Building understanding of elements, compounds and mixtures

Has a student ever asked you where water is on the periodic table, and you wondered how the heck they don’t know water is a compound and not an element? Knowing the difference between elements, compounds and mixtures is key to students understanding so much of our instruction in chemistry.

By the time they reach their terminal exams at age 16, they don’t have to specifically identify elements, compounds and mixtures, but they encounter a range of names, formulas, diagrams and models. By the time students take their exams, we assume knowledge of these different modes of communicating information. But we need to teach carefully to provide strong foundations for more challenging concepts later on. For example, it’s no good learning about how to separate the components of a mixture if you can’t work out whether something is a mixture or a compound.

So how can we teach it so students get it? Here are five ideas:

1. State the obvious

As teachers, we’re experts at navigating our way around the periodic table and we hold a vast amount of general chemical common sense in our heads, so we can easily decode whether something is an element or not. Our students are novices and time spent helping students gain familiarity with the periodic table is never wasted as it will speed them up when using it for other purposes.

Teach students that elements have names connected to astronomical objects or the gods (eg mercury or thorium), countries or places (eg francium, californium), famous scientists (eg curium, einsteinium) or properties of the element or minerals (eg silicon or potassium). Suffixes are also relatively standardised, especially for recently discovered elements. Most metallic elements have names ending in -ium, while non-metals tend to end in -on, -ine or -gen.

Get students to research the names and symbols of the elements and they will find it much easier to decide whether a substance is an element or not. What is obvious to us teachers is not necessarily obvious to our students.

2. Teach thoroughly

Students encounter many different representations of chemical substances as they progress through their chemical education. Consider when they will meet each one and whether it is best for your students that you teach all of this in your 11–14 unit on elements, compounds and mixtures, or rather as they learn about each new representation. I teach all representations explicitly in the 11–14 stage including some that are not needed until much later, like interpreting displayed formulas of organic molecules. If it is better for your students to introduce this in stages then make sure schemes of work are thorough, prompting teachers to teach each new representation and not assume students will have this as prior knowledge.

3. Create opportunities for concrete experiences

Chemical substances is an abstract concept which might seem incidental and unimportant to students, especially older ones who know they won’t see a clear question on it in their exam. Concrete experiences, from physical models to examples from everyday life, can help students find the meaning in the concept and scaffold the learning to support consistent progress.

4. Watch out for exceptions

I tend to tell my youngest students that compounds typically have ‘chemically-sounding names’ which have at least two parts to them. This helps for most examples. However, water can cause difficulties because it’s a compound with a common name everyone uses, including scientists – no scientist talks about solutions made up with dihydrogen monoxide. But water is not a systematic name. Don’t assume your students will naturally pick this up though, explicitly teach it.

5. Share common names

Acknowledge the existence of common names for compounds but emphasise that they won’t encounter them in exams. As scientists and teachers we like to be totally accurate with everything and skim over or bypass the idea that common names exist. But common names are a good way to help students connect the science they learn in school with the real world.

Show them some citric acid powder, give its chemical formula (C₆H₈O₇) and its systematic name (2-hydroxypropane-1,2,3-tricarboxylic acid) and show a few different diagrams and models of it. Pick apart how these representations show that citric acid is a compound and remind them that lots of them will consume citric acid every day in fruit and confectionary.