FOUNDATIONS OF CHEMISTRY CHAPTER 7.4 page 254 CHEMICAL PROPERTIES AND CHANGES marshall-arts

Questions: What is a chemical property? What are some signs of chemical change? What are chemical equations useful? What are some factors that affect the rate of chemical reactions?

Big Ideas Chemical Properties Signs of Chemical Change A chemical property, such as the ability to burn, is the capacity of a substance to undergo certain types of reactions. Chemical properties include the ability to burn, rot, and rust. For example, an iron post will rust over time as it reacts with moisture and oxygen in the air. Chemical changes occur all around us, and it is often through sensory information that we are able to detect those changes. Signs such a changes in color, odor, or energy indicate that a chemical reaction occurred. Formation of a precipitate is also evidence of chemical change. The yellow crystals that form when potassium iodide, a colorless liquid, is added to lead nitrate, another colorless liquid, is an example. Not all chemical changes are directly observable by human sensory systems. If the physical properties of the reactants and products are silimar, other means are often required to determine whether a chemical change occurred.

Big Ideas Chemical Reaction Rates A chemical reaction occurs when two molecules collide with enough energy to break bonds and make new ones. Certain factors, including concentration, temperature, and surface area, slow down or speed up the rate of a chemical change.

Chemical Properties Chemical property is a characteristic of matter that can be observed as it changes to a different type of matter. It has a new identity.

Chemical Properties Example : For example what are some chemical properties of a piece of paper? Can you tell just by looking at it that is will burn easily? The only way to know that paper burns is to bring a flame near the paper and watch it burn. When paper burns it turns to a different type of matter. The ability of a substance to burn is called chemical property. The ability to rust is another chemical property.

Comparing Properties Comparing properties is the comparing of physical and chemical properties. All objects have physical and chemical properties. For example, a piece of wood has the ability to burn, this is a chemical property. A piece of wood is round, heavy, rough and solid. These are the physical properties.

Comparing Properties l Sour taste- physical Flammability- chemical The color red- physical

Chemical Changes Chemical change- is a change in matter in which the substances that make up the matter change into other substances with new physical and chemical properties. Ex: When iron undergoes a chemical change with oxygen, rust forms. When substances that undergo a change no longer have the same identity.

Signs of Chemical Change Signs of chemical change include formations of bubbles or a change in odor, color, or energy. Some of these signs don’t always mean a chemical change occured. Think about when you heat water on a stove. Bubbles form as the water boils. Bubble show the water is changing state, which is a physical change.

Explaining Chemical Reactions Recall that particles in matter are in constant motion. As the particles are moving, they collide with enough force, the bonded atoms that make up all the particles can break apart. The atoms then rearrange and bond with other atoms. When new atoms bond together in new combinations, new substances form. This process is called chemical reaction.

Explaining Chemical Reactions Reactants are the substances that are present before a chemical reaction. Products are any new substances formed during a chemical reaction. The arrow is read as “yields.” It separates the reactants and the products and indicates that a reaction has taken place. A plus sign separates two reactants or products.

Balancing Chemical Equations When balancing an equation, you can not change the chemical formula of any reactants or products. If you change the formula you change the identity of the substance. Coefficients change the amount of reactants and products there.

Using Chemical Formulas Writing a chemical equation is a useful way to understand what happens during a chemical reaction. It shows the chemical formula of each substance in a reaction. The formula Fe + S is reactants The formula FeS is Product Reactant- A substance that takes part in and undergoes change during a reaction. Product- Are any new substances formed during a chemical reaction.

Rate of Reactions A higher temperature usually increases the rate of reaction. Concentration is the amount of substance in certain volume. A reaction occurs faster if the concentration of at least one reactant increases. Surface area also affects reaction rate if at least one reactant is a solid.

Factors that affect the Rate of Chemical Reactions

In summary:

Physical vs. Chemical Properties Physical vs. Chemical Changes P - is observed with the senses P - identity does not change (ex. color, shape, mass, length, density, melting/boiling point, odor) C - how a substance reacts with something else. C - original substance looks like a different substance. (observation) (ex. Ability to rust, combust, burn) P - original substance still exists; it only changes form or state of matter. (ex. Cut paper, crunch a chip, paint a boat, melt ice) C - a new substance is produced. Energy changes always accompany chemical changes. (ex. Burn a match, digest pizza, rust a can)

Chemical or Physical Change?

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