Questions of Heat Energy, Matter and Density

The Thermometer

The Thermometer Mr. Martinez placed a thermometer in a jar of very hot water. His students watched what happened to the thermometer. Immediately the level of the red liquid in the thermometer went up. His students disagreed about why the red liquid in the thermometer rose when the thermometer was placed in the hot water. This is what they said:

Jean-Paul: “The hot water pushed it up” Pita: “The mass of the red liquid increased” Jonathan: “The heat inside the thermometer pulls it up” Jimena: “The air inside the thermometer pulls it up” Molly: “The molecules of the red liquid are further apart” Greta: “The number of molecules of the red liquid increased” Keanu: “The molecules of the red liquid are getting bigger”

Who is correct? Molly. The thermometer is a closed system. It operates on the principle that the fluid inside (usually alcohol) expands when heated and contracts when cooled. When the fluid is in contact with the warm water, energy from the hot water is transferred to the liquid inside the thermometer. The molecules of liquid inside the thermometer gain energy, start moving faster, bumping into each other, and move further apart using more space.

Comparing Cubes Sophia has two solid cubes made of the same material. One cube is very large, and the other cube is very small. Density equals mass divided by volume: d=m/v The density of the larger cube is greater than that of the smaller cube The density of the smaller cube is greater than that of the larger cube The density of both cubes are the same

Cubes Explained The reason why the large and small cubes have the same density is because they are made from the same material, and the molecules are arranged in the same exact way for both cubes. This is similar to the reason why water’s density is always 1 g/ml, regardless of how much or how little water you have!

Floating Logs A log was cut from a tree and put in the water. The log floated on its side so that half the log was above the water surface. Another log was cut from the same tree. This log was twice as long and twice as wide. How does the larger log float compared with the smaller log? More than half of the larger log floats above the water surface. Half of the larger log floats above the water surface. Less than half of the larger log floats above the water surface.

Floating Logs b) Half of the larger log floats above the surface. Density is a property of matter that does not depend on the amount of material. If one sample is very large and the other is very small, the mass to volume ratio is still the same, and the molecules are still arranged in the same way, so the density remains the same.

Floating High & Low Sam floated in a pool of water. He floated half way above and half way below the water level. What can Sam do to make himself float like Daffy Duck, who is floating just beneath the water surface?

Floating High & Low Sam needs to have less density than Daffy Sam needs to have more density than Daffy Sam could add weights (more mass) on his stomach Sam could add weights on Daffy’s stomach Add more water to the pool so it is deeper Add salt to the water Answers: b and c To make Sam float so that more of him is under the water, you must increase his density, which can be done by adding heavy weights to his body. Adding more water affects both Sam & Daffy, as does adding salt (increasing water’s density)

Ice Cubes in a Bag You have ice cubes in a sealed bag and record the mass. Ten minutes later, after the cubes have melted, you check the mass again. The mass of the water in the bag will be less than the mass of the ice in the bag The mass of the water in the bag will be more than the mass of the ice in the bag The mass of the water in the bag will be the same as the mass of the ice cubes in the bag

Ice Cubes In A Bag C). The mass of the water in the bag will be the same as the mass of the ice cubes in the bag, as long as none of the H2O molecules were allowed to escape the bag

Lemonade A glass of lemonade had a mass of 255 g. A spoonful of sugar with a mass of 25 g is stirred into the lemonade. Predict the mass of the lemonade with the sugar added. It will have a mass of slightly less than 255 grams but more than 230 grams Its mass will be slightly more than 255 grams but less than 280 grams Its mass will be 230 grams Its mass will be 280 grams Its mass will be 255 grams

Lemonade D) Its mass will be 280g, but its volume will remain unchanged. Therefore, the lemonade becomes more dense with more sugar!

Rusty Nails You have four dry iron nails and record the mass. You then put the nails in a moist, open dish and exposed to the air over several weeks. Will the mass of the rusted nails be different than the dry, unrusted nails? The mass of the dry rusted nails will be more than the mass of the dry nails before they are rusted The mass of the dry rusted nails will be less than the mass of the dry nails before they rusted The mass of the dry rusted nails will be the same as the mass of the dry nails before they rusted

Rusty Nails A) The mass of the dry rusted nails will be more than the mass of the dry nails before they are rusted. This is due to the fact that rust is the product of iron chemically reacting and combining with oxygen in the air. More molecules = more mass.

Cookie Crumbles You have a whole cookie that you aggressively crush into tiny pieces and crumbs. You measure the mass of all the pieces and crumbs. The whole cookie’s mass is greater than all of the cookie crumbs All of the cookie crumbs have more mass than the whole cookie The whole cookie and all of the cookie crumbs have equal mass

Cookie Crumbles c) The whole cookie and all of the cookie crumbs have the same mass, because no molecules are gained or lost in the crushing.

What’s in the Bubbles? Hannah is boiling water in a glass tea kettle. She notices bubbles forming on the bottom of the kettle that rise to the top and wonders what is in the bubbles. She asks her family and this is what they say: Dad: “They are bubbles of heat” Calvin: “The bubbles are filled with air” Grandma: “The bubbles are an invisible form of water” Mom: “The bubbles are empty, nothing inside them” Lucy: “The bubbles contain oxygen and hydrogen that separated from the water”

What’s in the Bubbles? Dad: “They are bubbles of heat” Calvin: “The bubbles are filled with air” Grandma: “The bubbles are an invisible form of water” Mom: “The bubbles are empty, nothing inside them” Lucy: “The bubbles contain oxygen and hydrogen that separated from the water” Grandma is correct! This invisible water is called water vapor. When heat energy is applied, it results in increased molecular movement. The heated molecules can no longer remain in the liquid state and they become gaseous water vapor. (Water vapor is unlike steam which contains some condensed liquid water.)

Ice Cold Lemonade It was a hot summer day. Mattie poured herself a glass of lemonade. The lemonade was warm, so Mattie put some ice in the glass. After 10 minutes, Mattie noticed that the ice was melting and the lemonade was cold. Mattie wondered what made the lemonade get cold. She had three different ideas. Which idea do you think best explains why the lemonade got cold?

Ice Cold Lemonade The coldness from the ice moved into the lemonade The heat from the lemonade moved into the ice The coldness and the heat moved back and forth until the lemonade cooled off

Remember how heat flows…. The heat from the lemonade moved into the ice. Heat energy will only move from a warmer object to a cooler object, never the other way around. The warm molecules of the lemonade came into contact with the cooler molecules of the ice. Heat energy flowed INTO the ice FROM the lemonade. This resulted in a the melting of the ice, which cooled the lemonade.

Mixing Water Melinda filled two glasses of equal size half-full with water. The water in one glass was 50 degrees Celsius. The water in the other glass was 10 degrees Celsius. She poured one glass into the other, stirred the liquid, and measured the temperature of the full glass of water. What do you think the temperature of the full glass of water will be after the water is mixed? 20 degrees Celsius 30 degrees Celsius 40 degrees Celsius 50 degrees Celsius 60 degrees Celsius

Mixing Water 20 degrees Celsius 30 degrees Celsius 40 degrees Celsius Temperature is a measure of the average motion of the particles that make up the water. When the cooler water and the warmer water are mixed together, a transfer of energy occurs.

Mixing Water The flow of heat moves from the molecules of the warmer water to the molecules in the cooler water until they have the same average energy (temperature). Since the two samples of water were identical in volume, the thermal equilibrium that is reached is an average of the two temperatures, 30 degrees Celcius.