90944 ass-2011

NCEA Level 1 Science (90944) 2011 — page 1 of 6
Assessment Schedule – 2011
Science: Demonstrate understanding of aspects of acids and bases (90944)
Evidence Statement
Question Evidence Achievement Merit Excellence
ONE
(a)
Description of ion
An ion is an atom or group of atoms that has lost or gained
electron(s) and therefore carries a charge / full outer shell /
stable.
Explanation of charges
Sodium has 11 protons and electron arrangement of 2,8,1.
Sodium has one electron in its outer shell, which it loses,
so its outer shell is full (2,8) and it has a charge of +1, as it
still has 11 protons (positive charges) and now has only 10
electrons (negative charges).
Oxygen has 8 protons and electron arrangement of 2,6.
Oxygen has 6 electrons in its outer shell, and so it gains
two electrons, so its outer shell is full (2,8) and so has a
charge –2, as it still has 8 protons (positive charges) and
now has 10 electrons (negative charges).
Evidence may come from diagrams
• Describes an ion correctly.
• Correctly gives the electron
arrangements of sodium and
oxygen ions.
• States that sodium loses one
electron and that oxygen gains
two electrons when forming
ions.
• States charges on sodium and
oxide ions.
• Explains the charge on either
sodium or oxygen in terms of
electron arrangement AND
atomic structure.
• Explains the charge on both
sodium and oxygen in terms of
electron arrangement and atomic
structure.
• Explains that sodium and oxide
ions have the same numbers of
electrons, as Na has lost one,
leaving 10 and oxygen has
gained two, giving 10 and
therefore the same arrangement.
• Explains why the two ions have
the same electron arrangement;
achieve full outer shells by
either gaining two electrons or
losing one electron, and
therefore having two more
electrons than the number of
protons or one less than the
number of protons.
(b) Ratio of X ions to OH–
ions
1:3
Charge of X
Hydroxide ion has a charge of –1. Because there are three
hydroxide ions overall, the total negative charge is –3.
Therefore to form a neutral compound, X must have a
charge of +3, as there is only one X ion in the formula.
Therefore X loses three electrons to have a charge of +3.
How to find group for X
The only element between 11–18 that has three electrons in
its valence shell is aluminium in group 13, OR it must be
in group 13 to have three valence electrons.
What is X?
Aluminium.
• Ratio of 1: 3 for X to (OH–
)
ions.
• The total negative charge in
(OH)3 is –3.
• States that X must have a
positive charge, as hydroxide
ions have a negative charge.
• States X has a charge of +3.
• States X is Aluminium (no
explanation).
• Hydroxide ion has a charge of
–1. Because there are three
hydroxide ions overall, the total
negative charge is –3.
• To form a neutral compound X
must have charge of +3 to
balance the –3 charge, so the
compound is neutral overall.
• Identifies X as aluminium.
AND
• Explains the balance of negative
and positive charges in the ions
in X(OH)3 and how the periodic
table shows that X must be in
group 13 because it is the only
element between 11 and 18 that
has three valence electrons.

Not achieved Achievement Achievement with Merit Achievement with Excellence
Q1 NØ = no evidence
or no relevant
evidence
N1 = one partially
correct answer,
eg one e–
arrangement.
e.g. an ion is an
atom with a charge.
e.g. electron
arrangement of Na
= 2,8,1.
N2 = 1 point
from
Achievement
A3 = 2 points A4 = 3 points M5 = 2 points M6 = 3 points E7 = 1 point E8 = 2 points

