A chemical is a core concept in the field of chemistry, plus it applies to everyday life. But what exactly is a chemical? Let’s dive in!
In everyday usage, a “chemical” often refers to an undesirable additive or contaminant. For example, pesticides in water or artificial colorings in food are “chemicals.” But, chemistry sees chemicals somewhat differently.
A chemical is a substance with a specific composition. It is an element, an alloy, or a compound made up of two or more elements combined in a fixed ratio. For instance, water (H2O) is a chemical because it consists of hydrogen and oxygen combined in a 2:1 ratio. All matter either is a chemical or else consists of chemicals.
Natural vs. Synthetic Chemicals
Some chemicals occur in nature, while others are synthesized. Some chemicals occur both from natural and manmade processes.
- Natural Chemicals: These are chemicals that occur in nature without human intervention. For example, fruits produce citric acid naturally.
- Synthetic Chemicals: Humans make synthetic chemicals via chemical reactions. A common example is plastic, which comes from petrochemicals.
It’s worth noting that the distinction between “natural” and “synthetic” doesn’t necessarily equate to “safe” and “dangerous.” Many natural chemicals are harmful, and many synthetic chemicals are perfectly safe. For example, arsenic is a natural element that is poisonous, while baking soda (sodium bicarbonate) is a synthetic chemical that we use in baking all the time.
Pure Chemical vs. Mixture
Chemicals occur on their own or in combination with other chemicals:
- Pure Chemical: A pure chemical or pure substance has a constant composition and cannot be separated into its constituent elements by physical methods. For example, pure water, regardless of its source, always has two hydrogen atoms bonded to one oxygen atom. However, chemical and nuclear reactions change chemicals into other chemicals.
- Mixture: A mixture, on the other hand, consists of two or more substances that are physically combined but not chemically combined. This means each substance in a mixture retains its own chemical properties. Air is an example of a mixture, which consists of oxygen, nitrogen, carbon dioxide, and other gases mixed together without any specific ratio. A bag of candies is another mixture example.
Mixture vs. Chemical Reaction
Mixtures and chemical reactions often get confused, but they’re fundamentally different:
- When substances form a mixture, they don’t change their identities. For instance, when you mix sugar and water, you can still separate and retrieve the original sugar and water through processes like evaporation.
- In a chemical reaction, substances react to form new substances with different properties. This involves breaking and forming chemical bonds. For example, when hydrogen gas reacts with oxygen gas, it forms water. Once formed, the water does not return to its original gases without another chemical reaction. Baking a cake also involves a chemical reaction. Chemically, the composition of a cake differs from that of its ingredients.
Examples: Chemical or Not?
While all matter consists of chemicals, not everything is a chemical.
- Chemicals: Water, oxygen, glucose, sodium chloride, vinegar, sterling silver, helium, gold.
- Not Chemicals: Light, heat, sound. These are forms of energy, not matter. Thoughts, dreams, and emotions are other examples of things that are not chemicals.
The Role of Chemicals in Everyday Life
Chemicals play a pivotal role in our daily lives:
- Medicine: Many medicines are chemicals formulated to treat specific illnesses. For example, aspirin is a chemical that relieves pain.
- Food: Chemical reactions are at the heart of cooking. For example, baking soda reacts with acids in vinegar or fruit juice to produce carbon dioxide, making cakes and bread rise.
- Digestion: The body uses chemical reactions for turning food into chemical energy and raw resources for growth and repairing tissues.
- Cleaning: Household cleaners contain chemicals that kill bacteria, remove stains, and clean surfaces.
- Environment: Photosynthesis, a natural chemical reaction, is how plants convert carbon dioxide into oxygen, which is essential for life on Earth.
- Energy: Combustion, another chemical reaction, releases energy from fuels. It provides heat, plus the energy aids in electricity production.
Because a chemical has a consistent composition, it has characteristic properties that identify it. Both physical and chemical properties help identify chemicals.
Physical properties are apparent without changing the chemical identity of a substance. Here are some common physical properties:
- Color: The visual appearance of a substance. For example, copper has a reddish-brown color.
- Odor: The smell of a substance. For instance, ammonia has a pungent odor.
- Density: The mass of a substance per unit volume.
- Melting Point: The temperature at which a solid turns into a liquid.
- Boiling Point: The temperature at which a liquid turns into a gas.
- Solubility: The ability of a substance to dissolve in another substance. For example, sugar is soluble in water.
- Malleability: The ability of a substance to be hammered or rolled into sheets.
- Ductility: The ability of a substance to be drawn into wires.
- Conductivity: The ability of a substance to conduct electricity or heat.
- State of Matter: Whether a substance is solid, liquid, or gas at a given temperature and pressure.
Chemical properties describe how a substance interacts with other substances, i.e., its ability to undergo chemical changes. These properties only become apparent during a chemical reaction. Some examples include:
- Reactivity: How readily a substance reacts, often with respect to another substance. For example, sodium reacts violently with water.
- Flammability: The ability of a substance to ignite or burn.
- Oxidation: How a substance reacts with oxygen. Rusting of iron is an example of oxidation.
- Acidity/Basicity (pH): Determines if a substance is acidic, basic, or neutral.
- Toxicity: How harmful a substance is to organisms.
- IUPAC (1997). “Chemical Substance.” Compendium of Chemical Terminology (the “Gold Book”) (2nd ed.). Oxford: Blackwell Scientific Publications. doi:10.1351/goldbook.C01039
- Petrucci, Ralph H.; Herring, F. Geoffrey; Madura, Jeffry D.; Bissonnette, Carey (2011). General Chemistry: Principles and Modern Applications. Pearson Canada. ISBN 9780137032129.