Why does salt dissolve slower in cold water

Updated April 27, 2018

By Natalie Andrews

At room temperature, you need at least 100 grams of water to dissolve around 35 grams of salt; however, if the temperature changes, the amount of salt that water can dissolve also changes. The point where water can no longer dissolve salt is called saturation, and this happens when the salt you add only goes to the bottom of the solution. There are also several factors that facilitate salt’s solubility in water.

In general, you can dissolve 35 grams of salt in 100 mL of water. However, increasing the temperature can help you dissolve more.

Most substances diffuse in water at direct proportion to temperature increase. Some elements also readily dissolve in water with the slightest temperature increase, like salt potassium nitrate. The solubility of sodium chloride or table salt is only slightly affected by temperature increase. Aside from this, salt also increases the temperature at which water boils. With 100 grams of almost-boiling water (around 200 to 212 degrees F), you can add around 40 grams of salt before it becomes saturated.

Salt dissolves faster in hot water than in cold water. Conversely to heating, salt lowers the temperature at which water freezes. Adding salt as the solute to water (solvent) at water’s freezing temperature disrupts the equilibrium of water. Salt molecules compete with and displace the water molecules, but will repel ice that is formed at this juncture. The salt increases the melting point of water, meaning salt slows down ice melting. Adding more salt will create a significantly lower melting and freezing point for water.

In an unsaturated salt solution, the solute molecules (salt) become hydrated by the solvent (water), thus decreasing the size of the salt crystals and eventually dissolving the salt. In a saturated solution, a point of equilibrium is reached where the crystal particles either keep getting dissipated or stick to the crystal, forming smaller sized crystals in water. In room temperature, the saturation point is reached when the water is no longer able to take in any salt molecules, thereby forming two separate layers of solute (salt) and solvent (water). At a number of degrees below the freezing point of water, about -5.98 degrees F, water can no longer hold any more salt molecules. At this point, a mixture of solid ice and crystal salt is observed.

One factor to consider when studying salt solubility in water is the kind of salt used. For example, rock salt diffuses less readily than table salt or canner’s salt. This is because rock salt has more impurities, which take more time for water molecules to break up.

Does salt dissolve quicker in hot or cold water?

Aim: To find out if the temperature of water affects the speed at which salt dissolves.
Hypothesis: The salt will dissolve faster is hot water rather than cold.
Equipment: 

• Cold water in a clear glass.
• Hot water in a clear glass.
• Two TBS of salt (one in each glass)
• Spoon for stirring.
• Timer
• Pen and paper to record results.

Method:

1. Gather equipment.
2. Make sure your two glasses have an equal amount of hot/cold water.
3. Place one TPS of salt in each glass.
4. Start your timer once salt is placed into your glass and once the salt is dissolved, record the time taken for this occurance.
5. Stir both glasses at the same speed until the salt dissolves.
6. Record the time taken for the salt to dissolve in both hot and cold water.

Variables:Dependant: Time taken for the salt to dissolve.Independent: Temperature of the water.Control: Same amount of water, same glass, same amount of salt, stirred at the same speed.

Risks:

Risk of burning yourself with hot water- Be extra cautious when dealing with hot water by keeping your distant from it and safely carry it.Spilling and slipping on liquid- Be cautious when handling, to prevent from spilling place you liquids safely on the table. If spilling occurs, clean up immediately.Breaking glass cups and cutting yourself- Be cautious when handling with glass. Wear long pants, sleeves and closed shoes to avoid cuts.Salt going in your ayes- Wear safety glasses to prevent the salt from going in your eye. If salt reaches your eye, rinse immediately. 

Results: The salt dissolved quicker in hot water.

Discussion: Salt would dissolve quicker in hot water because the molecules in hot water move faster than cold water because of the higher energy, therefore the molecules of hot water move fast, they can more easily break apart the other molecules of a different substance when they come into contact.
Conclusion: Salt dissolves better and faster in hot water.
Diagram: As you can see below, it has taken 42s for salt to dissolve in hot water (first image) but 1m 1s for cold water. (second image)

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• Adding energy (heating) increases molecular motion.
• Increased molecular motion competes with the attraction between solute molecules and tends to make them come apart more easily.
• Increased molecular motion causes more solvent molecules to contact solute molecules and pull on them with more force, usually resulting in more dissolving.
• Since different substances are made from different atoms, ions, or molecules, increased temperature will affect their dissolving to different extents.

