What is Evaporation — Reading Comprehension

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"This science passage for middle school students (grades 6-8) explores the fundamental physical science concept of evaporation. Aligned with NGSS standards, the reading details how a liquid substance changes into a gas below its boiling point, a process crucial to the Earth's water cycle. The passage explains the role of kinetic energy and molecular motion in evaporation, distinguishing it from boiling. It provides relatable, real-world examples, such as sweat cooling the body and wet clothes drying. Factors affecting the rate of evaporation, like temperature and surface area, are also discussed. This resource is designed to enhance student understanding of physical science concepts through a direct, informative style suitable for self-study and classroom use. The content supports key NGSS physical science standards related to matter and its interactions."

CONTENT PREVIEW

Evaporation as a key part of the water cycle, showing water molecules rising from a liquid surface.

Evaporation is a fundamental process in which a substance changes from a liquid to a gaseous state. This phase change occurs at the surface of a liquid, below its boiling point. The water cycle provides a perfect example: water from oceans, lakes, and rivers absorbs energy, causing it to change into a gas known as water vapor, which then rises into the atmosphere. This continuous movement of water is essential for Earth’s climate and ecosystems. Understanding evaporation helps explain many everyday phenomena and is a cornerstone of physical science.

All molecules in a liquid are constantly in motion, but some move faster than others. The molecules at the surface of a liquid are in a position to escape. When these surface molecules absorb enough kinetic energy (energy of motion) from their surroundings, they overcome the forces of attraction holding them together. Once they have enough energy to break free, they transition into a gaseous state and disperse into the air. This process is different from boiling, which occurs when a liquid is heated to a specific temperature and vapor forms throughout the entire liquid, not just at the surface.

Evaporation is responsible for many of the changes we observe daily. For instance, when wet clothes are hung outside, the water molecules on the fabric’s surface absorb energy from the sun and the surrounding air, eventually turning into water vapor and drying the clothes. Similarly, a puddle on the sidewalk disappears after a rainy day as the liquid water slowly evaporates. The human body also uses evaporation as a cooling mechanism. When we sweat, the sweat on our skin absorbs body heat to evaporate, which effectively cools us down.

The rate of evaporation is influenced by several factors, including temperature, surface area, and humidity. Higher temperatures increase molecular energy, speeding up the process. A larger surface area exposes more liquid molecules to the air, increasing the rate of evaporation. In contrast, high humidity slows evaporation because the air is already saturated with water vapor and cannot hold much more.

Fun Fact: Evaporation is used in large-scale solar evaporation ponds to produce salt. Seawater is funneled into shallow ponds, where the sun evaporates the water, leaving behind the salt crystals.

What is the main idea of the passage?

The difference between evaporation and boiling.The process and importance of evaporation.How humidity affects the water cycle.The use of evaporation in producing salt.

According to the passage, what state of matter is water vapor?

LiquidGasSolidPlasma

Evaporation differs from boiling primarily because evaporation occurs:

only on very hot days.at the boiling point of the liquid.at the surface of the liquid.throughout the entire liquid.

Which of the following is not mentioned as a factor that influences the rate of evaporation?

TemperatureSurface areaWind speedHumidity

How does sweating help to cool the human body?

It adds moisture to the skin.Sweat absorbs body heat to evaporate.The liquid sweat cools the skin directly.It increases the body’s kinetic energy.

A student places two identical cups of water in a room. One cup is in a sunny spot, and the other is in a shaded spot. Based on the passage, what is the most likely outcome?

The water in the shaded cup will evaporate faster.Both cups will evaporate at the same rate.The water in the sunny cup will evaporate faster.Neither cup will experience evaporation.

In the context of the passage, what is kinetic energy?

The energy of a substance in a gaseous state.The energy of attraction between molecules.The energy a molecule has to break free.The energy of motion.

Why does a puddle on the sidewalk eventually disappear on a warm, sunny day?

The water soaks into the sidewalk.The sun's light destroys the water molecules.The water freezes and sublimates.The water molecules evaporate into the air.