In the previous lesson, we learned that identical substances have identical properties. The subject of chemistry concerns itself with the properties of those substances, and what occurs when they change. Let’s begin by examining the types of properties that exist.

There are two types of properties. The first type are physical properties. Physical properties are properties of a substance that can be measured or observed without changing the substance. So, that is to say, without changing what element or compound the substance is. By contrast, chemical properties are the properties of a substance that allow it to undergo chemical reactions, forming new substances. Let’s look more in depth at each of these properties, starting with physical properties.

Physical properties can be observed without changing the substance, which make these types of properties very useful for identifying substances. These are properties such as color, or density, or melting point. You can find the melting point of something without chemically changing what that something is. You can observe the color of an object without changing the object itself.

So, while physical properties can be observed without changing the substance, chemical properties you do have to chemically change the substance to measure chemical properties. These properties tell us how a substance will behave in a given chemical reaction. So some examples of these are whether a substance will rust, or whether it will burn, or react with acid. The only way to find out definitively if a substance will rust is to cause that substance to rust, and by doing that you have chemically changed the substance itself. You’ve turned metal into rust, or paper into ash. So both physical and chemical properties of a substance can change as that substance changes. Ad just like there are physical and chemical properties, there are physical and chemical changes that can occur. And again, let’s look at physical changes first.

Physical changes alter a material without changing its chemical composition. That is to say, after physical change, the substance is still the exact same. So some examples of this would be melting, or cutting, or bending, or pulverizing, which is turning something into a powder. If you melt ice into water, it’s still chemically the exact same. If you cut a piece of paper in half, after the cut it is still chemically identical to what it was before.

By contrast, chemical changes alter the chemical composition of the substance itself, creating a new substance. So during a chemical change, the elements and compounds are rearranged during a process called a chemical reaction. So some examples of this would be an object burning, or rusting, or rotting. During each of these, a new substance was created, something that wasn’t there before. And all this occurs during a process called a chemical reaction.

Chemical reactions are process by which one or more substance are changed into at least one new substance. One well-known reaction is hydrogen reacting with oxygen to form water. And this reaction occurs according this chemical equation here: hydrogen combines with oxygen to create H2O, water. We’ll learn how to create those types of chemical equations later on in this course. But we can read this equation to mean “these substances here combine and produce this substance here.” You can think of it as starting substance react to form the substances formed, and the terms we use for that are reactants and products. So reactants react to form products.

But during a chemical reaction, an important law must be obeyed, and that’s the law of conservation of mass. That tells us that during a reaction, mass cannot be created or destroyed. That is to say that in any chemical reaction, the mass of the reactants is equal to the mass of the products.