Changes in Matter• Physical Changes

– Altering a substance without changing its composition

• Cutting up paper• Breaking a pencil• Crushing rocks• Changing states of matter

» Ice –melting liquid» Water –melting water

Changes in Matter• Chemical Changes

– Process where one or more substance changes into a new substance

• Rusting• Fire

• Reactants- substances that we begin with

• Products-substances that we end with

• Reactant Products

Changes in Matter

• Evidence of chemical reaction:1. Gas is produced2. Color Change3. Change in smell4. Formation of solid—precipitate5. Light is produced6. Temperature change

-exothermic-endothermic

Law of Conservation of Mass

• Mass is neither created nor destroyed

• Mass reactants = Mass products

Reactants Products

H2 + O2 H2O

18 g + 22 g ?g H2O

Steps to solve Law of Conservation of Mass problems

1. Write what you know• chemical reaction• reactants and their masses• products and their masses

2. What are you solving for?

3. Solve for your unknown

• From a laboratory experiment designed to separate water into hydrogen and oxygen gas, a student collected 10.0 g of hydrogen and 79.4 g of oxygen. How much water was initially involved in the process?

Chemical Reaction

H2O H2 + O2

Reactants Products

What do we know?

H2 = 10.0 g

O2 = 79.4 gWhat is our unknown?

H2O = ??? g

Law of Conservation of Mass says…

Mass reactants = Mass of ProductsH2Omass = H2 mass + O2 mass

H2Omass = 10.0 g + 79.4 g

H2O mass = 89.4 g

• A student carefully placed 15.6 g of sodium in a reactor supplied with an excess quantity of chlorine gas. When the reaction was complete, the student obtained 39.7 g of sodium chloride. How many grams of chlorine gas reacted?

Chemical Reaction

Reactants Products

Sodium + Chlorine Sodium Chloride

What do we know? Sodium = 15.6 g Sodium Chloride = 39.7 g

What is our unknown? Chlorine = ??? g

Law of Conservation of Mass says…

Mass reactants = Mass of Products Sodium mass + Chlorine mass = Sodium Chloride mass

15.6 grams + ? grams = 39.7 grams

? grams = 24.1 grams of Chlorine

• In a flask, 10.3 g of aluminum reacted with 100 g of liquid bromine to form aluminum bromide. After the reaction, no aluminum remained, and 8.5 grams of bromine remained unreacted. How many grams of compound were formed?

Chemical Reaction

Reactants Products Aluminum + Bromine Aluminum Bromide

What do we know? Aluminum = 10.3 g Bromine = 100 g Bromine left over = 8.5 g Bromine used = ??

= 100 – 8.5 g = 91.5 g

What is our unknown? Aluminum Bromide = ??? g

Law of Conservation of Mass says…

Mass reactants = Mass of Products

Aluminum used+ Bromine used = Aluminum Bromide made

10.3 grams + 91.5 grams = ??? grams

101. 8 grams = ? Grams of Aluminum Bromide