Learning difficulties

chemistry

Prof. J. Peña

Complutense University of Madrid

The language of chemistry

The language of chemistry

UCM - Faculty of Education

Introduction

Substance

Empty words

Specifically...

Conclusions

Essentials of Chemistry

introduction

UCM - Faculty of Education

Essentials of Chemistry

How should concepts be taught in an abstract discipline like chemistry, which has its own scientific language?

Chemical substance versus Chemical compound

Example:

The concept of substance

UCM - Faculty of Education

Essentials of Chemistry

UCM - Faculty of Education

Essentials of Chemistry

Substance definition

Substance main features

The concept of substance

UCM - Faculty of Education

Essentials of Chemistry

Chemical changes

Physical changes

Characteristics

Examples

Physical changes

Key

Examples

The concept of substance

UCM - Faculty of Education

Essentials of Chemistry

Chemical changes

Physical changes

Summary

Example for Primary School

The concept of substance

UCM - Faculty of Education

Essentials of Chemistry

Are water and ice different substances? And what about a piece of aluminum versus powdered aluminum?

Primary School

Example

The concept of substance

UCM - Faculty of Education

Essentials of Chemistry

If students do not understand the basic concepts, they will very likely have trouble grasping more advance ideas

The concept of substance

UCM - Faculty of Education

Essentials of Chemistry

Words "that mean nothing" to the student. The meaning of words is consolidated if there is a connection between them and the knowledge already acquired; if this process does not occur, it is likely that words are heard or read as empty.

empty words

Examples

UCM - Faculty of Education

Essentials of Chemistry

For elementary students, a helpful way to explain it would be to say that pure substances are materials that are made up of only one type of "ingredient"—like a perfectly consistent recipe with only one item in it. On the other hand, mixtures are made up of two or more different ingredients that are combined but not chemically bonded. However, since they contain a mix of different parts, they may be identified as “impure” or “non-pure” substance.

pure versus non pure substances?

UCM - Faculty of Education

Essentials of Chemistry

The language of chemistry can be complex, and misunderstandings at an early stage can lead to misconceptions that may persist and make advanced topics harder to grasp. By using clear, relatable language and taking care to address misunderstandings early on, we can help students build a strong foundation.

conclusions

mixtures

UCM - Faculty of Education

In chemistry, when two or more substances mix with each other without participating in a chemical change, the resulting substance is called a Mixture.

Properties

Types

Map

Essentials of Chemistry

pure substances

UCM - Faculty of Education

Essentials of Chemistry

• Elements: Substances made of only one type of atom (e.g., oxygen, O₂).
• Compounds: Substances made of two or more elements chemically combined in fixed proportions (e.g., water, H₂O).

Pure vs mixture

solutions

UCM - Faculty of Education

• A solution is a homogeneous mixture of two or more components. The dissolving agent is the solvent. The substance that is dissolved is the solute.

• According to the amount of solute, we have dilute (contains a small amount of solute) or concentrated solutions (contains a large amount of solute relative to the solvent).

Essentials of Chemistry

other types of mixtures

UCM - Faculty of Education

• Suspensions: The particles in suspensions are larger than those found in solutions. Example: Oil and water
• Colloids: Particles intermediate in size between those found in solutions and suspensions. Example: Milk

Essentials of Chemistry

A colloidal dispersion is a heterogeneous system consisting of a continuous phase (the dispersing medium) and a dispersed phase made up of small regions (weakly connected or not connected at all), which are distributed more or less homogeneously

Mayonnaise is a colloidal dispersion, i.e., it is an emulsion, which is a type of colloid where tiny droplets of one liquid (oil) are dispersed in another liquid (water, vinegar, or lemon juice) that they are immiscible with. The emulsifying agent, typically egg yolk, helps to stabilize this dispersion, preventing the oil and water from separating.

A solution or homogeneous mixture is a homogeneous system upon subdivision but heterogeneous upon a change of state. For example, seawater is a homogeneous system upon subdivision; any portion of the system has the same properties.

Solvent particle

Solute particle

• Example of an element in the liquid state: Mercury (Hg). This is the only metal that is liquid at room temperature (around 20°C).
• Another element that can be in liquid form under slightly elevated temperatures is bromine (Br), a non-metal that becomes a liquid just above room temperature.

A mixture of sand mixed with salt is an example of a heterogeneous mixture. Heterogeneous mixtures possess different properties and compositions in various parts i.e. the properties are not uniform throughout the mixture.

