From sweets and seeds to atoms and reactions

Yasmin Jayathirtha

Once students have an idea of relative atomic mass (RAM) and have learnt about physical chemical change, they can begin to study reactions and learn about equations. Unfortunately, the activities that help this process are lab based, using various chemicals, hard glass test tubes and bunsen burners.

I will cover these activities teaching point by teaching point.

1. The idea of formula mass
Take down as many bottles of chemicals from the shelves as possible and ask the students to read the labels. Ask them if they know any formulae. Most should know H₂O and CO₂. Can they determine the formula mass of these compounds? Ask them to pick a bottle, find the formula on the label and calculate the formula mass using the periodic table.

2. Changes that occur when reactions happen
We will use heat as a way to bring about change and will heat single compounds. The common gases that can be evolved are oxygen, O₂ and carbon dioxide, CO₂. Show the students how to test for these gases.

Choose salts with water of hydration like hydrated copper sulfate (CuSO₄.5H₂O), magnesium sulfate (MgSO₄.6H₂O) and any others you find on the shelf. Choose carbonates and hydrogen carbonates like calcium carbonate (CaCO₃), magnesium carbonate (MgCO₃), copper carbonate (CuCO₃) and sodium hydrogen carbonate (NaHCO₃), which will give interesting results.

Ask the students if the formula contains the elements to generate oxygen and carbon dioxide. Usually, all of these will contain the atoms to make O₂, and some will contain the atoms to make CO₂ and H₂O.

Let the students heat the compounds, first gently then strongly. If they work in pairs, one can heat the compound while the others test for the gases and they can then swap. They should note down all that they have observed. Remind them that no change is also an observation. Then we can build equations for the reaction, both word and formula.

Observations, word equation and formula equation may look like this for heating copper carbonate;

Observations:
A blue green solid became black on heating; the gas evolved turned lime water milky.

Word equation:
Copper carbonate –> black solid + carbon dioxide

Formula equation:
CuCO₃ –> CuO + CO₂

It is apparent that taking away CO₂ leaves behind CuO. It is not Cu, because no oxygen evolved and Cu is not black. This should be elicited from the students.

We will consider a more complicated reaction, the heating of sodium hydrogen carbonate:
White solid, stays white on heating. Gas is evolved and colourless droplets condense on the cooler part of the test tube. Test for carbon dioxide positive.

Sodium hydrogencarbonate –> white solid + water + carbon dioxide

NaHCO₃ –> X + H₂O + CO₂

Looking at this equation, we realize that there aren’t enough hydrogen atoms, and the equation is written as:
2NaHCO₃ –> X + H₂O + CO₂

We are now left with 2 Na and 1CO₃. This can be either Na₂CO₃ or Na₂O + CO₂.

So further tests are needed or we can consider the formula masses.

NaHCO₃ formula mass – 84
CO₂ formula mass – 44
H₂O formula mass – 18

We can write out the two equations as:

i) 2NaHCO₃ –> Na₂CO₃ + H₂O + CO₂
168 106 18 44

ii) 2NaHCO₃ –> Na₂O + H₂O + 2 CO₂
168 62 18 88

So heating a known mass of sodium hydrogencarbonate and determining the mass of the residue will tell us which reaction occurs.

Take 16.8 g of NaHCO₃ in a hard glass test tube and heat for 4-5 minutes. Allow to cool and find the mass. Repeat the heating, cooling and weighing till there is no change in the mass. The mass of the residue, either 10 or 6 g will tell us which reaction is occurring.

This is the precursor to both balancing equations and mole calculations for high school classes.

The author works with Centre for Learning, Bengaluru. She can be reached at yasmin.cfl@gmail.com.