What is the main idea of the passage?

A thermos slows several routes of heat transfer through a vacuum gap, reflective surfaces, and a tight stopper. (The passage assigns different heat-transfer limits to the gap, surfaces, and stopper.)

Why does the vacuum slow heat transfer?

It contains very few particles to pass energy through the gap. (Conduction and circulation require matter, and the vacuum greatly reduces that matter.)

What does “vacuum” mean here?

a space containing very little matter (The first paragraph directly defines the space between the walls.)

What helps prevent air or vapor leaving through the top?

a tight stopper (The third paragraph explains why the stopper matters at the opening.)

The Empty Space in a Thermos: Level 1 Reading Comprehension Questions

A thermos often has one container inside another. Between their walls is a space with very little matter. This vacuum slows the movement of heat.

Heat can travel when moving particles bump into nearby particles. With almost no air between the walls, there are fewer particles to carry energy by conduction or circulation. The gap does not stop all heat.

Shiny surfaces can reflect some thermal radiation. A tight stopper also matters because an open top would let warm air or vapor move. Each part slows a different route.

A thermos does not create cold or heat. It helps a drink keep its starting temperature longer by reducing energy transfer. Hot soup gradually cools, and ice water gradually warms. The empty-looking gap is useful because what is missing, most of the air, leaves heat fewer paths to follow.

Comprehension questions