Unit notes and practice questions
Everything here is for unit 5 of the Grade 9 Chemistry textbook: what the chapter covers, the short notes, practice questions with their answers, and the lesson video where one exists.
The cards filed under this chapter on the Grade 9 Chemistry page. Open one at a time.
When you use it
Use when a bonded electron pair is donated entirely by one atom rather than shared mutually.
Watch out
Do not assume both bonded atoms must each supply an electron. In a dative bond, a donor with a lone pair supplies both electrons to an acceptor atom with an empty valence shell.
Drafted from Grade 9 Chemistry, page 143 onwards, then checked twice before it went up
| Electronegativity Difference | Bond Type | Example |
|---|---|---|
| Nonpolar covalent | , | |
| Polar covalent | , | |
| \ge 2.0 | Ionic | , |
When you use it
Use this guideline to classify a bond as nonpolar covalent, polar covalent, or ionic from the electronegativity values of bonded atoms.
Drafted from Grade 9 Chemistry, page 143 onwards, then checked twice before it went up
| Property | Ionic Compounds | Explanation |
|---|---|---|
| Physical State | Hard, brittle crystalline solids | Strong electrostatic attraction; repulsive forces shatter lattice when struck. |
| Melting/Boiling Points | Very high (e.g. NaCl melts at ) | Requires large thermal energy to break strong electrostatic bonds. |
| Electrical Conductivity | Conducts in molten state and solution only | Ions are locked in solids but become free-moving mobile charges when melted or dissolved. |
| Solubility | Soluble in polar solvents | Polar solvent molecules interact with charged cations and anions. |
When you use it
Use this summary to explain physical characteristics and conductivity behaviour of ionic salts.
Drafted from Grade 9 Chemistry, page 143 onwards, then checked twice before it went up
When you use it
Use when determining whether a molecule containing polar bonds is overall polar or nonpolar.
Watch out
Do not assume a molecule is polar just because it contains polar bonds. Symmetrical geometries such as linear in CO2 or tetrahedral in CCl4 cause opposing bond dipoles to cancel out, resulting in a nonpolar molecule.
Drafted from Grade 9 Chemistry, page 143 onwards, then checked twice before it went up
| Property | What it is | What to remember |
|---|---|---|
| Shape and volume | A liquid keeps its own volume but takes the shape of whatever container holds it. | Its particles are close together and are still free to slide past one another. |
| Viscosity and fluidity | Viscosity is a liquid's resistance to flowing and fluidity is how easily it flows. One is the reciprocal of the other, so no liquid is high in both. | Viscosity comes from the attraction between the particles and from long tangled molecules that cannot slide easily. Water is low, honey is high. |
| Viscosity and temperature | A liquid's viscosity falls as the liquid gets hotter, so it is not a fixed number for a substance. | Warmed honey pours like syrup and engine oil is thick on a cold morning. Gases do the opposite: their viscosity rises with temperature. |
| Surface tension | A molecule inside the liquid is pulled in every direction by its neighbours and the pulls cancel. One at the surface has no neighbours above it, so it feels a net inward pull and the surface shrinks to the smallest area it can. | Water is about 0.072 newtons per metre at 20 degrees Celsius, thanks to hydrogen bonding. Ethanol, whose molecules attract each other more weakly, is about 0.022. |
| The paper clip test | Steel is about eight times denser than water, so the clip has to sink once the surface gives way. | It stays up because the unbroken film bends under it and pushes back. One drop of detergent weakens the film and the clip goes straight down. |
| Diffusion | Liquid particles spread into one another, but far more slowly than gas particles do. | Each particle is hemmed in by its neighbours and can only take short crooked steps between collisions. |
| Density | A liquid is about a thousand times denser than a gas at ordinary pressure, because the particles are touching instead of far apart. | Compare a solid with a liquid only when the same substance is named. Ice at 0.92 against liquid water at 1.00 grams per cubic centimetre is the exception to know, and it is why ice floats. |
| Compressibility | Liquids are almost incompressible. | There is almost no empty space left between the particles. Water shrinks by well under one percent even at a hundred times atmospheric pressure. |
When you use it
Reach for this when a question asks why honey pours slowly, why a paper clip rests on water without sinking, or why a liquid keeps its volume but takes the shape of its container. Every answer here comes back to how strongly the particles pull on each other.
