Physical & Chemical Changes

Density, GRASP, Physical and Chemical Changes

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.

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+

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.

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.

Density?

• How can we calculated density?• Mass x volume?• Mass + volume?• Mass – volume?• Mass ÷ volume?

• What will its units be?

Example Question 1.

• A block of aluminum occupies a volume of 15.0 mL and weighs 40.5 g. What is its density?

GRASP MethodGRASP (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. • Use units to help you.

• 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.

Example SolutionA 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 mLD = 2.67 g/mL

• Paraphrase: The block of aluminum has a density of 26.7 g/mL.

Example Question 2

• What is the weight of the ethanol that exactly fills a 500.0 mL container if the density of ethanol is 0.789 g/mL?

Example SolutionWhat is the weight of the ethanol that exactly fills a 500.0 mL container

if the density of ethanol is 0.789 g/mL?

• Givens: V = 500.0 mL, D = 0.789 g/mL

• Required: M

• Analysis: D = M/VM = D x V

• Solution: M = 0.789 g/mL x 500.0 mLM = 394.5 g

• Paraphrase: The mass of the ethanol to fill the container is 394.5 g.

Physical Changes:

• These are processes that combine substances into mixtures, separate mixtures, and change the objectproperties 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).

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 ofthe 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.

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

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 occursexample: Silver Nitrate and Sodium Hydroxide

iii) a new odour is producedexample: ammonium bicarbonate and vinegariv) a gas is created (bubbles or fizzing)example: Baking soda and vinegarv) 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

Exceptions

• None of these clues are fool-proof. Write down one exception for each of the clues.i) water freezes ii) paint driesiii) 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