TWO
(a)
Fizzing / bubbling occurs, marble chips decrease in size,
heat is produced.
• ONE correct observation.
(b) Why mass decreases
The marble chips decreases as one of the products is CO2
gas. This gas escapes and so the mass of the flask and
contents is reduced.
What’s happening
As the reactant particles collide, they form product
particles. As the reaction proceeds, there are fewer and
fewer reactant particles left to collide and so the rate of
reaction becomes slower.
At the start (section X) of the reaction, more product
particles are being formed, then more gas is being formed,
therefore more gas escapes at first and so the mass of the
flask and contents decreases more rapidly. At X, the rate of
reaction is fast but decreasing with time.
In section Y there are now fewer (less) reactants and so
there are fewer collisions per second (unit time) and so less
product is formed, ie less gas being released, so the mass
does not decrease as rapidly. At Y, the rate of reaction is
slow and slowing.
In section Z the reaction has stopped, as one of the
reactants (marble chips or HCl) has run out, so there are no
particles left to react.
• Mentions CO2 / gas escaping.
• Describes that when there are
more collisions, a faster reaction
occurs.
• Correctly describes the rate of
reaction at either X, Y or Z.
• When there are more reactants
there are more collisions.
• Has the idea that as the reaction
proceeds, CO2 gas is released
and so it escapes, meaning the
overall mass is less.
• Correctly explains the rate at X
(fast), Y (slowing) and Z
(stopped) linked to reactants or
products.
• Fully explains the rate of
reaction at the three stages,
AND has a link between the
amount of particles colliding
and reaction rate, so affecting
the amount of CO2 gas escaping.
(c) When more concentrated acid is used, there are more acid
particles in the same volume of the acid. Because of this,
there are more particles to collide with the calcium
carbonate. Because there are more to collide, the rate of
reaction is faster.
ONE of:
• More concentrated acid has
more acid particles to react.
OR
• When there are more reactants
there are more collisions.
• Explains that there are more
collisions due to more acid
particles being present when the
more concentrated acid is used,
and hence the reaction rate is
faster.
• Explains why the reaction is
faster, by linking the idea that
there are more acid particles in
the same unit volume available
for collisions, and hence there
will be more collisions, causing
the faster reaction rate.

or no relevant
evidence
N1 = partially
correct
statement
N2 = 1 point
from
Achievement
A3 = 2 points A4 = 3 points M5 = 2 points M6 = 3 points E7 = 1 point E8 = 2 points

THREE Type of reaction
Acid-Base reaction or neutralisation.
Word Equation
hydrochloric acid + magnesium oxide → magnesium
chloride + water.
Balanced Equation
2HCl + MgO → MgCl2 + H2O
How to make it
Add magnesium oxide to hydrochloric acid in a beaker.
Heat slightly and pour this into an evaporating dish and
leave somewhere warm (e.g. window sill) for a few days or
heat over Bunsen to speed up the reaction, so that the water
can evaporate, leaving magnesium chloride salt. Safe lab
procedure would include dealing appropriately with
chemicals such as HCl or the wearing of safety glasses.
• Identifies reaction type.
• Correct word equation.
• Correct symbol equation with
one mistake in an ionic formula.
• Describes one stage correctly in
the making of magnesium
chloride.
• Correct symbol equation, but not
balanced.
• Explains the two steps of
reacting and evaporating in the
making of magnesium chloride
either in solution or as a solid
• Correctly balanced symbol
equation.
• Fully discusses making
magnesium chloride salt, either
in solution or as a solid
• Discusses correct equipment or
safe lab procedure for making
MgCl2 from MgO and HCl.
or no relevant
evidence
N1 = partially
correct, eg word
equation
without water
N2 = 1 point
from
Achievement
A3 = 2 points A4 = 3 points M5 = 1 points M6 = 2 points E7 = 2 points E8 = 3 points

FOUR Explanations
When the solution is red, the pH is 1–2. The colour is due
to a high concentration of H+
ions.
When the solution is green, the solution is neutral pH 7 and
the hydroxide ions have reacted with the hydrogen ions,
forming water.
When the solution is purple, the pH is 12–13 and there is a
high concentration of hydroxide ions.
Word equation
Nitric acid + sodium hydroxide → sodium nitrate + water
Balanced equation
HNO3 + NaOH → NaNO3 + H2O
• Link TWO pHs values to colour
(eg, red = acid, purple = base)
• States when colour is green and
the pH is 7 the solution is
neutral.
• States that the acid provides
hydrogen ions.
• States that the base provides
hydroxide ions.
• Correct word equation.
• Explains that H+
and OH–
form
water.
• Explains that before sodium
hydroxide is added, H+
ion
concentration is high and once 20
mL of sodium hydroxide has
been added the concentration of
OH–
ions is high.
• Correct symbol equation (not
balanced).
• Links the colour change to the
pH by relating it to the ions that
are present.
• Shows that when 10mL of
sodium hydroxide has been
added the number of hydrogen
ions are equal to hydroxide ions,
water / neutral solution is
formed.
• Writes a balanced chemical
reaction.
or no relevant
evidence
N1 = 1 partially
correct point
N2 = 1 point
from
Achievement
A3 = 2 points A4 = 3 points M5 = 2 points M6 = 3 points E7 = 2 points E8 = 3 points
Judgement Statement
Not Achieved Achievement
Achievement
with Merit
Achievement
with Excellence
Score range 0 – 9 10 – 18 19 – 24 25 – 32

90944 ass-2011

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90944 ass-2011