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Lesson 5 - Chemical Reaction

A chemical reaction involves the transformation of reactants into products with different properties, exemplified by baking a cake or iron rusting. Signs of a chemical reaction include color change, gas production, temperature change, and formation of a precipitate. Key laws governing chemical reactions are the Law of Conservation of Mass, which states mass is conserved, and the Law of Constant Proportions, which indicates elements in a compound are always in a fixed ratio.

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13 views2 pages

Lesson 5 - Chemical Reaction

A chemical reaction involves the transformation of reactants into products with different properties, exemplified by baking a cake or iron rusting. Signs of a chemical reaction include color change, gas production, temperature change, and formation of a precipitate. Key laws governing chemical reactions are the Law of Conservation of Mass, which states mass is conserved, and the Law of Constant Proportions, which indicates elements in a compound are always in a fixed ratio.

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chafiq jellas
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© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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I. What is a Chemical Reaction?

A chemical reaction is a process where one or more substances (called Reactants) are
transformed into new substances (called Products) with different properties. Think of
it like rearranging the pieces of a puzzle to create a completely new picture.

For example:
- When you bake a cake, the ingredients (flour, eggs, sugar) mix and change into
something new (the cake).
- When iron rusts, the iron reacts with oxygen in the air to form rust, which is a
completely different substance.

II. How Do You Know a Chemical Reaction Has Happened?

There are a few key signs to look for:

- Color Change: A new color appears. For example, when iron rusts, it turns from
shiny silver to reddish-brown.
- Gas Production: Bubbles or fizzing occur. For example, when you mix baking
soda and vinegar, you see bubbles of carbon dioxide gas.
- Temperature Change: The reaction feels hot or cold. For example, when you
light a match, it releases heat.
- Formation of a Precipitate: A solid forms in a liquid. For example, mixing two
clear solutions might create a cloudy solid.

III. Chemical Reactions vs. Physical Changes

It’s important to know the difference between a chemical reaction and a physical
change:
- Physical Change: The substance looks different, but it’s still the same material.
For example:

o Cutting a piece of paper (it’s still paper, just smaller).

o Melting ice (it’s still water, just in liquid form).

- Chemical Reaction: A new substance is formed. For example:

o Burning wood (it turns into ash and smoke, which are new substances).

o Baking a cake (the ingredients change into something new).

IV. Laws of Chemical Reactions

When atoms and molecules interact in chemical reactions, they follow specific laws.
The two most important laws are:

1. Law of Conservation of Mass

Definition : This law states that mass cannot be created or destroyed in a chemical
reaction. The total mass of the reactants (substances you start with) is always equal to
the total mass of the products (substances formed).

Example : When dihydrogen (H₂) reacts with oxygen (O) to form water (H₂O), the total
mass of hydrogen and oxygen before the reaction equals the total mass of water
produced.

2. Law of Constant Proportions

Definition: This law states that in a given compound, the elements are always present
in a definite proportion by mass. No matter how much of the compound you have,
the ratio of the elements remains the same.

Example: Water (H₂O) always consists of hydrogen and oxygen in a 2:1 ratio by
volume.

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