- Introduce the activity by discussing the freezing temperatures of water and the melting temperatures of ice.
Ask the children to make predictions about the melting point of ice.
- At what temperature does water freeze? 32°F (0°C). Ask them about their experiences with ice. Where do they find it? In their freezers. How cold is the freezer? Have they ever observed ice freezing outside? What was the temperature? The children may not know the numerical answer, but they will discover it in the activity. Explain to the children that this is called the "Freezing Point" of water.
- At what temperature do they think ice melts? Again, the children may or may not know. Invite the children to think about their experiences. Would ice melt on a hot summer day outside in the Sun? What might that warm outside temperature be? Would it melt in the room in which the children are at this moment? Probably (hopefully!). What is the temperature of the room?
Facilitator's Note: Changes in temperature can cause water to change state, and these changes occur at specific temperatures. Fresh water transitions between the solid and liquid states at 32°F (0°C) at sea level. At temperatures below 32°F (0°C), liquid water freezes; 32°F (0°C) is the freezing point of water. At temperatures above 32°F (0°C), pure water ice melts and changes state from a solid to a liquid (water); 32°F (0°C) is the melting point. For most substances, the melting and freezing points are about the same temperature.
Share with the children that they are about to undertake some experiments to determine the freezing point of water and the melting point of ice. Can they come up with some ideas for an experiment to find this information?
- Divide the children into groups of four to six and provide them with their Ice Investigator Journals, the cups of crushed ice, thermometers, pencils or pens, and paper towels.
- Prompt them to place the thermometer so that the bulb is sitting in the middle of the cup of ice, toward the bottom. It is important that the thermometer does not touch the sides or bottom of the cup and that the ice is tightly packed in the cup.
Invite the children to record their predictions in their Ice Investigator Journals.
After the thermometer has been in the ice for a few minutes, the children can attempt to get a temperature of their ice. The readings they collect may be close to 32°F (0°C), but may vary for several reasons. The warmer air in the room can circulate in the air pockets between the pieces of crushed ice, causing their readings to be warmer. Warmer readings may also arise because the thermometer was in contact with the sides or bottom of the cup, or it wasn't buried deeply enough in the ice, or the children may have held it out of the ice for too long before reading it. Colder reading may occur because standard household freezers are generally kept at about 0°F (-18°C) and the ice will also be that cold. In addition, some thermometers read differently.
Remind them that 32° Fahrenheit is the same temperature as 0° Celsius. They are just two names for the same temperature on different temperature scales. Scientists use the Celsius scale.
- Allow the ice to melt for 5 to 10 minutes. Have the children collect a temperature reading once there is sufficient water to immerse the thermometer bulb in the water.
- Invite each team to share the temperature readings from the water.
- What readings did they find? The readings should be pretty close to 32°F (0°C).
Share with the children that the temperature at which ice melts is called the melting point. The melting point is the temperature at which a solid turns to a liquid. The melting point at which ice — a solid — turns to water — a liquid — is 32°F (0°C). Invite the children to record their responses in their Ice Investigator Journals.
- Were they all similar? What does this tell us about the temperature at which ice melts — or the melting point of ice? It 32°F (0°C).
- If there is time, allow the ice to melt for another 5 to 10 minutes.
- What do the children think the temperature will be? Some of the children may voice that the temperature will be warmer as more ice has melted.
- Have the children take another temperature reading.
- What do they find? The temperature is still the same as the original reading, about 32°F (0°C). The temperature will remain at 32°F (0°C) until all the ice is melted. Once the ice is melted, the temperature of the water will warm until it reaches the temperature of the room
Facilitator's Note: Contrary to our instincts, the melt water's temperature will not increase as long as ice is present. The warm room continues to add energy to the cup, but all of that energy continues to go into melting the ice rather than raising the temperature. Once all the ice has melted, that energy can begin to increase the water's temperature.
Invite the children to record their responses in their Ice Investigator Journals.
- Ask the children how they might re-freeze the melted water without putting the cup back into the freezer.
- Is there something they can do, based on what they learned in the Melting Point Activity?
- Do all substances have the same melting point?
- Provide the children with the larger container, ice cubes, foil, and salt. Demonstrate the different steps of the experiment and have the children repeat what you do.
Remove the foil from the cup, place the lid on the container, and allow the experiment to sit undisturbed for about 10 minutes.
- Cover the entire bottom of the large container with 1/3 of the ice cubes.
