contents slowly until you see some crystals (solids) begin to form. Record the temperature for when the solid begins to form in the data table on the board. Continue to stir the contents slowly until you see some crystals (solids) begin to form. Record the temperature for when the solid begins to form in the data table on the board. Repeat steps 1-3 in order to record a total of 2 trials. Repeat steps 1-3 in order to record a total of 2 trials. Calculate and record the solubility of KNO 3 in units of “g/100g of water” in the data table. Calculate and record the solubility of KNO 3 in units of “g/100g of water” in the data table. When you are done, work on textbook homework and worksheet that is due tomorrow. When you are done, work on textbook homework and worksheet that is due tomorrow.