1. Physical & Chemical Changes
Density, GRASP, Physical and Chemical Changes

2. Recall
Mixtures can be separated into pure substances whose properties are uniform and consistent through out.
Pure substances can be combined in certain ways to produce new substances that also are uniform and consistent in their properties.
These new substances also must be pure substances!
Since they are made by compounding other pure substances together, they are called compounds
The processes that we use to make these new substances are called chemical changes.

3. Two Types of Pure Substances
Elements
One type of atom
Type of atom determines its properties entirely.
Compounds
One set ratio of specific types of atoms.
Type of atoms and their ratio determines its properties entirely.
1 Sodium +
1 Chlorine
1 Sodium Chloride

4. Law of Conservation of Mass
In chemical changes, the reacting particles (atoms) are not destroyed, because we can get them back in subsequent chemical changes.

5. How do we know that a new substance is created?
How do we identify substances in the first place?
Substances can be identified by observing physical properties. Examples:
One of the best of these for identifying substances is density.
This property is determined by the mass of the particles and how closely they are together.

6. Density? How can we calculated density? What will its units be?
Mass x volume?
Mass + volume?
Mass – volume?
Mass ÷ volume?
What will its units be?

7. Example Question 1.
A block of aluminum occupies a volume of mL and weighs 40.5 g. What is its density?

8. GRASP Method
GRASP (or GRASS) is THE method you will use to solve all numerical calculations in this class (and probably all others)
Givens
Write down the numbers given and assign appropriate variable symbols to them.
Use units to help you.
Required
Write down what you are being asked to find and assign appropriate variable symbol.
Analysis
Chose appropriate equation(s)
Use algebra to obtain in proper form
Required is isolated on one side, givens are all on the other.
Solution
Insert numbers appropriately and use BEDMAS and a calculator to get answer showing needed calculation steps.
Numbers should have appropriate units attached.
Paraphrase
State what you have found in a brief and correct sentence.

9. Example Solution
A block of aluminum occupies a volume of 15.0 mL and weighs 40.5 g. What is its density?
Givens: V = 15.0 mL, M = 40.0 g
Required: D
Analysis: D = M/V
Solution: D = 40.0 g/ 15.0 mL
D = 2.67 g/mL
Paraphrase: The block of aluminum has a density of 26.7 g/mL.

10. Example Question 2
What is the weight of the ethanol that exactly fills a mL container if the density of ethanol is g/mL?

11. Example Solution
What is the weight of the ethanol that exactly fills a mL container if the density of ethanol is g/mL?
Givens: V = mL, D = g/mL
Required: M
Analysis: D = M/V
M = D x V
Solution: M = g/mL x mL
M = g
Paraphrase: The mass of the ethanol to fill the container is g.

12. Physical Changes:
These are processes that combine substances into mixtures, separate mixtures, and change the object properties of substances.
Physical changes cause different types of particles to mix or separate.
Changes do not alter the particles making up the substance
In a physical change, no new substances are made.
Examples: mix sand and water, chop potatoes, break a coffee mug,evaporate salt water to make pure salt and pure water, freeze water (any change in state).

13. Chemical Changes:
These are processes that change pure substances into other pure substances. (chemical reactions)
A substance that was not present in the sample before the process starts must be present at the end of the process.
The particles of the substance are themselves changed.
Chemical changes cause particles to join or separate to make new types of particles.
A new type of particle means a new substance.
Examples: cooking food, digesting food, burning anything.

14. How do you tell if a process is a chemical change or a physical change?
Identify a new substance.
This can be time consuming, but it is the only certain way.
Look for new states.
Look for new chemical properties.
Types of chemical reactions that the substance will take part in
Reacts with oxygen (combustion)
Reacts with water
Reacts with acids or bases

15. A Chemical Reaction May Have Occurred IF…
There are some clues that suggest that a chemical change has occurred:
i) large amounts of energy are released or absorbed (sample gets hot or cold) example: chlorine and sodium
ii) a permanent colour change occurs example: Silver Nitrate and Sodium Hydroxide
iii) a new odour is produced example: ammonium bicarbonate and vinegar iv) a gas is created (bubbles or fizzing) example: Baking soda and vinegar v) a solid precipitate is made (when solutions are mixed) example: Sodium Iodide and Mercury Chloride
vi) the process is difficult to reverse. example: Burn paper

16. Exceptions
None of these clues are fool-proof. Write down one exception for each of the clues.
i) water freezes
ii) paint dries
iii) melt plastic and you start to smell it
iv) water boils (it is still water, just in the gas state)
v) salt precipitates when salt water evaporates
vi) a coffee mug breaks