Key Concepts

The procedure of dissolving deserve to be endothermic (temperature go down) or exothermic (temperature goes up).When water dissolves a substance, the water molecule attract and “bond” to the particles (molecules or ions) that the substance leading to the particles to different from each other. The “bond” that a water molecule provides is not a covalent or ionic bond. It is a solid attraction brought about by water’s polarity.It takes energy to break the bonds in between the molecule or ions of the solute. Energy is released when water molecules bond come the solute molecule or ions.If the takes an ext energy to separate the corpuscle of the solute than is released as soon as the water molecule bond come the particles, climate the temperature goes under (endothermic). If it takes less energy to different the particles of the solute 보다 is released when the water molecules bond to the particles, then the temperature goes increase (exothermic).


Students will certainly feel the temperature readjust that occurs when a cold pack and a hot pack space activated. Castle will watch that these temperature transforms are as result of a solid substance dissolving in water. Students will then to compare the temperature alters that happen as four various solutes dissolve in water and also classify these as either endothermic or exothermic. Students will certainly be presented to the concept that it takes energy to rest bonds and also energy is released when bonds room formed during the process of dissolving.


Students will have the ability to identify variables in one experiment to uncover out exactly how much the temperature boosts or decreases as each of 4 solutes disappear in water. College student will have the ability to correctly divide the process of dissolving together either exothermic or endothermic for each solute. College student will have the ability to explain that the temperature transforms in dissolving room a an outcome of the quantity of power released contrasted to the lot of energy used together “bonds” are formed and broken.

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Download the student activity sheet, and also distribute one every student when specified in the activity. The task sheet will certainly serve together the “Evaluate” component of each 5-E great plan.


Be certain you and the students wear effectively fitting goggles. Overabundance dry product from the hot and also cold packs can be placed in the trash.

Materials because that the Demonstrations

2 disposable cold packs2 disposable warm packsGraduated cylinderWater (room temperature)2 clear plastic cups2 thermometers

Materials for Each Group

Potassium chlorideCalcium chlorideSodium carbonateSodium bicarbonateWater5 tiny cupsGraduated cylinderThermometerGram balance

Notes around the Materials

The hand warmer shown in the video clip in the Extend includes a super-saturated equipment of salt acetate v a tiny metal disk inside a clean plastic bag. This warmer is activated by bending the metal disk slightly. This hand warmers, dubbed The heat Solution, are accessible from Flinn Scientific, product number AP1933, and a variety of various other vendors. Potassium chloride might be purchased in ~ a grocery keep under the brand surname Nu-Salt Salt instead of or indigenous Flinn Scientific, product number P0042.Calcium chloride might be purchased in ~ a hardware keep under the brand name Damp-Rid or stimulate calcium chloride, anhydrous product number C0016 native Flinn Scientific.Sodium carbonate, anhydrous may be purchased native Flinn Scientific, product number S0052. Salt bicarbonate is baking soda. That may also be purchased from Flinn Scientific, product number S0043.

Read more about the energy changes in making and also breaking bonds in the context of dissolve in the teacher elevator section.

Note: This task deals v a concept that is not frequently addressed in center school—that a temperature adjust occurs throughout the process of dissolving. Most center school textbooks and also curricula combine a adjust in temperature only with chemical change. Dissolving is usually thought about a physical adjust but additionally can an outcome in a change in temperature. This change in temperature is based on the energy changes associated in breaking and making “bonds” in the procedure of dissolving.


Allow student to feel the temperature readjust in an caused cold pack and also an activated hot pack.

Tell students that they will check out how some hot and also cold packs work. Offer them a hint the it needs to do v dissolving, i beg your pardon they have been studying in this chapter.

Materials for the Demonstration

1 disposable cold pack1 disposable hot pack


Select 2 student volunteers—one to activate one cold pack and also another come activate one hot pack. Have each student feel each bag and also guess what is inside each. Castle should notice a dry pellet-like solid and a fluid-filled bag. Allude out the the bags perform not feeling cold or hot yet.

