CHEMISTRY 11
Chapter 1
Observing Matter
Solutions for Practice Problems
Student Textbook page 18
1. Problem
Write the following quantities in your notebook. Beside each quantity, record the
number of significant digits.
(a) 24.7 kg
(b) 244.7 mL
(c) 247.701 mg
(d) 0.247 01 L
(e) 8.930 × 105 km (f) 2.5 g
(g) 0.0003 mL
(h) 923.2 g
What Is Required?
You need to give the number of significant digits in each measure listed.
What Is Given?
The exact measure is given.
Plan Your Strategy
Follow the Rules for Determining Significant Digits, listed in Table 1.4, Student
Textbook page 17. Briefly:
Rule 1 All non-zero numbers are significant
Rule 2 Zeros between non-zero numbers are significant
Rule 3 Zeros to the left of non-zero numbers are not significant
Rule 4 Zeros to the right of non-zero numbers are significant if they have not been
made to a 10th power exponent.
Act on Your Strategy
(a) 3
(b) 4
(c) 6
(d) 5
(e) 4 (note how the last zero was not included to the 10th power exponent of 5, so it
is significant)
(f) 2
(g) 3 (note how all the zeros before 3 are not significant)
(h) 4
Check Your Solution
Reexamine the numbers again and make sure they follow the rules of Table 1.4, Rules
for Determining Significant Digits, in the student textbook.
2. Problem
Consider the quantity 2400 g.
(a) Assume that you measured this quantity. How many significant digits does it
have?
(b) Now assume you have no knowledge of how it was obtained. How many signifi-
cant digits does it have?
What Is Required?
You have to give the number of significant digits based on whether you had made the
measurement yourself or not.
Chapter 1 Observing Matter • MHR
1
CHEMISTRY 11
What Is Given?
The measure is given.
Plan Your Strategy
The key to answering this question is your level of certainty of the measurement. If
you had taken the measurement yourself, you know for certain that this is the exact
measurement, and that all the zeros after 4 are digits that have shown up on your
measuring scale. The zeros are therefore all significant.
However, if you did not measure the quantity yourself, you cannot be sure that
the zeros after 4 did indeed show up on the scale, so these zeros create a level of
uncertainty. The measure can then have up to three sets of significant digits,
depending on where the level of uncertainty ends.
Act on Your Strategy
(a) All four numbers are significant.
(b) The zero numbers could have been represented as 2400 g (also 2.400 × 103)
or 2.400 × 103 g or 2.400 × 103 g, giving them 4, 3, or 2 significant digits,
respectively.
Check Your Solution
Reexamine the answers again and make sure they follow the rules of Table 1.4, Rules
for Determining Significant Digits, in the student textbook.
Solutions for Practice Problems
Student Textbook page 22
3. Problem
Do the following calculations Express each answer using the correct number of
significant digits.
(a) 55.671 g + 45.78 g
(b) 1.9 mm + 0.62 mm
(c) 87.9478 L − 86.25 L
(d) 0.350 mL + 1.70 mL + 1.019 mL
(e) 5.841 g × 6.03 g
(f) 0.6 kg ÷ 15 L
(g) 17.51 g ÷ 2.2 cm3
What Is Required?
You calculate the problems listed, giving the correct significant digits in the answer.
What Is Given?
A specific math calculation is given.
Plan Your Strategy
Apply the Rules of Reporting Significant Digits in Calculations, given in Table 1.5,
student textbook page 20. Briefly:
Rule 1 When multiplying or dividing, the answer has the same number of
significant digits as the value with the lowest number of significant digits
in the calculation.
Rule 2 When adding or subtracting, the answer has the same number of decimal
places as the number with the fewest decimal places in the calculation.
Rule 3 If the least significant digit is greater than 5, drop the 5 and increase the preceding digit by 1. If the least significant digit is less than 5, then leave the
numbers untouched. If the least significant digit is equal to 5, drop the 5 and
only increase the preceding digit by 1 if it was an odd number.
Act on Your Strategy
(a) 55.671 g + 45.78 g = 101.45 g (rounded to 2 decimal places)
(b) 1.9 mm + 0.62 mm = 2.5 mm (rounded to 1 decimal place)
Chapter 1 Observing Matter • MHR
2
CHEMISTRY 11
(c) 87.9478 L − 86.25 L = 1.70 L (rounded to 2 decimal places)
(d) 0.350 mL + 1.70 mL + 1.019 mL = 3.07 mL (rounded to 2 decimal places)
(e) 5.841 × 6.03 g = 35.2 g (rounded to 3 significant digits)
0.6 kg
= 0.04
15 L
17.51 g
= 8.0
(g)
2.2 cm3
(f)
kg/L (rounded to 1 significant digit)
g/cm3 (rounded to 2 significant digits)
Check Your Solution
Check all the units. None of them should cancel out.
Check that your answer follows the Rules of Reporting Significant Digits in
Calculations in the Student textbook.
Solutions for Practice Problems
Student Textbook page 26
4. Problem
Classify each situation as either a physical change or a chemical change. Explain
your reasoning.
(a) A rose bush grows from a seed that you have planted and nourished.
(b) A green coating forms on a copper statue when the statue is exposed to air.
(c) Your sweat evaporates to help balance your body temperature.
(d) Frost forms on the inside of a freezer.
(e) Salt is added to clear chicken broth.
(f) Your body breaks down the food you eat to provide energy for your body’s cells.
(g) Juice crystals dissolve in water.
(h) An ice-cream cone melts on a hot day.
What Is Required?
You have to indicate whether the phenomena listed are physical or chemical changes.
What Is Given?
A particular phenomenon is given in each case.
Plan Your Strategy
A physical change is a change that only affects the substance’s appearance, but not its
chemical composition. A chemical change will affect both the appearance and chemical composition of the substance.
Act on Your Strategy
(a) “A rose bush grows from a seed that you have planted and nourished.”
This is a chemical change as new products are being made and it cannot be
reversed.
(b) “A copper statue forms a green coating when exposed to air.”
This is a chemical change which is the result of the copper oxidizing slowly in air
in the presence of CO2 to produce Cu2(OH)2CO3 , which is a green coating. The
outer surface of the Statue of Liberty is made of copper, and this reacted compound is what gives it its green colour.
(c) “Your sweat evaporates to help keep your body temperature within a normal
range.”
Sweat evaporating is a physical change. The sweat is just changing state from liquid to gas.
(d) “Frost forms on the inside of a freezer.”
Frost forming in the freezer is the result of water vapour in the air subliming
(turning from gas directly to solid) to form ice on the walls of the freezer.
Sublimation is a physical change.
Chapter 1 Observing Matter • MHR
3
CHEMISTRY 11
(e) “You add salt to clear chicken broth.”
The added salt will dissolve which is a physical change.
(f) “Your body breaks down the food you eat to provide energy for your body’s cells.”
This is definitely a chemical change as the broken down food cannot be reverted
to its original form in the body. Chemical breakdown of food is the main power
source for the human body.
(g) “Juice crystals dissolve in water. ”
Juice crystals will likely contain mostly sugar molecules and vitamin C, both of
which do not undergo any specific chemical reactions with water. Just in the same
way as salt dissolves in clear broth, the dissolution of juice crystals in water is a
physical change.
(h) “An ice-cream cone melts on a hot day. ”
Melting, like sublimation and evaporation, is a physical change.
Check Your Solution
Check that your answers meet with the definitions of physical and chemical changes.
Other reference books and Internet sources may reveal the results too.
Chapter 1 Observing Matter • MHR
4