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what is a Chemical Reaction — Reading Comprehension

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This middle school science reading passage, aligned with NGSS standards, provides a clear and direct explanation of what a chemical reaction is. The passage defines chemical reactions as processes that rearrange atoms to form new substances, in contrast to physical changes. It introduces key vocabulary such as reactants and products and explains the fundamental principle of the Law of Conservation of Mass. Students will learn to identify common signs of a chemical reaction, such as a change in color, temperature, or the formation of a gas or precipitate. The text connects these concepts to real-world examples, including photosynthesis and the rusting of iron, making the topic relatable for students. This content is designed to support middle school curriculum, focusing on core ideas about matter and its interactions.

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A science experiment showing two liquids mixing in a beaker, resulting in a bubbling, foamy new substance. — A chemical reaction, where reactants (two liquids) combine to form new products with different properties.

A chemical reaction is a process that rearranges the atoms of one or more substances, called reactants, to form new and different substances, known as products. Unlike a physical change, which alters a substance's appearance but not its chemical identity (like melting ice), a chemical reaction creates something entirely new. The atoms themselves are not created or destroyed; they are simply rearranged to form new molecules with new properties. This fundamental principle is known as the Law of Conservation of Mass, which states that the total mass of the reactants before a reaction is equal to the total mass of the products after the reaction.

Evidence of a chemical reaction can often be observed in everyday life. For example, when you bake a cake, the liquid batter (reactants) transforms into a solid, fluffy cake (products). This is a result of a complex series of chemical reactions. Similarly, when an iron nail is left outside, it reacts with oxygen in the air to form rust. This is a visible color change, a common sign that a chemical reaction has occurred. Other signs include the production of a gas (like fizzing from an antacid tablet in water), a change in temperature (like an instant cold pack getting cold), or the formation of a solid precipitate in a liquid.

Chemical reactions are essential to life and technology. Photosynthesis is a crucial chemical reaction where plants use sunlight to convert carbon dioxide and water into glucose (sugar) and oxygen, providing food for themselves and oxygen for us to breathe. Another example is the combustion reaction that happens in a car engine, where gasoline reacts with oxygen to release energy, powering the car. Understanding these reactions allows scientists and engineers to create new materials, develop medicines, and solve complex problems.

Fun Fact: The human body uses thousands of different chemical reactions every second to digest food, move muscles, and even think!

What is the main difference between a chemical reaction and a physical change?

A physical change rearranges atoms.A chemical reaction creates new substances.A chemical reaction changes a substance's state.A physical change is always reversible.

What are the starting substances in a chemical reaction called?

MoleculesProductsReactantsCompounds

The Law of Conservation of Mass states that during a chemical reaction, the total mass of the reactants is...

less than the total mass of the products.greater than the total mass of the products.not related to the total mass of the products.equal to the total mass of the products.

Which of the following is an example of a chemical reaction mentioned in the passage?

Ice melting into water.Sawing a piece of wood.Water boiling into steam.An iron nail rusting.

Which of the following would be a sign that a chemical reaction has occurred?

A substance changes color.A substance is cut into smaller pieces.A substance freezes.A substance dissolves in water.

Based on the passage, what is the primary function of chemical reactions in living organisms, such as plants?

To create physical changes.To produce new materials.To release energy.To rearrange atoms to form new molecules.

What is the central idea of the passage?

Chemical reactions are always visible.Photosynthesis is the most important reaction.Chemical reactions are processes that create new substances by rearranging atoms.The Law of Conservation of Mass is a complex scientific theory.

You mix two clear liquids together, and a white, powdery substance suddenly forms at the bottom of the container. What has most likely happened?

A physical change has occurred.The liquids have dissolved.The liquids have frozen.A chemical reaction has formed a precipitate.