Students revisit the dissolving M&M activity from Lesson 4. They will design an experiment to see if temperature affects the amount of dissolving of the sugar coating of an M&M.

Objective

Students will be able to identify and control variables to design an experiment to see whether the temperature of a solvent affects the speed at which a solute dissolves. Students will be able to explain, on the molecular level, why increasing temperature increases the rate of dissolving.

Evaluation

Download the student activity sheet, and distribute one per student when specified in the activity. The activity sheet will serve as the “Evaluate” component of each 5-E lesson plan.

Safety

Be sure you and the students wear properly fitting goggles. Warn students not to eat the M&M’s.

Materials for Each Group

• M&M's (3 same color)
• 3 clear plastic cups
• 1 sheet of white paper
• Room-temperature water
• Hot water (about 50 °C)
• Cold water (about 5 °C)

Materials for the Demonstration

• 4 graduated cylinders (50 mL)
• Hot water (about 50 °C)
• Cold water (about 5 °C)
• Salt
• Sugar
• Tablespoon
• 2 small cups

1. Remind students of the experiment they did in Lesson 4 in which they placed an M&M in water and watched the colored sugar coating dissolve. In that experiment, students used water and then alcohol and oil to see if the solvent used affects the dissolving of the M&M coating.

   Ask students:

   What could you investigate about M&M’s dissolving in water? If students do not suggest changing the temperature of the water, ask whether they think the temperature of the water affects the amount of coating that dissolves.

   Give each student an activity sheet.

   Students will describe their experimental design, record their observations, and answer questions about the activity on the activity sheet. The Explain It with Atoms & Molecules and Take It Further sections of the activity sheet will either be completed as a class, in groups, or individually, depending on your instructions. Look at the teacher version of the activity sheet to find the questions and answers.

   Have student groups discuss the following questions:

   • How could you investigate whether the temperature of water affects the amount of coating that dissolves from an M&M?
   • What are the variables in this experiment, and how will you control them?

   As you visit the groups and listen to their discussions, check to see if students are thinking about variables such as:

   • Kind of container
   • Amount of water
   • Color of the M&M’s
   • When the M&M’s are placed in the water
   • Location of the M&M’s in each cup

   All these variables should be kept the same. Students should realize that the only variable that should be changed is the temperature of the water.

2. As each group presents their plans, have the class identify how each plan controls variables. Some groups may have planned to test M&M’s in hot and cold water but didn’t consider using room-temperature water, too. Encourage all groups to test an M&M in all three temperatures of water. The room-temperature water serves as a control and can help students see the difference in how temperature affects dissolving.

3. Does the temperature of water affect the amount of coating that dissolves from an M&M?

   Materials for Each Group

   • 3 same-colored M&M’s
   • 3 clear plastic cups
   • 1 sheet of white paper
   • Room-temperature water
   • Hot water (about 50 °C)
   • Cold water (about 5 °C)

   Procedure

   1. Pour cold, room-temperature, and hot water into the cups so that the water is deep enough to cover an M&M.
   2. Place the three cups on the white paper. Write Cold, Room-temp, and Hot near its cup.

   3. With the help of your partners, place a same-colored M&M in the center of each cup at the same time. Observe for about 1 minute.

   Expected Results

   More color and sugar dissolve from the M&M in the hot water and less in the room-temperature and cold water. This means there is more chocolate visible on the M&M in the hot water than there is in the room-temperature and cold water. The color and sugar in the room-temperature water dissolve somewhere between the cold and hot water, but are more similar to the cold than the hot.

   Note: There are actually two processes happening in this activity. The color and sugar are dissolving in the water but they are also diffusing in the water. The temperature of the water affects the amount of dissolving but it also affects the rate of diffusion. Students should focus on the surface of the M&M to judge the amount of color and sugar that dissolves.

4. Ask students:

   • Does the temperature of the water affect the amount of colored coating that dissolves from an M&M? How do you know?
   • Based on their observations, students should conclude that the hotter water causes more dissolving. Students may have noticed a greater difference in the amount of dissolving between the hot and the room-temperature water than between the room-temperature and the cold water. If no one comments on this, suggest that there is a difference.

