Melting and Freezing - First Grade Science
Have you ever held an ice cream cone on a hot summer afternoon and watched in dismay as creamy drops dripped down your fingers? Or have you filled a plastic tray with tap water, placed it into the freezer, and returned hours later to find hard, solid ice cubes? These everyday changes are magnificent examples of how temperature affects matter. By adding thermal heat or taking heat away, matter can change back and forth between different states. Two of the most important temperature changes you observe every day are melting and freezing.
Heating and Cooling: Adding and Removing Energy
Temperature is a measure of how warm or cool matter is. Adding heat warms matter up; cooling matter takes heat away.
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| THE HEATING AND COOLING CYCLE |
| |
| [ SOLID ICE ] |
| (Cold & rigid) |
| | |
| HEAT ADDED (Melting) |
| v |
| [ LIQUID WATER ] |
| (Flowing & fluid) |
| | |
| HEAT REMOVED (Freezing) |
| v |
| [ SOLID ICE ] |
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Melting: From Solid to Liquid
Melting is the physical process where a solid absorbs heat energy and turns into a liquid.
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| HOW MELTING WORKS MICROSCOPICALLY |
| |
| 1. SOLID AT REST Particles are packed tight, vibrating gently. |
| 2. HEAT IS APPLIED Particles absorb thermal energy and vibrate |
| faster and more violently! |
| 3. BONDS BREAK FREE The particles break out of their fixed rows |
| and begin sliding and rolling past each other. |
| 4. LIQUID STATE! The solid melts into flowing liquid. |
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The Melting Point
The specific temperature at which a solid turns into a liquid is called its melting point. For water ice, the melting point is 0 degrees Celsius (32 degrees Fahrenheit). Once ice warms above this temperature, it begins melting into liquid water.
What Else Can Melt?
Water is not the only matter that melts: Butter melts into a golden liquid when heated in a warm frying pan. Candle wax softens and melts into clear liquid near the flame. Crayons melt into colorful wax puddles if left on a hot sunny car dashboard! Metals like gold, copper, and iron can also melt, but only inside blistering industrial furnaces heated to thousands of degrees!
Freezing: From Liquid to Solid
Freezing (also called solidifying) is the exact reverse of melting. Freezing occurs when a liquid loses heat energy, cools down, and turns into a solid.
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| HOW FREEZING WORKS MICROSCOPICALLY |
| |
| 1. FLOWING LIQUID Particles slide and roll around freely. |
| 2. HEAT IS REMOVED As the liquid cools, particles lose energy and |
| slow down significantly. |
| 3. PARTICLES LOCK Slowed particles are pulled together by strong |
| bonds, locking into tight, rigid rows. |
| 4. SOLID STATE! The liquid hardens into a firm solid block! |
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The Freezing Point
The freezing point is the temperature at which a liquid turns into a solid. For water, the freezing point is exactly 0 degrees Celsius (32 degrees Fahrenheit)—the same temperature as its melting point! If you chill liquid water below 0 degrees Celsius, it freezes into solid ice.
Other Liquids That Freeze
Liquid candle wax freezes back into solid wax as soon as it cools down away from the flame. Melted chocolate cools and solidifies into firm candy bars in the refrigerator. Melted lava pouring from a volcano cools in the cold air, freezing into hard black basalt rock!
Reversible Changes (Physical Changes)
Melting and freezing are called reversible physical changes. A physical change alters the form or appearance of a material, but does not turn it into a different chemical substance.
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| REVERSIBLE PHYSICAL CHANGE OF WATER |
| |
| SOLID ICE ========(Warm up)=======> LIQUID WATER |
| LIQUID WATER ======(Cool down)=====> SOLID ICE |
| |
| Ice is water, and liquid water is water! It never becomes a new |
| chemical substance; you can melt and freeze it over and over again! |
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Unlike baking a cake or burning a piece of paper (which are irreversible chemical changes that cannot be undone), you can melt an ice cube and freeze it right back into an ice cube as many times as you like!
Chapter Review Exercises
Question 1: What is melting, and what must be added to a solid to make it melt?
Question 2: What is freezing, and what temperature is the freezing point of liquid water?
Question 3: Describe what happens to the microscopic particles in a solid as it is heated up into a liquid.
Question 4: Why are melting and freezing classified as reversible physical changes rather than chemical changes?
Solutions and Explanations
Solution 1: Melting is the physical change of a solid turning into a liquid. Thermal heat energy must be added to the solid to make it melt.
Solution 2: Freezing is the physical process of a liquid cooling down and turning into a solid. The freezing point of liquid water is 0 degrees Celsius (32 degrees Fahrenheit).
Solution 3: When a solid is heated, its particles absorb heat energy and vibrate faster. Eventually, they gain enough energy to break out of their fixed rigid positions and begin sliding and rolling freely past one another as a flowing liquid.
Solution 4: Melting and freezing are reversible physical changes because the substance itself remains unchanged chemically—water is still water whether it is frozen as ice or flowing as liquid—and the process can be reversed repeatedly simply by changing the temperature.