Sugar mixed with water is the most common example of a homogeneous mixture. Homogeneous mixtures can be defined as the mixtures which possess the same properties and combination throughout their mass.

• The components of a mixture each keep their original properties.
• The separation of components can be easily done.
• The proportion of the components is variable.

A suspension refers to a solid dispersed in a liquid medium. In this case, when at rest, the particles settle and can be separated by filtration, resulting in a turbid preparation. Examples include carbon particles in water or antibiotics in suspension.

In chemistry, a substance is a form of matter that has a specific chemical composition and distinct properties. Substances are characterized by their uniform and definite composition, meaning they contain only one type of particle—either an element or a compound—and their properties remain consistent throughout the sample.

• Uniform composition: Substances have the same chemical composition and properties throughout.
• Definite properties: Substances have specific physical and chemical properties, like melting point, boiling point, density, and reactivity, which do not vary within the sample.
• Cannot be separated by physical means: Unlike mixtures, pure substances cannot be separated into other substances by physical methods (e.g., filtration, distillation) because their particles are chemically bonded.

A chemical change (or chemical reaction) is a process that alters the chemical composition of a substance, resulting in the formation of one or more new substances with different properties.

• New substances formed: Chemical changes result in the creation of substances with different chemical compositions from the original substances.
• Irreversible by simple means: Most chemical changes are not easily reversed by simple physical processes.
• Energy changes: Chemical changes often involve the release or absorption of energy (e.g., heat, light, or sound).
• Signs of a chemical change: Common indicators include a change in color, the production of gas (bubbling or fizzing), formation of a precipitate (solid), or emission of heat/light.

Examples

• Rusting of iron
• Burning of wood
• Baking a cake (transforms ingredients chemically)
• Vinegar reacting with baking soda to produce carbon dioxide gas

A physical change is a change in the form, shape, or state of matter that does not alter the substance's chemical composition. The molecules remain the same, but the arrangement or physical appearance of the particles changes.

• No new substances: Physical changes do not produce new substances.
• Often reversible: Physical changes are often reversible (e.g., freezing and melting).
• Energy changes: Physical changes may involve energy changes, but they are usually less intense than in chemical changes.
• No chemical bonds broken or formed: The molecular structure of the substance remains intact.

Examples

• Melting of ice
• Boiling of water
• Dissolving sugar in water
• Cutting or crushing a material

Physical vs chemical

For primary school students, a good way to differentiate a chemical change from a physical change is to explain that a chemical change is like when we add dye to a piece of fabric—it leaves a stain that can’t be removed unless we use special chemicals. In contrast, a physical change is like sewing two pieces of fabric together—they can be taken apart by simply undoing the stitches.

Physical vs chemical

• For students, elements, the ultimate components of any material, are the only pure substances.
• Mixtures are confused with substances.
• Everyday context: 'it was a viscous substance,' 'it had a black substance,' etc., giving the meaning of substance to a material that very likely contains multiple substances.

“That substance made up of different atoms”

This definition leads one to suppose that mixtures would also be included, since in these types of systems there are also different atoms.

• Without a clear idea of substances, it's hard to understand what changes during a chemical reaction.
• Understanding the difference between pure substances (elements and compounds) and mixtures helps in analysing matter, learning about separation techniques, and understanding why different materials behave differently.
• Recognizing that substances have characteristic properties like boiling point, and reactivity makes it easier to understand advanced concepts like bonding and molecular structure.

• Concepts like molarity, stoichiometry, and molar mass calculations rely on understanding substances at the molecular level, especially when balancing chemical equations.

• "The study of chemistry is the science that explores matter and its changes."
• "Matter is made up of atoms of different types combined in various ways."
• "Matter is classified into mixtures and pure substances."
• "Mixtures can be physically separated into pure substances."
• "Pure substances can be compounds or elements."
• "Compounds can be chemically separated into elements."

Typical senteces

Sugar, salt, and water are examples because each one has a specific chemical makeup (like all sugar molecules are the same).

Pure substances

Calling a mixture a "non-pure substance" can lead students to think that a mixture is simply another type of substance, rather than a distinct category on its own. This is a subtle but important point—mixtures are not substances; they’re combinations of substances that retain their individual properties.

Saltwater (a mix of salt and water) and trail mix (a mix of nuts, seeds, etc.) are examples of mixtures because they contain different components mixed together.

Mixtures