5. Ask students:

   What are the differences in the way water molecules move in cold, room-temperature, and hot water? Students should remember that water molecules move faster in hot water than in cold. Why do you think sugar dissolves better in hot water than in cold water? The reason why sugar dissolves at a faster rate in hot water has to do with increased molecular motion. The added energy in the hot water causes water molecules to move faster and sucrose molecules to vibrate faster. This added movement tends to make the bonds between sucrose molecules easier to overcome. When faster-moving water molecules attach to sucrose molecules, a higher proportion of these sucrose-water interactions have enough energy to pull sucrose molecules away from other sucrose molecules, so the rate of dissolving increases. Why do you think there is a greater difference in the amount of dissolving between the hot and room-temperature water than between the room-temperature and cold water? There is a greater difference in the rate of dissolving because there is a greater difference in temperature between hot and room-temperature water (about 30 degrees) than between room-temperature and cold water (about 15 degrees).

6. Ask students to make a prediction:

   In the activity, you have seen that hot water dissolves sugar better than cold water. Do you think that salt will dissolve much better in hot water than in cold, like sugar does?

   Project the image Hot and Cold Water Dissolve Salt vs. Sugar.

   If you choose to do the demonstration, follow the procedure below.

   Question to Investigate

   Will salt dissolve much better in hot water than in cold, like sugar does?

   Materials for the Demonstration

   • 4 graduated cylinders (50 or 100 mL)
   • Hot water (about 50 °C)
   • Cold water (about 5 °C)
   • Salt
   • Sugar
   • Tablespoon
   • 4 small cups

   Teacher Preparation

   • Label two small cups salt and two cups sugar.
   • Place 1 tablespoon of salt and 1 tablespoon of sugar in each of their labeled cups.

   Procedure

   1. 1. Place 25 mL of hot water and 25 mL of cold water in two separate graduated cylinders.
      2. At the same time, pour one tablespoon of salt into each graduated cylinder. Do not swirl, shake, or stir. Set these graduated cylinders aside.
   2. 1. Place 25 mL of hot water and 25 mL of cold water in two separate graduated cylinders.

      2. At the same time, pour one tablespoon of sugar into each graduated cylinder. Do not swirl, shake, or stir. Set these graduated cylinders aside.
   3. 1. Show students the graduated cylinders with the salt.
      2. Show students the graduated cylinders with the sugar.

   Expected Results

   Less sugar is visible in the hot water than in the cold, meaning that more sugar dissolves in the hot water than in the cold water. There is no obvious difference between the amount of salt that dissolves in the hot water compared to the cold water. This shows that temperature affects the dissolving of sugar more than it affects the dissolving of salt.

7. Project the graph Solubility of Salt and Sugar.

   Help students understand that the graph shows that more sugar dissolves in water as the temperature of the water increases. Also help them to see that the dissolving of salt also increases as the temperature of the water increases. But the dissolving of salt does not increase nearly as much as sugar.

   Table 1. Solubility of Sodium Chloride and Sucrose in Water from 0–100 °C. Temperature (° C) Sodium Chloride Sucrose

   0	20	40	60	80	100
   35.5	36	36.5	37.5	38	39
   179	204	241	288	363	487

   Note: Students may ask why so much more sugar dissolves at higher temperatures compared to salt. This is not easy to explain on the molecular level at a middle school, high school, or even college level. Tell students that since substances are composed of different atoms, ions, and molecules, they are held together differently and interact with water differently. Changing temperature also affects the motion of the atoms, ions, or molecules of the substance and affects the interaction between water molecules and the particles of the substance. There are so many factors involved that it is difficult to explain why the solubility of one substance is affected more than another by an increase in temperature.

   Ask students:

   The demonstration showed that temperature affects the dissolving of sugar more than it affects the dissolving of salt. Explain how the graph shows this. As the temperature increases, more and more sugar can be dissolved, but only slightly more salt can be dissolved. How much sugar dissolves in 100 mL of water at 50 °C? About 260 g of sugar will dissolve. How much salt dissolves in 100 mL of water at 50 °C? About 37 g of salt will dissolve.

8. Tell students that they will plot the solubility of potassium chloride on a graph provided on the activity sheet. Explain that potassium chloride is used as a salt substitute for people who should not eat regular salt (sodium chloride.)

   Table 2. Solubility of Potassium Chloride in Water from 0–100 °C. Temperature (° C) Potassium Chloride

   0	20	40	60	80	100
   28	33	38	44	50	55

   Ask students:

   • At what temperature would you say that the solubility of sodium chloride and potassium chloride are about the same?
   • At 0 °C, which substance is the least soluble?
   • At 0 °C, which substance is the most soluble?