Last week, during our Incr-Edible Science, the children and I discovered the joys of making atoms, molecules and compounds out of sweeties! Better than that we all actually understood the concepts. Although I took chemistry at advanced level, I always found it hard to understand. I think I only passed by working incredibly hard learning everything verbatim. This would have been the only subject I truly would have wondered if I had it in me to teach. Having a son who just ADORES science I’m so pleased to find that 18 years later I understand things which when I was younger I found so bewildering!
It was the chemistry of Sodium Bicarbonate that I was particularly interested in the children understanding, and its reaction with various acids. We had already learnt about what happens when it mixes with water:
NaHCO3 + HOH ——————————> Na^+1 (aq) + OH^-1 (aq) + H2CO3 (aq)
We also learnt that the Carbonic acid turned into water and Carbon Dioxide:
H2CO3 (aq) —> CO2 (g) + H2O (l)
Therefore when Sodium Bicarbonate is added to water the following reaction occurs:
NaHCO3 + HOH —> Na+1 (aq) + OH-1 (aq) + CO2 (g) + H2O (l)
These resulting products of CO2 and OH-1 ions were key for the children in:
- Understanding pH (measure by the activity of H+ and OH- ions in solution)
- Realising it is the gaseous property of Carbon Dioxide which is exploited in home cooking
The whole point of these sessions doing edible science is to teach L10 (who wants a career in cooking/baking) the principles of food chemistry. It is almost too good to be true, but she is really understanding and enjoying it. She is the first to answer a lot of the questions (even the maths ones-gasp!) and I seem to have her full attention all of the time.
This week I began to teach them about the acid we would be using to make lemonade (Citric Acid); why it is an acid; how we measure if something is an acid or alkaline using universal indicator paper and why we are able to make lemonade from adding the Bicarb (an alkaline) to the lemon juice (an acid) to create a fizzy drink.
They already knew, to some extent, that it was the OH- ions present when Bicarb is in solution which made it an alkaline. We looked at the Citric acid ionization equation to determine what made it an acid:
C6H8O7——————–>C6H5O7 (-3) + 3H+
They correctly identified the H+ ions as determining its acidity. I wrote pH on the board and asked what it might mean. C10 said ‘positive Hydrogen’ which I thought was quite a good guess! I told them potential Hydrogen and asked what that might mean. L10 thought it might be the amount of H+ ions potentially free in a solution. The best and clearest explanation I found came from this website and it is from this I quote and taught:
The water molecule has the property of dissociating into two ionic components in aqueous solutions.
H2O <-> H+ + OH–
The H+ ion is termed hydrogen ion or proton, the OH– ion hydroxide ion. The pH value describes the activity of hydrogen ions in aqueous solutions typically on a scale of 0 to 14. Based on this pH scale, liquids are characterized as being acidic, alkaline or neutral: a solution which is neither acidic nor alkaline is neutral. This corresponds to a value of 7 on the pH scale. Acidity indicates a higher activity of hydrogen ions and a pH measurement value lower than 7. Alkaline solutions are characterized by a lower hydrogen ion activity or higher hydroxide ion activity, respectively and a pH measurement value above 7.
The children knew about Universal Indicator paper, from our pond study the week before. So I made a chart on the white board and had the children fill jars with a bicarb solution, water and a citric acid solution. They were then required to guess the relative pH of each liquid (<7, =7, >7):
We talked about the reaction that might occur between the acid and alkali. They had been involved in their younger sister’s science that morning and knew that acid (in this case vinegar) when added to Bicarb caused a big fizz. I asked them what caused the fizz, T11 answered CO2. I told them that was the gas in soda drinks and that we would be making our own lemonade today. I asked them to narrate to me the chemistry that would occur when the Bicarb was mixed with the lemon juice-which they did well.
T11 mentioned that he thought just lemon juice would taste a bit sour no matter how fizzy it was and suggested we add sugar. Then he wondered if sugar would effect the pH and therefore the fizz; so being the intrepid experimenters that we are (or at least have become in the last few weeks!), we decided to test it to see. We filled three jam jars with a lemon juice solution. To one we added 1 tsp of sugar; to the next we added 3 tsp of sugar and to the last we didn’t add any:
Now for the fun! Each jar was brought up to the same sweetness (3tsp sugar) and one by one they added a tsp of Bicarb:
I’m fairly certain there’s a proper recipe out there that if you follow would not obtain results such as these. Ehem! So we put them in a tray – much quicker than looking up said recipe:
Then the children drank it (!):
I set T11 a challenge by writing one side of the equation on the white board and asking him to complete the right hand side using his knowledge and deduction:
All the children agreed that the lemonade tasted salty (the sodium citrate). I had thought this might be the case so I had decided to set them a quest. By next Monday I wanted them to have designed and implemented a method to make lemonade which would not taste of Sodium Citrate. I knew this would be right up T11 alley, but I also knew it might worry L10, so I gave the girls on opt in or out option and set them the easier task of designing apparatus which, using the reaction from todays activity, would blow up a balloon. They went to work straight away, VERY enthusiastically. I’ll post next week what they come up with!