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The concept of chemical reaction (transformation)


One or more substances, present in the initial state of a system, become one or more different substances, which will be present in the final state, the transformation is a chemical reaction, or chemical transformation.

In other words, Chemical reaction is a process in which new substances are formed from others.

You may be wondering: How are chemists sure new substances have been formed?

A chemical is characterized by its properties such as density, melting point, boiling point, color, odor etc. So, To know if there was a chemical reaction, we need to compare the properties of the substances present in the system, in the initial and final states.

Imagine that the system chosen for study is an iron pipe and that it is observed before and after being sawn in half. The initially present substance, iron, has exactly the same properties as the substance present at the end, which is also iron. Sawing an iron pipe is therefore not a chemical transformation as no new substance has been formed.

When an object falls, a piece of paper is torn, a piece of sand is mixed with water, a chalk is crushed to dust, and a nail is stuck in the wood, we are seeing examples of transformations that no They are chemical reactions.

Example of chemical reaction: combustion of ethanol

Studying the burning of ethanol (common alcohol), chemists could identify the substances present in the initial and final states of the system.

Ethanol combustion requires the presence of oxygen gas (eg air). Both substances become, during combustion, two new substances: water and carbon dioxide (also called carbon dioxide: it is the gas that forms bubbles in refrigerants).

In equation:

In this representation of ethanol combustion, the signs of more (+) can be read as "and". The arrow ( ) can be read as “React to form”.

In words: Ethanol and oxygen gas react to form carbon dioxide and water.

Chemists identify these substances through two properties. They confirm that, in fact, substances exist in the final state are different from those in the initial state.

See some of these properties:

ethanol + oxygen gas carbon dioxide + water
78ºC-183ºC-78ºC100ºCBoiling point
liquidgaseousgaseousliquid

Physical state at 20ºC

0.79 g / cm30.0013 g / cm30.0018 g / cm31.0 g / cm3Density at 20ºC
colorlesscolorlesscolorlesscolorlessColor