A chemical reaction in which to or more substance undergo a chemical union to form more complex substance is a what reaction?

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A chemical reaction in which two or more substances undergo a chemical union to form a more complex substance is:

a Direct Combination or Synthesis Reaction

A chemical reaction in which two or more substances combine to form a more complex substance is known as a synthesis reaction or a combination reaction.

To identify and understand different types of chemical reactions, you need to be familiar with their general characteristics and reaction patterns. Here's how you can determine the type of reaction:

1. Read the question or problem carefully, looking for key information like reactants and products.
2. Examine the chemical equation provided. Note the elements or compounds present on either side of the arrow.
3. In a synthesis reaction, the reactants combine to form a single, more complex product. The general form of a synthesis reaction is A + B → AB, where A and B represent different reactants.
4. Look for clues such as one reactant being a metal or a non-metal oxide, as these are common in synthesis reactions. For example, the reaction of sodium (Na) with chlorine (Cl) to form sodium chloride (NaCl) is a synthesis reaction: 2Na + Cl₂ → 2NaCl.
5. Finally, confirm that the reaction follows the general pattern of a synthesis reaction, where the reactants combine to form a more complex product.

Remember, practice and familiarity with different types of reactions will aid you in recognizing and classifying them correctly.