When students find a concept in chemistry a bit difficult the teacher may try to lighten things up with the following joke:

Chemistry is like cooking ... just don't lick the spoon.

The meaning is quite clear: cooking isn't hard to do, so neither is chemistry.

Often teachers drop the simile and get straight back into explaining the chemistry, but maybe we should continue to use cooking to help us explain some fundamental chemical concepts...

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Chemistry, like cooking, is a practical subject so let's start this discussion with a recipe ...

This recipe is a tried and tested set of instructions for making chocolate fudge. Recipes, such as this, are published in cook books and they enable other people to follow the instructions to make their own chocolate fudge. When a chemist writes down a set of instructions so that other people can reproduce a chemical reaction, we don't call it a recipe, we call it a lab report, a scientific report, or a scientific article depending on where the information is published.

There are a lot of similarities between this recipe for cooking chocolate fudge, and a Chemist's idea of how to set out a lab report.

Firstly, the ingredients are separated from the instructions. This makes it easier to determine if you have all the ingredients you need, or if you need to visit a supplier before you begin.

Second, the quantity of each ingredient is given in appropriate units. This is so you can check that you have a sufficient quantity of each ingredient before you start following the method.

Third, the method for making the final product (chocolate fudge) is given as a set of step-by-step instructions.

Fourth, the step-by-step instructions are given in the third person passive voice, no pronouns are used, and, in general, each step starts with a verb (line, stir, pour).

If we wanted to write this up as a lab report, only a few things would need to be changed:

• change the heading from "ingredients" to "materials"
• add a separate heading for "equipment" and list the equipment needed
• change the heading from "method" to "procedure" (optional) and remove unnecessary descriptions

So here is our recipe for chocolate fudge written in the format of a chemistry lab report:

Procedure

1. Place baking paper in bottom of cake tin.
2. Add 1 packet of chocolate melts to the heatproof bowl then place inside the microwave oven.
3. Heat for 1 minute. Stir.
4. Add the remaining chocolate and keep stirring. Heat in 10 second bursts until all the chocolate has melted.
5. Stir in condensed milk until well combined.
6. Pour mixture into cake tin and refrigerate until set.

If someone at a party asked you how you made chocolate fudge you wouldn't start telling them the ingredients then give them detailed instructions for each step would you? No, you'd give them an abbreviated version ...

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If you needed to quickly tell someone how to make chocolate fudge you'd probably say something like,

"To make fudge, mix a tin of condensed milk into 2 packets of molten chocolate then cool it."

In cooking, the condensed milk and the chocolate are referred to as "ingredients", but in chemistry these items are referred to as reactants. Reactants react together to produce a product, in this case the product is chocolate fudge.

Now we borrow a couple of symbols from mathematics; a plus sign (addition sign, +) and an arrow (→).
So, instead of saying "add condensed milk to molten chocolate", we could write

molten chocolate + condensed milk

and instead of saying "these react to produce" fudge would could use an arrow (→):

molten chocolate + condensed milk → chocolate fudge

This is called a "word equation" in chemistry. The reactants, separated one from another by a plus sign, are always written on the left hand side of the arrow. The arrow always points to the right to indicate the direction of the reaction, that is the reactants will produce the product. The products, separated by a plus sign if there is more than product, will always be written to the right of the arrow.

The reactants, molten chocolate plus condensed milk, react to produce the product, chocolate fudge.

A word equation is an overall description of the chemical reaction, but it tells us nothing about how much of each reactant to use nor how much product is theoretically achievable. To do this, we need another type of short-hand ...

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Cooks love abbreviations! Here are some common examples:

Chemists also love abbreviations, and not just for units of measurement!

Chocolate, for example, is used in lots of different recipes such as chocolate mousse, rocky road, chocolate chip cookies, chocolate brownies, to name just a few. So instead of writing "chocolate" every time we could allocate a symbol, such as a black circle, to represent chocolate as shown below:

⚫ = chocolate

Similarly, condensed milk is a common cooking ingredient. It can be found in coconut ice, caramel, slices, cheesecakes, etc. So instead of writing "condensed milk" every time we could allocate a symbol, such as a white circle, to represent condensed milk as shown below:

⚪ = condensed milk

But how will we represent the product, the chocolate fudge? Clearly it is made up of both chocolate and condensed milk so we could just join a black circle and a white circle as shown below:

⚫⚪

except that we used 2 packages of chocolate melts for every 1 package (tin) of condensed milk, so our fudge product must contain twice as many chocolate "particles" as condensed milk "particles". We would therefore represent the fudge product as 2 black circles and 1 white circle joined together as shown below:

⚫⚪⚫

Now we can replace all the "words" in our word equation in the section above with our new shorthand symbols:

But wait! On the left hand (reactant) side of the equation there is only 1 package of chocolate, but on the right hand (product) side of the equation there are 2. What should we do? Add another package of chocolate to the left hand side?

Now this equation shows us that we need 2 packages of chocolate for every 1 package of condensed milk to produce chocolate fudge.

