# Introduction to chemical reactions

### Introduction to chemical reactions

#### Lessons

• Chemical reactions involve collisions between molecules – they are far too small to see directly with the naked eye. Instead we make observations which are evidence that a reaction has happened or is happening.

• For chemists, an observation would be something that we can tell directly using our senses, for example seeing a color change, feeling a temperature change (like a glass beaker warming up) or seeing gas being released from a test tube.

• The reason this happens is because different chemical compounds have different properties that we can see when that chemical is produced. For example a color change from brown to colorless might mean a reactant chemical that is brown is being converted into a product that is colorless.

• There are two ways to write that a reaction is taking place:
$\circ$ a word equation
$\circ$ a chemical (symbol) equation.

• The → arrow found in a chemical equation is very important; it shows that a reaction is taking place. The reactants (what gets put in) of a reaction are always on the left-hand side of it, and the products (the new chemical that gets made) of a reaction are always on the right-hand side.

In this lesson, we will learn:
• How to determine whether a chemical reaction has occurred.
• The methods of writing a chemical reaction.
• 1.
Introduction to chemical reactions
a)
What is a chemical reaction?

b)
How do we know reactions happen?

c)
How to write a word equation

d)
How to write a chemical equation

• 2.
Write word and chemical equations to describe basic chemical reactions.
Some hydrochloric acid (HCl) was added to a beaker containing sodium hydroxide (NaOH). A reaction occurred and produced sodium chloride (NaCl) and water (H$_2$O).
a)
Write a word equation to show this reaction.

b)
Write a chemical equation to show this reaction happening.

c)
Explain why sodium chloride is written on the right hand side of the reaction arrow.

• 3.
Recognise evidence of a chemical change or reaction.
Read the notes from each lab experiment below and highlight which part of the experiment notes shows evidence that a chemical reaction is happening:

Experiment 1: Hexene was added to bromine water in a beaker and stirred thoroughly. Afterward, the brown color disappeared and the mixture became colorless.

Experiment 2: Using a delivery tube and gas syringe, carbon dioxide was bubbled through a solution containing lime water. Over a short time, the lime water became cloudy.

Experiment 3: Sodium hydroxide and sulfuric acid were added to a test tube which was shaken thoroughly. The test tube began to feel warm to the touch.

Experiment 4: Bubbling and fizzing occurred when a strip of magnesium metal was dropped in a solution of hydrochloric acid.

Experiment 5: Iron sulfide powder was added to a solution of hydrochloric acid and a bad smell of rotten eggs was noticed.