A cold drink sits in a warm room. An hour later it is warmer, not colder. It is warmer every single time.
Why only that direction? Could the drink ever chill itself while the room warms?
In earlier grades you saw that heat flows from hot to cold.
The warm room's air is the hotter thing, so energy leaves the air for the colder drink. The drink warms.
The reverse never happens on its own. The drink never transfers energy into the already-warmer room and chills itself.
A bowl of hot stew in cool air obeys the same rule the other way around: the stew is the hotter thing, so heat flows from the stew to the air.
Cold is not a stuff that flows. When your hand chills against a cold window, no cold seeps into your hand: heat leaves your warm hand for the colder glass.
The direction is the whole claim: from hotter to colder, never the reverse on its own.
A plate of leftovers at 60 °C sits in a 20 °C kitchen. Which way does heat flow?
The leftovers are hotter than the kitchen air.
Heat flows spontaneously from the hotter object to the colder one.
Heat flows from the leftovers into the kitchen air, and the leftovers cool.
A juice pouch at 8 °C sits in the 25 °C air of a classroom. Could the pouch get colder on its own?
Getting colder would need heat to flow from the colder pouch into the hotter air.
Heat never flows from a colder object to a hotter one on its own.
No. In the warmer classroom the pouch warms; it never chills itself.
A metal spoon at 20 °C is placed into soup at 70 °C. Which way does heat flow?
Freja says: 'A glass of lemonade at 5 °C in a 22 °C kitchen always warms up; it never gets colder by itself.' Is she correct?
An ice pack is laid on Keane's sprained ankle, and the ankle's skin chills. Keane says: 'Cold flowed out of the ice pack into my ankle.' Is he correct?
A stick of butter at 5 °C sits on a counter in a 22 °C kitchen. What happens?
Which of the following statements about heat flow is correct?
A baking sheet at 180 °C is stacked against a baking sheet at 20 °C. What happens?