- Cover the top of the cup that contains crushed ice and water from the previous experiments with foil. Place it in the middle of the container on top of the ice.
- Sprinkle half of the salt over the ice cubes. Create another layer of ice over the salt using 1/3 of the ice cubes. Sprinkle this layer with the rest of the salt. Cover it with the remaining 1/3 of the ice cubes. Make sure that the top of the cup with crushed ice is a little above the layers of ice and salt.
- As the facilitator, repeat the experimental design for one additional container, but leave out the salt. This container will hold only ice cubes and the cup of crushed ice and melt. Share what you are doing with the children.
- Invite the children to predict what will happen to the cups of crushed ice that are in the containers of salt and ice.
- Will the crushed ice continue to melt?
- Will it stop melting?
- Will it freeze?
- What will happen to the cup of crushed ice in the large container that does not have any salt?
Facilitator's Note: Adding salt lowers the melting point of water.
Prompt the children to brainstorm why they think their predictions will be correct.If you have already completed the companion activity, The Melting Point, remind them of their experiences with the salt on the ice cubes.
Pure water and ice, kept insulated from the warm outside world, come to equilibrium over time. On a molecular level, water molecules are freezing onto the ice at the same rate as they are melting off of it. The entire water/ice solution is at the melting/freezing point, 32°F (0°C). Adding rock salt — or any substance that dissolves in water — disrupts this equilibrium. Fewer water molecules are interacting with the ice at any given moment, so the freezing rate is slowed. The salt has no effect on the melting rate, so more melting occurs than freezing — melting "wins" — and the ice melts. In doing so, heat energy is used to break the hydrogen bonds that hold the molecules in the ice together. In other words, the ice "uses up" some warmth from the solution and the temperature drops. Melting and freezing again match rates ("tie") once the temperature has dropped to the new melting point.
The greater the amount of salt, the lower the freezing point (to a point; once there is sufficient salt that no more will dissolve, the freezing point no longer decreases). Ocean water is about 3.5% salt; sea water freezes at about 28°F (-2°C). A 10% salt solution freezes at about 20°F (-6°C), and a 20% solution freezes at 2°F (-16°C).
- After 5 to 10 minutes, check to see if the predictions were correct! Note that the experiment should be stopped when some of the water has refrozen, not all. Have the children uncover their containers and lift the cup of crushed ice out of the bowl.
Have the children take another temperature reading of the remaining water.
- What do the children observe about the water that had been in the bottom of their cup of crushed ice? Some of it has frozen (again).
- What do they find? The water temperature is 32°F (0°C).
- If some of the water has frozen, what does this mean that the freezing point of water is? 32°F (0°C).
- Share the results from the container of ice without salt.
- What do the children observe about the water in the bottom of the cup of crushed ice? It is not frozen. More may have melted.
- Why did melted ice in the ice/salt mixtures refreeze and the melted ice in the other container not? Because the temperature of the ice/salt mixture was cold enough to refreeze the water.
- Invite them to find the temperature of the ice/salt mixture using one of their thermometers. At the same time, have them take the temperature of the crushed ice again.
Have the children continue to make notes about their findings in their Ice Investigator Journals.
- What do they find? The temperature of the ice/salt mixture is lower than 32°F (0°C); it may be as low as 0°F (-18°C). Note, the temperature will vary, depending on how much salt is used, how well the salt dissolves, and the temperature of your freezer.
- Discuss their experimental findings.
- At what temperature does fresh water turn to ice? 32°F (0°C). Share that the temperature at which fresh water freezes is called the freezing point. The freezing point is the temperature at which a liquid turns to a solid. The freezing point at which water — a liquid — turns to ice — a solid — is 32°F (0°C).
- Based on their experiments melting the crushed ice, at what temperature does ice melt to make water — what is the melting point of ice? 32°F (0°C).
- How can the melting and freezing temperatures be the same? The melting point and freezing point are the same temperature for any particular substance: 32°F (0°C) for water. This temperature is referred to as the melting point when temperatures rise above 32°F (0°C), causing ice to melt and change state from a solid to a liquid (water), as happened with the cup of crushed ice at room temperature. It is referred to as the freezing point when temperatures decrease below 32°F (0°C), causing water to change state from a liquid to a solid (ice), as happened with the cup of crushed ice immersed in the very cold salt/ice mixture.
- Why is the ice/salt mixture colder? Salt lowers the freezing point of water. Salty water stays liquid at lower temperatures — or freezes at a colder temperature — than fresh water.