Salt Lowers the Freezing Point of Water

Transcription

1 Salt Lowers the Freezing Point of Water Topic Sodium chloride (NaCl), salt, lowers the freezing point of water. Introduction Salt is added to ice in ice cream freezers because salt lowers the freezing point of water and increases the rate at which an ice cream mixture will change to a solid. If salt is not added to the ice, ice cream remains in a mushy state because it does not get cold enough to freeze. Salt is used in many other freezing and refrigeration processes. Another type of salt, calcium chloride (CaCl 2 ), is used as a road deicer. When CaCl 2 is spread on ice, it lowers the freezing point of water below 0 C, so the ice turns into its liquid form, which is much safer to drive on. Time Required 40 minutes Materials salt tablespoon (tbsp) stirrer ice water 2 small disposable cups (that will fit in the mouth of 2 larger disposable cups) 2 larger disposable cups 2 thermometers graduated cylinder

2 SALT LOWERS THE FREEZING POINT OF WATER 2 crushed ice masking tape or peel-and-stick labels black pen colored pencils graph paper Safety Note Please review and follow the safety guidelines. Be careful when punching holes in the bottoms of the small disposable cups. Procedure 1. With a black pen and masking tape or labels, label the paper cups as A and B. 2. Use the tip of a pen or pencil to punch five small holes in the bottom of each cup. 3. Fill each cup two-thirds full of crushed ice. 4. Add 1 tbsp of salt to cup B, then stir to mix. 5. Place each small cup on the mouth of a larger disposable cup so that the water dripping from the holes of the small cup will collect in the large one. 6. Insert a thermometer into each cup of ice (Figure 1). thermometer small cup with crushed ice A B water dripping into large cup Figure 1

3 SALT LOWERS THE FREEZING POINT OF WATER 3 7. Every 2 minutes (min), for the next 20 min, check the temperature in each cup. Record those temperatures on the data table. 8. At the end of 20 min, pour the water that has collected in cup A into a graduated cylinder to determine its volume. Record that volume in your science notebook. Do the same for the water in cup B. 9. Create a graph showing time on the X axis and temperature of ice on the Y axis. Graph the information on the data table. Draw the line for cup A in red and the line for cup B in blue. Data Table Time, minutes Temperature ( C) of ice in cups Cup A Cup B Analysis 1. In which cup did the temperature drop the lowest? 2. Which cup produced the most ice water? 3. What general statement can you make about the effect of salt on ice? 4. In the winter when roads are icy, the Department of Transportation prefers to spread calcium chloride instead of sodium chloride on slick surfaces. Propose an experiment in which you could compare the effectiveness of calcium chloride to sodium chloride in lowering the melting point of water.

4 SALT LOWERS THE FREEZING POINT OF WATER 4 What s Going On? When salt was added to the ice, it lowered the freezing point of water. This is because sodium chloride is a strong electrolyte. When it dissolves, it produces sodium and chloride ions that interfere with the ability of water molecules to get close together so that they can form a solid, crystalline structure. Want to Know More? See Our Findings.

5 OUR FINDINGS SALT LOWERS THE FREEZING POINT OF WATER Analysis 1. cup B 2. cup B 3. Salt lowers the freezing point of water. Even as temperatures drop below 0 C, the ice melts. 4. Student answers will vary; they might suggest a test similar to the one they just completed, putting calcium chloride in one of the cups and sodium chloride in the other.