Direct the students come activate your packs by complying with the accuse on the package. Have actually them shiver the package to obtain the liquid to spread throughout the bag.

Pass the cold and hot packs around the room.

Expected results

The cold pack conveniently becomes cold when the hot pack quickly becomes hot.

Do a show to show how cold and also hot packs work.

Note: In this demonstration, 1 tespoon of each substance is liquified in 10 mL that water. Due to the fact that the function of this demonstration is to present whether the temperature merely goes up or down, this form of volume measure is fine. But to compare which substance is an ext or much less exothermic or endothermic 보다 another, as students will carry out in the activity, the solute will certainly be measured in grams.

Materials for the Demonstration

1 disposable cold pack1 disposable warm packGraduated cylinder, 50 mL or smallerWater (room temperature)2 clean plastic cups2 thermometers1 teaspoon


Carefully reduced open one cold pack and also one warm pack. Display students the contents, however do not take care of or enable students to manage the heavy substance inside the packs. Tell students that the liquid within the fluid-filled bags in both the cold and also hot package is water. Pour around 10 mL of room-temperature water in two separate clear plastic cups. Ar a thermometer in each cup and also select two student volunteers to call the course the starting temperature the the water in each cup.

With the thermometer still in the cup, add about 1 teaspoon of the solid problem from the cold load to the water in one cup. Tenderness swirl the cup to assist the substance dissolve. Have the class watch the thermometer and then questioning a student to call the class the lowest temperature the the solution.

With the thermometer still in the cup, place about 1 tespoon of the problem from the warm pack in the water in the various other cup. Gently swirl the cup to assist the substance dissolve. Have actually the course watch the thermometer and also then asking a college student to call the class the greatest temperature of the solution.

Expected Results

Dissolving the substance from the cold load will cause the temperature to decrease to less than 10 °C (endothermic). Dissolve the problem from the warm pack will reason the temperature to rise to over 40 °C (exothermic). Results might vary.

Introduce the terms endothermic and exothermic.

Tell students that scientists describe temperature transforms that occur when substances interact as one of two people endothermic or exothermic. As soon as the temperature decreases, as it walk in the cold pack, the procedure is endothermic. As soon as the temperature increases, as it go in the hot pack, the process is exothermic.


Introduce the dissolving task students will execute and help students recognize the variables.

Tell students that they will certainly compare exactly how much the temperature changes when four family substances dissolve in water. Introduce the crystals students will dissolve:

Potassium chloride is a usual salt substitute.Calcium chloride is supplied to absorb moisture from the air. The is likewise included in part ice-melt mixtures to act icy sidewalks throughout winter. Salt carbonate is a usual ingredient in detergents for dishwashing machines.

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Have students monitor alters in temperature as they dissolve four different household solutes in water.

Question come Investigate

Which solute dissolves the most endothermically and which dissolves the most exothermically in water?

Materials because that Each Group

Potassium chlorideCalcium chlorideSodium carbonateSodium bicarbonateWater5 little cupsPermanent mite or masking tape and penGraduated cylinderThermometerGram balance


Label the little plastic cup Potassium Chloride, Calcium chloride, salt carbonate, and also Sodium bicarbonate.Weigh 2 g of each solute and place them in your labeled cups. Add 10 mL that water come the small unlabeled cup and place a thermometer in the water. Document this initial temperature in the chart on the task sheet.

Pour the potassium chloride into the water and swirl the cup. Clock the thermometer.

When the temperature stop changing, document the final temperature. Repeat procedures 3–5 for each solute.

Expected Results

Potassium chloride dissolved the most endothermically, and also calcium chloride liquified the many exothermically. Student temperature readings will certainly vary, but will most likely be comparable to the following:

Discuss student observations.

Ask students:

Which solutes dissolved endothermically? Potassium chloride and also sodium bicarbonateWhich solute dissolved the most endothermically?Potassium chlorideWhich solutes liquified exothermically?Calcium chloride and also sodium carbonateWhich solute liquified the most exothermically?Calcium chloride