What is this material made of? (the question of Chemical Identity)
One of the most important questions that chemical thinking allows us to answer is "What is this substance?" Chemical models, reasoning, and experimental methods can be used to find the identities of the thousands of substances in our surroundings and inside our bodies. These ways of thinking and acting are used daily by professionals seeking to determine which pollutants are in the air we breathe or in the water we drink, detect the levels of metabolites in our bodies, or identify unknown materials found at a crime scene. The question of chemical identity is the driver for chemical analysis directed at identifying, detecting and quantifying substances both in the natural world or synthesized in chemical laboratories.The search for the chemical identity of the substances with which we interact is based on the fundamental assumption that each substance, no matter how simple or complex, has at least one measurable property that can be used to differentiate it from other substances. Differentiating characteristics often allows us to narrow down the chemical categories to which an unknown substance belongs, which facilitates its identification. For this reason, chemistry relies on a variety of classification systems (e.g., acids and bases; molecular, ionic, or metallic). Differentiating characteristics tend to be intensive properties with unique values for each targeted substance, such as its normal melting and boiling points, its density, its solubility in water, or its electrical conductivity. Current chemical methods of analysis take advantage of the unique ways different substances absorb and give off different kinds of light. A substance's distinctive emission or absorption of electromagnetic radiation serve as fingerprints during chemical analysis and provide clues about the chemical composition and structure of the submicroscopic particles that make up a substance. The molecular structure of a substance is the ultimate differentiating characteristic, which is why a significant amount of chemical thinking and action is dedicated to uncovering it.
|What's in my food?|
Central Ideas: Food products contain different proportions of carbohydrates, fats, and proteins. These types of substances have different physical and chemical properties that can be used to detect, separate, and quantifying them.
Science Practices: Planning and carrying out investigations; analyzing and interpreting data; designing solutions and building arguments.
Crosscutting Reasoning: Patterns; systems and system models; structure and function.
Performance Expectation: Students will design an experiment to separate the fats from the carbohydrates and proteins present in different food products of their choice, quantify the mass of fat per unit mass of food product, and compare their results with reported data.
There is a wide variety of questions and problems of interest to students and the communities in which they live related to the question of chemical identity. As illustrated in the example below, these questions tend to involve the identification of unknown substances in a system, the detection of known substances, the separation of components, and the quantification of amounts. These types of anchoring problems create opportunities for students to characterize physical and chemical properties, analyze data, look for patterns, design experiments, evaluate different analytical strategies, and build arguments based on experimental evidence. Problems with various levels of difficulty can be utilized depending on students’ interests, communities, and backgrounds, or the nature of the resources available. For example, basic identification, detection, separation, and quantification tasks can be completed using solubility as a differentiating characteristic and simple volumetric and gravimetric equipment. Affordable spectrometric equipment can further enrich the types of chemical identity problems that can be tackled.
Eliciting and Exploring
Existing research in science and chemistry education suggests that young children struggle to differentiate between objects (e.g., a window) and materials (e.g., glass), and often use object-relevant properties (e.g. size, shape) to classify different kinds of substances. Novice learners typically do not distinguish between mixtures and pure substances, and their reasoning about chemical identity is influenced by three major factors: (a) appearance, (b) usage, and (c) history. For example, younger students tend to pay attention to perceivable properties of materials such as shape, color, texture, and smell, to make judgments about category membership. They often assume that materials known to have similar functions (e.g. glues, oils) share the same intrinsic nature. Novice learners also may judge substances as different if they come from distinct sources or result from different processes (e.g., thinking that natural and synthetic forms of a substance are different).
Students are likely to use a mixture of extensive (i.e. dependent on size) and intensive (i.e. independent of size) properties to identify materials. Many learners may see some properties (e.g. color, taste, smell) as separable from the actual substances. They may think of such properties as "ethereal" things that may be added or removed without change in a substance’s identity. These views are likely to hinder students' ability to differentiate between single substances and mixtures of substances, particularly when dealing with homogeneous materials. Once students are introduced to atomic models and chemical symbology to represent substances, they are known to pay more attention to compositional (what components are present) than structural (how these components are organized) cues to differentiate them. Their focus on chemical composition rather than molecular structure to assign identity often constraints students' thinking (e.g., failing to differentiate between water and a mixture of hydrogen and oxygen).
Advancing and Connecting
The road toward chemical thinking in the area of chemical identity seems to demand several shifts in the ways that students reason about materials and their properties. They need to: a) understand that substances are the underlying constituents of objects in their surroundings, b) pay attention to intensive properties of materials to characterize and categorize them, c) recognize the importance of experimental testing of selected differentiating properties (e.g. melting points) to make differentiations, and d) add a structural conceptualization to their compositional understanding of the submicroscopic world. These shifts demand multiple opportunities for students to engage with problems focused on the identification, detection, separation, and quantification of the components of systems of interest, designing analytical experiments, collecting and analyzing data, and building arguments supported by evidence and chemical rationales of different types (e.g., phenomenological based on the analysis of measured properties, structural based on models of matter at the submicroscopic level)