The ACT Science section does not require extensive background knowledge to achieve a high score; students need to understand a few simple concepts and be familiar with the format of the passages. One basic concept used quite often is pH and how it applies to various solutions.
The term “pH” stands for “Potential of Hydrogen” and represents the mathematical measurement of free-floating hydrogen ions (H+) in an aqueous solution. The basis for pH stems from how water (H2O) dissociates into H+ cations and OH- (hydroxide) anions. When this occurs the free H+ can be measured and applied to the categories of acids, bases, or neutral substances.
Although there is a mathematical formula used in chemistry experiments, it is not a formula that needs to be memorized for the ACT. There may be some passages that use the formula in their scientific concept section, but that will be stated in the concept area.
What students need to know for the exam is that there is a standardized scale that ranges from 0 to 14. Solutions that have a pH of 7 are considered neutral and include pure water. A pH below 7 is identified as an acid; and a pH above 7 is identified as a base (or alkaline).
The pH of a solution is determined by the concentration of hydrogen ions (H+) in the solution, relative to the concentration of hydroxide ions. Acids release hydrogen ions in water, increasing the concentration of H+ ions and lowering the pH. Bases either take up hydrogen ions or release hydroxide ions (OH-), decreasing the concentration of H+ ions and raising the pH. A simple concept to remember is:
Reading the pH scale involves understanding the relationship between the pH value and the acidity or alkalinity of a solution. The following are some key points for reading and interpreting the pH scale:
- Neutral pH: A pH of 7 is considered neutral. This means that the concentration of hydrogen ions (H+) is equal to the concentration of hydroxide ions (OH-) in the solution. Water is an example of a neutral substance, with a pH of 7.
- Acidic pH: pH values below 7 indicate acidity. The lower the pH value, the more acidic the solution. For example, lemon juice has a pH of around 2, which is highly acidic. Vinegar typically has a pH of around 3, while orange juice has a pH of around 3.5-4.
- Alkaline pH: pH values above 7 indicate alkalinity. The higher the pH value, the more alkaline the solution. For example, baking soda dissolved in water has a pH of around 8-9, making it alkaline. Ammonia has a pH of around 11-12, indicating a strong alkaline nature.
- Logarithmic scale: Remember that the pH scale is logarithmic, meaning that each whole number change on the scale represents a tenfold change in acidity or alkalinity. For instance, a solution with a pH of 3 is ten times more acidic than a solution with a pH of 4.
- pH indicators: In some cases, pH indicators such as litmus paper or pH meters are used to measure the pH of a solution more accurately. These indicators change color based on the pH of the solution, providing a visual representation of the acidity or alkalinity.
- Buffers: Buffers are substances that prevent the pH from changing (either up or down) by either binding excess H+ being added or donating extra H+ if needed. An example would be the bicarbonate ion (HCO3-) found in the human blood system to keep our blood pH stable.
By understanding these principles and being familiar with common substances and their pH values, students can effectively read and interpret the pH scale and successfully answer pH questions on the ACT Science section.
