Active Read - What Burns, What Melts?

​What Burns, What Melts?

​Imagine this: You’re camping with your family. It’s a beautiful night, just perfect for sitting around the campfire and making s’mores under the stars. You layer a graham cracker, a piece of chocolate, a marshmallow you’ve toasted over the fire, and another graham cracker—a sweet, sticky treat!

Multiple Choice

What is a s'more made of?

• cookies, marshmallows and chocolate

• graham crackers, marshmallows and chocolate

• cookies, marshmallows and candies

• graham crackers, marshmallows and ice cream

​Why does one burn?

​You might even leave your whole s’more sitting near the fire to help melt the chocolate. In that moment, the campfire is transferring energy to the wood in the fire and to the chocolate, but they’re responding in very different ways. The wood is burning, while the chocolate in the s’more is melting. What’s the deal?

Multiple Choice

When energy from a fire is transferred, the wood and the chocolate respond how?

• in the same way

• they don't respond

• In different ways

​Melting - Solid to Liquid

Melting is a phase change from the solid phase to the liquid phase. The molecules in a solid chocolate bar are packed tightly together and can only move in place. When energy is added to the solid chocolate—like thermal energy from a fire—the molecules gain energy and begin to move around more.

Multiple Choice

"Melting" is a phase change going from.....

• a gas to a liquid

• a solid to a gas

• a liquid to a solid

• a solid to a liquid

​Changing Phase Does Not Change The Substance

​When they have enough energy and freedom of movement to flow around each other, the chocolate becomes a liquid. It’s important to remember that melting doesn’t change substances into other substances. . The molecules that make up your chocolate bar haven’t changed; they’re just moving around more.

Multiple Choice

When a substance goes through a phase change....

• the molecules in the substance disappear

• the molecules in the substance move differently

• the molecules in the substance turn into a different substance

​Combustion

​The wood in your campfire is also receiving energy from the fire, but it definitely isn’t melting into a liquid. Instead, it’s burning. Burning, also known as combustion, is a chemical reaction that happens when some materials reach a certain temperature and their molecules react with oxygen. The fire transfers energy to the wood, raising its temperature.

Multiple Choice

 Burning, or combustion, occurs when materials reach a certain temperature and...

• their molecules react with oxygen

• their molecules react with hydrogen

• their molecules react with carbon

​Now That, Is Hot!

​The fire transfers energy to the wood, raising its temperature. When the wood reaches about 150°C (302°F), its molecules begin to break down. Some change into the gas phase and escape into the air; others react with oxygen in the air and give off light and heat. Some parts of the wood don’t burn and are left behind as ash.

Multiple Choice

At high temperatures, wood molecules begin to break down and 

• some change into gas

• some react with oxygen and give off light and heat

• some become ash

• all answers are correct

​What Do Fires Need to Burn?

​ The combustion reaction keeps repeating itself until it runs out of fuel (like wood) to burn, until its supply of oxygen is cut off, or until its temperature is no longer hot enough to keep breaking down the molecules in the wood. That’s why the fire goes out a little while after you stop adding new logs. Without fuel to burn, the reaction that causes the fire can’t continue.

Multiple Choice

In addition to oxygen and high temperatures, fires need what in order to burn?

• warm weather

• fuel

• water

• earth

​Melts vs Burns

​So the chocolate on your s’more melts, while the wood in the campfire burns. But why? Every material has a different temperature at which it melts (its melting point) and a temperature at which it burns (its flash point). How the material reacts to heat depends on which of those temperatures is lower.

Multiple Choice

Every material has a different temperature at which it melts or burns.  These temperatures are called...

• melting point and gas point

• melting point and freezing point

• melting point and flash point

• melting point and solid point

​Flash Point

If a material’s flash point is lower than its melting point, that material will burn before it melts.

If a material’s melting point is lower than its flash point, it will melt before it burns.

​

Wood’s flash point is lower than its melting point, so it burns. Chocolate’s melting point is lower than its flash point, so it melts.

Multiple Choice

Chocolate melts because...

• its flash point is lower than its melting point

• its flash point and melting points are the same

• its melting point is lower than its flash point

Multiple Choice

Wood burns because...

• its flash point is lower than its melting point

• its flash point and melting points are the same

• its melting point is lower than its flash point

Multiple Choice

What determines whether a material melts or burns when heated?

• how much heat is added

• the molecule phase

• melting point and flash point

• the molecule size