We can simplify this further, just as you would when you do algebra in mathematics; x + x is the same as 2 × x which is the same as 2x. In our case, ⚫ + ⚫ is the same as 2 × ⚫ or just 2⚫ so that our symbol equation becomes:

We could even simplify the symbol for the fudge product since it is made up of 1 condensed milk "particle" and 2 chocolate "particles", but where do we put the "2" so that it won't cause confusion?
We can't write 2⚫⚪ because this would mean we need to have to 2 new particles each made up of a black circle and a white circle (⚫⚪ + ⚫⚪).
And we can't write ⚫⚪2 because, since we read from left to right, this 2 has nothing following it ... two times what?
Chemists solve this problem by using a subscript 2 immediately after (that is, to the right of) the symbol used to represent the "particle" that is present more than once in the product. Our fudge "particle" would therefore be represented as ⚫₂⚪
Our complete symbol equation to represent the chemical "reaction" used to produce fudge is now as follows:

2⚫ + ⚪ → ⚫₂⚪

This equation tells us that to make 1 package of chocolate fudge we need to combine 2 packages of chocolate melts and 1 package of condensed milk.
This symbol equation is said to be balanced; the number of black circles on the left hand side of the arrow is the same as the number of black circles on the right hand side of the arrow and the number of white circles on the left hand side of the arrow is the same as the number of white circles on the right hand side of the arrow.
You should also notice that if only 1 "package" of a reactant is needed, there is no need to write the number "1" in front of the symbol for the reactant. Similarly, if there is only 1 "package" of product produced we do not need to write the number "1" in front of the symbol for the product.

But what if we wanted to make twice as much fudge, or half as much fudge? How much chocolate and condensed milk would we need?

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In the section above we developed a word equation, then a symbol equation, as a really quick way to tell someone how to make a 20 cm × 20 cm tin of chocolate fudge:

2 packages of molten chocolate added to 1 package of condensed milk produces 1 package of chocolate fudge.

What if I wanted to make two packages of fudge, that is, two 20 cm × 20 cm tins of fudge? How much chocolate and condensed milk would I need?
I could just multiply all the "reactants" and all the "products" by 2:

4 packages molten chocolate + 2 packages condensed milk → 2 packages fudge

To make 2 tins of fudge I need 4 packets of chocolate melts and 2 tins of condensed milk.

In chemistry, the ratio of the quantities of chocolate to condensed milk to fudge produced is called the stoichiometric ratio⁽¹⁾ (or the mole ratio):

chocolate : milk : fudge is 2 : 1 : 1

In mathematics this relationship is known as a part-part ratio, 2 parts chocolate to 1 part condensed milk to 1 part fudge.
In our example 1 part was equal to 1 package.
So this ratio was 2 packages chocolate to 1 package condensed milk to 1 package chocolate fudge.

We can use this stoichiometric ratio (part-part ratio) to make any qantity of fudge. If you want to produce half a package of fudge, we just multiply all the terms in the equation by ½

We can also use this stoichiometric ratio (part-part ratio) to determine how much fudge could be produced by a given amount of either reactant.
If you have 6 packages of chocolate melts, how much fudge could you produce? How much condensed milk would you need to produce this amount of fudge?

The stoichiometric ratio (part-part ratio) of chocolate melts to fudge is 2:1
We could divide each term ny 2 so that the ratio of chocolate melts to fudge is ²/₂ : ¹/₂ or 1 : ½
So, if we have 6 packages of chocolate melts, we could multiply each term by 6 and the ratio of chocolate melts to fudge is 6 ×1 : 6 × ½ or 6 : 3
If we have 6 packages of chocolate melts we can make 3 packages of fudge.

The stoichiometric ratio (part-part ratio) of fudge to condensed milk is 1 : 1
So, to make 3 packages of fudge we need to use 3 packages of condensed milk.

OK, so we've learnt how to make different quantities of fudge, but where's the chemistry?

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Just like using coloured circles to represent the "elements" used in making fudge, each chemical element known to us is represented by a 1 or 2 letter symbol. For example, the symbol for potassium is K while the symbol for sulfur is S.
We could replace all the black balls in the fudge equation with the symbol K and all the white balls with the symbol S as shown below:

Now we have a balanced chemical equation to represent the synthesis of potassium sulfide (K₂S) from the elements potassium (K) and sulfur (S).
The "packages" of reactants and products are generally referred to as "molecules". So 2 "molecules" of K react with 1 "molecule" of S to produce 1 "molecule" of K₂S.
We can use the part-part ratios of reactants (K and S) to product (K₂S), or stoichiometric ratios (K : S : K₂S is 2:1:1) to determine how many product "molecules" can be produced from a given number of "molecules" of either reactant, and, we can determine the number of "molecules" of each reactant we need to use to produce a given number of product "molecules".

Ofcourse, if you wanted to make a different product, you would have to start with different reactants (or ingredients) and write a new balanced chemical equation.

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Refer to the balanced chemical equation below:

4KNO₃ + 7C + S → 3CO₂ + 3CO + 2N₂ + K₂CO₃ + K₂S

Match the description to its corresponding example.

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