Experiences

Understand of the solutions pH

Katarína Javorová

Slovakia

Lower Secondary School

Chemistry

Teaching in class

Direct

The theme of the solutions pH is discussed in the 8th grade elementary school. At this time it is not defined what exactly pH is. Pupils only indicate that it is a quantity that expresses the acidity of solutions and represents the value of 0-14. The acidic solutions have pH  7, neutral pH = 7 and pH  7 are called alkaline solutions. I teach gifted students who often study by themselves a lot and ask a lot of scope and subject matter questions that often exceeds elementary school curriculum. They have met with the concept of pH when it was defined as the negative logarithm of the concentration of oxonium cations. Since the logarithm terms and the concentration are part of the high school curriculum, students have difficulty imagining what pH is. To clarify the issue I use a free application pH scale that can be downloaded for free from the page http://phet.colorado.edu/en/simulation/ph-scale. It presents an interactive simulation of various solutions with different pH. A student can fill or let go water and thus can be tested whether changing the pH change in volume of the substance or changing the amount of water (dilution).
The simulation can be used in different variants: macro (Fig.1), micro (Fig. 2) custom (Fig. 3).
The macro variant is the easiest one. A student chooses from the offered solutions, pours it in the tank and using a pH meter measures the solution pH. He may pour in water and watch how the dilution changes the pH value.
Micro and custom variants are slightly more difficult. Here are used the variables as concentration and amount of substance. For illustration there are included particles of oxonium cations and hydroxide anions. Students can watch how their quantity is changing as well as how changes the pH scale.
In the variant custom it can be easily seen at the case of neutral solution that the amount of oxonium cations is the same as the hydroxide anions. Pupils really enjoy working with simulations. They can track the changes by themselves that are not possible to view with a naked eye. Thus it is easier to both capture and understand the phenomenon. The advantage is that a given simulation is freely downloadable and students can use it at home.
This type of experience I consider to be successful-positive.
Fig. 1 pH scale - macro

Fig. 2 pH scale - micro

Fig. 3 pH scale custom – an example of a neutral solution