Heating, Melting, and Freezing - Second Grade Science
Matter does not stay in one state forever. A hard ice cube can turn into splashing puddle water on a hot sidewalk, and liquid water inside a kettle can turn into invisible steam floating into the kitchen air. What causes these dramatic transformations? The answer is thermal energy, which we commonly experience as heat. Adding heat energy or removing heat energy causes matter to change from one state to another.
Thermal Energy and Particle Speed
Heat is the flow of thermal energy. Temperature measures how fast the microscopic particles inside a substance are vibrating and moving around.
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| HOW HEAT CHANGES PARTICLE MOTION |
| |
| COOLING (Heat Removed) HEATING (Heat Added) |
| Particles slow down, Particles speed up, |
| move closer together, spread further apart, |
| and lock in place! and slide or fly freely! |
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Adding Heat Speeds Particles Up
When you heat a substance, you pump thermal energy into it. The particles absorb this energy, vibrate faster and faster, and push further apart from one another.
Removing Heat Slows Particles Down
When you cool a substance by placing it in a refrigerator or freezer, thermal energy leaves the matter. As the particles lose energy, they slow down and nestle closer together.
The Phase Changes: Melting, Freezing, Boiling, and Condensing
A phase change occurs when matter transitions from one state (solid, liquid, or gas) to another state due to temperature changes.
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| THE PHASE CHANGE CYCLE |
| |
| +-----------> MELTING ----------->+ |
| | (Solid turns to Liquid) | |
| [SOLID] [LIQUID] |
| ^ | |
| +<----------- FREEZING <----------+ |
| (Liquid turns to Solid) |
| |
| +----------> EVAPORATING --------->+ |
| | (Liquid turns to Gas) | |
| [LIQUID] [GAS] |
| ^ | |
| +<--------- CONDENSING <-----------+ |
| (Gas turns to Liquid) |
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Melting: Solid to Liquid
Melting is the process where a solid absorbs heat and turns into a liquid. When a solid reaches its melting point, its particles gain enough kinetic energy to break out of their rigid positions and begin rolling freely past one another. Ice to Water: Water ice melts at temperatures above 0 degrees Celsius (32 degrees Fahrenheit). Candle Wax: Solid candle wax melts into liquid wax near the hot flame. Chocolate: Chocolate melts quickly in your warm hand because human body temperature (about 37°C or 98.6°F) is higher than the melting point of cocoa butter.
Freezing: Liquid to Solid
Freezing, also called solidification, is the process where a liquid loses heat energy and turns into a solid. As temperature drops to the freezing point, particles lose energy, slow down, and lock tightly into rigid positions. Water to Ice: Placing liquid water into a freezer at or below 0°C cools the liquid until it solidifies into hard ice cubes. Lava to Rock: Molten liquid rock spewed from a volcano cools in the air and solidifies into hard volcanic basalt rock.
Evaporating and Boiling: Liquid to Gas
When liquid water is heated, particles gain enough energy to escape the liquid surface and become a gas called water vapor. Evaporation happens slowly at the surface of puddles under the sun, while boiling happens rapidly throughout the whole liquid when water reaches 100°C (212°F).
Condensation: Gas to Liquid
When hot invisible water vapor in the air touches a cold surface, such as the outside of a glass filled with iced tea, the gas particles lose energy instantly. They slow down, cluster together, and condense into shining liquid droplets on the outside of the glass.
Measuring Melting Points
Different substances melt and freeze at wildly different temperatures depending on how strongly their molecules cling to each other.
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| MELTING POINTS OF COMMON SUBSTANCES |
| |
| SUBSTANCE MELTING POINT (°C) STATE AT ROOM TEMP (20°C) |
| Water Ice 0°C Liquid |
| Butter 32°C Solid (Soft) |
| Candle Wax 55°C Solid |
| Aluminum Metal 660°C Solid (Extremely hard) |
| Iron 1,538°C Solid |
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Chapter Review Exercises
Question 1: What happens to the microscopic particles inside an ice cube when heat energy is added to it?
Question 2: Define melting and freezing, and identify the temperature in degrees Celsius at which both processes occur for pure water.
Question 3: Why do tiny beads of liquid water appear on the outside of a cold soda can on a warm summer day?
Question 4: Explain why butter melts quickly in a hot frying pan while an iron frying pan does not melt at all.
Solutions and Explanations
Solution 1: When heat energy is added to an ice cube, its particles absorb the thermal energy and begin vibrating faster and faster. Eventually, they gain enough energy to break out of their rigid solid crystal structure and begin rolling freely past one another as liquid water.
Solution 2: Melting is the change of state from a solid to a liquid as heat is added. Freezing is the change of state from a liquid to a solid as heat is removed. For pure water, both melting and freezing occur at 0 degrees Celsius (32 degrees Fahrenheit).
Solution 3: Liquid droplets appear because invisible water vapor gas in the warm surrounding air touches the cold metal can. The cold surface pulls thermal energy away from the gas particles, causing them to slow down and condense into liquid water.
Solution 4: Butter has a low melting point (around 32°C), which is easily reached over low stove heat. Iron has an extraordinarily high melting point (over 1,500°C), so typical cooking temperatures cannot provide enough thermal energy to melt iron.