The physical properties of a substance

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The physical properties of a substance

A physical change is one that affects only the physical properties of a substance, while a chemical change alters a substance at the molecular level.

Physical Properties: A property that can be observed or measured without changing the chemical identity of the substance.

Physical properties can be categorized as either intensive or extensive.


An element or compound’s reactivity is its ability or capacity to react, or bond, with another substance. As you continue through this course, you will observe and compare this particular chemical property in a variety of scenarios.


One type of chemical reaction, called combustion, involves the burning of various compounds and elements. Some substances are more combustible, or able to be burned, than other substances.

Oxidation state:

The difference in the number of electrons associated with an atom as a pure element as compared with that atom in a compound. An element’s electronegativity value and other properties affect its oxidation state in different compounds.


Some compounds decompose into more than one different element or compound as bonds are broken. Hydrogen peroxide (H2O2), a chemical you may have around your house, is stored in a dark container to slow down decomposition, which can be caused by heat and light.

clues that can indicate that a chemical change has occurred include: production of flames, color change, bubbling or fizzing, smoke, production of heat or light, and formation of a substance in a different state (for example, a solid from the mixing of two liquids)

Chemical equations: a representation, using formulas and symbols, of a chemical reaction. Reactants: The substances that are present at the beginning and undergo a chemical reaction. Products: The new substances that are formed or produced by a chemical reaction. (reactantsproducts)

-(aq) for aqueous solution

Law of Conservation of Mass: Matter is neither created nor destroyed during an ordinary physical change or chemical reaction.

–To balance a chemical equation, we use whole numbers, called coefficients, written in front of the reactants and products. A coefficient in front of a formula indicates the number of molecules or formula units that take part in the chemical reaction. The coefficients can also represent the mole ratio in which the compounds combine and form. Double Replacement Reaction: A type of reaction in which the ions of two compounds exchange places in an aqueous solution to form two new compounds.

–Double replacement reactions usually occur in aqueous solutions. Remember that aqueous means dissolved in water and is represented by the subscript (aq) in the chemical formula. Because the reactants are dissolved in water, they are able to move and collide together to form new products.

–In a single-replacement reaction, one element replaces a similar element in a compound. Most single replacement reactions occur in aqueous solution. The general forms for a single replacement reaction are shown below. A + BY → AY + B

Z + AX → AZ + X

Synthesis reactions, sometimes called composition reactions, involve two or more reactants combining to form one new substance. This means that in a synthesis equation, you will see two or more reactants on the left side of the equation, and only one product formed on the right side of the equation. A general equation for synthesis reactions can be represented by the following: A + B → AB

In this equation, A and B can represent elements or compounds, and AB represents a compound made of two or more elements.

Types of synthesis reactions: Metal with a nonmetal, Two nonmetals, Metal oxide with water, Nonmetal oxide with water

A decomposition reaction is the opposite of a synthesis reaction; it starts with only one reactant but ends with more than one product. Most decomposition reactions require energy, usually in the form of heat or electricity, to be added in order for the reaction to occur. In a decomposition reaction, a single compound undergoes a reaction that produces more than one product. The following general equation illustrates decomposition reactions: AB → A + B

AB is a compound made of two or more elements. A and B are individual elements or compounds.

Types of Decomposition Reactions: Binary compound, metal carbonate, metal chlorate, metal hydroxide, oxyacids In a combustion reaction, a compound or element reacts with oxygen, releasing a large amount of energy in the form of light and heat. –they are exothermic, which means that energy is released by the reaction.

Many combustion reactions involve the burning of organic compounds called hydrocarbons, compounds containing the elements carbon and hydrogen, or other organic compounds. When a hydrocarbon is burned completely, the products of the combustion reaction are usually carbon dioxide and water. Natural gas, propane, and wood all burn to produce carbon dioxide and water as the products.

Not all combustion reactions involve organic compounds made of carbon and hydrogen (nonorganic compounds), which means that the products are not always carbon dioxide and water. Other substances, both compounds and elements, can burn in oxygen. These reactions can often be categorized as synthesis reactions, but they are considered combustion reactions as well because a substance is reacting with oxygen and releasing energy.

Stoichiometry is the use of dimensional analysis to calculate relationships between the amounts of reactants and products in chemical reactions. (stoichion, which means element, and metron, which means measure.) Limiting Reactant: The reactant that controls the amount of product able to be produced by a chemical reaction because it is used up completely. Excess reactant: the reactant that is not used up completely in a chemical reaction. Theoretical Yield: The maximum amount of product that can be produced by a given amount of reactant(s).

The measured amount of a product actually obtained from a reaction is called the actual yield.

Percent Yield: The ratio of the actual yield to the theoretical yield, multiplied by 100 to make it a percentage.

percent yield = × 100%


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