6 SAFETY PRECAUTIONS Review Before Starting Any Experiment Each experiment includes special safety precautions that are relevant to that particular project. These do not include all the basic safety precautions that are necessary whenever you are working on a scientific experiment. For this reason, it is necessary that you read and remain mindful of the General Safety Precautions that follow. Experimental science can be dangerous, and good laboratory procedure always includes carefully following basic safety rules. Things can happen very quickly while you are performing an experiment. Materials can spill, break, or even catch fire. There will be no time after the fact to protect yourself. Always prepare for unexpected dangers by following the basic safety guidelines during the entire experiment, whether or not something seems dangerous to you at a given moment. We have been quite sparing in prescribing safety precautions for the individual experiments. For one reason, we want you to take very seriously every safety precaution that is printed in this book. If you see it written here, you can be sure that it is here because it is absolutely critical. Read the safety precautions here and at the beginning of each experiment before performing each activity. It is difficult to remember a long set of general rules. By rereading these general precautions every time you set up an experiment, you will be reminding yourself that lab safety is critically important. In addition, use your good judgment and pay close attention when performing potentially dangerous procedures. Just because the text does not say be careful with hot liquids or don t cut yourself with a knife does not mean that you can be careless when boiling water or punching holes in plastic bottles. Notes in the text are special precautions to which you must pay special attention. GENERAL SAFETY PRECAUTIONS Accidents caused by carelessness, haste, insufficient knowledge, or taking an unnecessary risk can be avoided by practicing safety procedures and being alert while conducting experiments. Be sure to check the individual experiments in this book for additional safety regulations and adult supervision requirements. If you will be working in a lab, do not work alone. When you are working off site, keep in

7 SAFETY PRECAUTIONS 2 groups with a minimum of three students per group, and follow school rules and state legal requirements for the number of supervisors required. Ask an adult supervisor with basic training in first aid to carry a small first-aid kit. Make sure everyone knows where this person will be during the experiment. PREPARING Clear all surfaces before beginning experiments. Read the instructions before you start. Know the hazards of the experiments and anticipate dangers. PROTECTING YOURSELF Follow the directions step-by-step. Do only one experiment at a time. Locate exits, fire blanket and extinguisher, master gas and electricity shut-offs, eyewash, and first-aid kit. Make sure there is adequate ventilation. Do not horseplay. Keep floor and workspace neat, clean, and dry. Clean up spills immediately. If glassware breaks, do not clean it up; ask for teacher assistance. Tie back long hair. Never eat, drink, or smoke in the laboratory or workspace. Do not eat or drink any substances tested unless expressly permitted to do so by a knowledgeable adult. USING EQUIPMENT WITH CARE Set up apparatus far from the edge of the desk. Use knives or other sharp-pointed instruments with care. Pull plugs, not cords, when removing electrical plugs. Clean glassware before and after use. Check glassware for scratches, cracks, and sharp edges.

8 SAFETY PRECAUTIONS 3 Clean up broken glassware immediately. Do not touch metal conductors. Do not use reflected sunlight to illuminate your microscope. Use alcohol-filled thermometers, not mercury-filled thermometers. USING CHEMICALS Never taste or inhale chemicals Label all bottles and apparatus containing chemicals Read labels carefully. Avoid chemical contact with skin and eyes (wear safety glasses, lab apron, and gloves). Do not touch chemical solutions. Wash hands before and after using solutions. Wipe up spills thoroughly. HEATING SUBSTANCES Wear safety glasses, apron, and gloves when boiling water. Keep your face away from test tubes and beakers. Use test tubes, beakers, and other glassware made of Pyrex glass. Never leave apparatus unattended. Use safety tongs and heat-resistant gloves. If your laboratory does not have heat-proof workbenches, put your Bunsen burner on a heat-proof mat before lighting it. Take care when lighting your Bunsen burner; light it with the airhole closed, and use a Bunsen burner lighter in preference to wooden matches. Turn off hot plates, Bunsen burners, and gas when you are done. Keep flammable substances away from flames and other sources of heat. Have a fire extinguisher on hand.

9 SAFETY PRECAUTIONS 4 FINISHING UP Thoroughly clean your work area and any glassware used. Wash your hands. Be careful not to return chemicals or contaminated reagents to the wrong containers. Do not dispose of materials in the sink unless instructed to do so. Clean up all residues and put them in proper containers for disposal. Dispose of all chemicals according to all local, state, and federal laws. BE SAFETY CONSCIOUS AT ALL TIMES!