Physical & Chemical Changes Standards 5.a.b.c.d.e.

Topic 1: Changes in Substances Standards 5.a.d.

Substances Change !   Substances do not always stay the same, sometimes they

change.

!   When ice melts, the solid water becomes a liquid. When iron rusts, the metal becomes a powdery read compound.

!   Substances can change by either a chemical change or a physical change.

Physical Changes

!   A physical change is any change in size, shape, or state of matter in which the identity of the substance remains the same.

!   For example, liquid water can change into a solid (ice). The substance did not change, only the movement of it’s molecules changed.

Chemical Changes

!   A chemical change is the change of one or more substances into other substances. It is also called a chemical reaction.

!   In a chemical change, atoms rearrange and make one or more new substances. They do this by breaking bonds and forming new ones.

!   The new substances have different properties than their original form.

!   When hydrogen and oxygen molecules bond together they form the new substance water.

Think-Check

!   Liquid water changing into a gas is a physical change. Why is this?

A.  It created new substances.

B.  The particles changed their arrangements.

C.  The temperature was increased.

!   What makes a physical change different from a chemical change?

A.  It changes material without changing its substance.

B.  It changes a substance into other substances.

C.  It changes the temperature of the substances.

Topic 2: Chemical Reactions Standards 5.a.b.

Chemical Equations

!   A chemical equation is a convenient way to describe what happens in a chemical reaction.

!   The reactants are the starting substances in a chemical reaction, which are written on the left side of the arrow.

!   The products are the new substances formed in a chemical reaction, which are written on the right side of the arrow.

Conservation of Matter

!   The conservation of matter states that the total number of atoms before a chemical reaction is equal to the total number of atoms after the reaction.

Balancing Chemical Equations

!   The number of atoms in the reactants must equal the number of atoms in the products. This is due to the simple fact that matter can not be created or destroyed.

!   When the number of reactant atoms equals the number of product atoms, then the chemical equation is balanced.

!   Simply, in a chemical reaction the atoms are rearranged making new substances.

Think-Check

!   What substance is a reactant?

A.  CO2

B.  H2O

C.  CH4

!   Which equation is balanced?

A.  Reaction A

B.  Reaction B

C.  Reaction C

Reaction Chemical Equation

A CH4 + O2 à CO2 + H2O

B CH4 + 2O2 à CO2 + 2H2O

C CH4 + O2 à CO2 + 2H2O

Topic 3: Energy & Change Standards 5.c.

Energy & Bonds

!   Energy is involved in chemical reactions.

!   When bonds are made or broken, then energy can be either released or absorbed as heat.

Exothermic Process

!   An exothermic chemical reaction is when heat energy is released after all bonds have been broken and made.

!   For example, when wood is burned it releases a lot of heat energy.

Endothermic Process

!   An endothermic chemical reaction is when heat energy is absorbed after all bonds have been broken and made.

!   For example, the use of a chemical ice pack absorbs heat energy.

Physical Change & Energy

!   Energy is not only part of chemical changes, it is also part of physical changes. Energy can be added or taken away to cause a change in the state of matter.

! Heating a substance can cause it to change from a solid, into a liquid, and then into a gas.

! Cooling a substance can cause it to change from a gas, into a liquid, and then into a solid.

Physical Change & Energy !   The temperature at which a solid becomes a liquid is the melting

point.

!   The temperature at which a liquid becomes a solid is the freezing point.

!   The temperature at which a liquid becomes a gas is the boiling point.

!   Increasing and decreasing the temperature will cause a physical change in the states of matter.

Think-Check

!   What kind of energy is released or absorbed in most chemical reactions?

A.  Heat energy

B.  Light energy

C.  Electrical energy

!   What type of reaction releases heat?

A.  Endothermic

B.  Exothermic

C.  Expothermic

Topic 4: Acids, Bases, and Neutral Substances Standards 5.e.

Liquid Substances

!   Almost every liquid substance is an acid or base.

!   By analyzing chemical reactions of substances it can be determined whether a substance is an acid or a base.

!   Typically in a chemical reaction, an acid will donate H+ ions and a base will donate OH- ions.

HCl (acid) + NaOH (base) à H2O + NaCl

pH

!   Substances can be acidic, basic, or neutral.

!   A pH scale is used to determine whether a substance is acidic, basic, or neutral.

Acids

!   An acid is a substance that produces positively charged hydrogen ions (H+) when it reacts with water.

!   An acid has a pH below 7.

Bases

!   A base is a substance that produces hydroxide ions (OH−) when reacting with water.

!   A base has a pH over 7.

Neutral

!   A neutral substance produces an equal number of hydrogen and hydroxide ions when reacting with water.

!   A neutral substance has a pH of 7.

Think-Check

!   A substance has a high concentration of hydrogen ions (H+). What kind of pH will this substance have?

A.  9.2

B.  7.1

C.  4.5

!   Human blood has been found to have a pH of 7.4. What can be concluded from this?

A.  The blood is slightly basic

B.  The blood is very basic

C.  The blood is very acidic