Watch out
Students read the honey and water picture as a picture about density and write that honey flows slowly because it is heavier. Density and viscosity are separate properties, and one does not follow the other. Mercury is about 13.5 times denser than water and still pours almost as freely, roughly 1.5 times water's viscosity, while honey is only about 1.4 times denser than water and thousands of times more viscous. The question to ask is how strongly the particles grip each other as they slide. How heavy the liquid is does not answer it.
| Ion | Formula | Where you meet it |
|---|---|---|
| Hydrogen | every acid in water | |
| Sodium | table salt, caustic soda | |
| Potassium | fertiliser, potash | |
| Silver | photographic salts | |
| Ammonium | fertiliser, the one polyatomic cation | |
| Magnesium | hard water, chlorophyll | |
| Calcium | limestone, bone, cement | |
| Barium | the sulfate used in X-ray meals | |
| Zinc | galvanised sheet, dry cells | |
| Iron(II) | the pale green solution | |
| Copper(II) | the blue solution | |
| Lead(II) | car batteries, old paint | |
| Aluminium | alum, water treatment | |
| Iron(III) | rust, the yellow brown solution |
When you use it
Use whenever you have to write a formula from a name. Take the charge of the metal from this table and the charge of the non-metal from its own, then swap the numbers so the total charge comes to zero.
Watch out
Iron and copper each form two ions, so the name must carry a Roman numeral: iron(II) is 2+ and iron(III) is 3+, and writing plain iron leaves the formula undecidable. Ammonium is the only positive ion here made of several atoms, and it behaves as one unit, so two of them are written as two brackets rather than eight hydrogens.
| Ion | Formula | Where you meet it |
|---|---|---|
| Fluoride | toothpaste, drinking water | |
| Chloride | table salt, hydrochloric acid | |
| Bromide | sea water, photographic film | |
| Iodide | iodised salt, thyroid medicine | |
| Hydroxide | every alkali in water | |
| Nitrate | fertiliser, all of them soluble | |
| Nitrite | meat curing salts | |
| Hydrogencarbonate | baking soda, blood buffering | |
| Permanganate | the deep purple oxidising agent |
When you use it
Use for the right hand half of a formula. An ion of one minus needs one positive charge to balance it, so it pairs one to one with sodium and two to one with calcium.
Watch out
Nitrate and nitrite differ by a single oxygen and by one letter, and the ate ending is always the one with more oxygen. The same rule runs through sulfate and sulfite, and reading the wrong one changes the compound rather than just its name. Hydrogencarbonate is still written bicarbonate on shop packets, and both mean this ion.
| Ion | Formula | Where you meet it |
|---|---|---|
| Oxide | rust, quicklime, most minerals | |
| Sulfide | ores, the rotten egg smell | |
| Sulfate | gypsum, copper sulfate | |
| Sulfite | food preservative | |
| Carbonate | limestone, washing soda | |
| Chromate | the yellow solution | |
| Dichromate | the orange oxidising agent | |
| Nitride | hard ceramic coatings | |
| Phosphate | fertiliser, bone, DNA |
When you use it
Use when the formula needs brackets. An ion of this size takes two or three positive charges to balance, which is where subscripts outside a bracket start appearing.
Watch out
Brackets go round a polyatomic ion before the subscript, so calcium phosphate is written with the phosphate bracketed and a two outside it. Writing the subscript inside changes which atoms are multiplied and gives a different substance. Chromate and dichromate both carry two minus even though one holds twice the chromium, which is worth checking rather than guessing from the size.
10 multiple-choice questions written against this chapter, each with its answer and the reason. Try one before you open the answer.
Question 1
According to the octet rule proposed by Gilbert Lewis, atoms tend to form compounds in order to achieve how many valence electrons?
Answer: D Eight valence electrons
Why
The octet rule states that atoms tend to form compounds in ways that give them eight valence electrons, matching the stable electron configuration of a noble gas.
Question 2
What is an anion?
Answer: B An atom or group of atoms with more electrons than protons, giving a net negative charge
Why
Anions are atoms or groups of atoms that have more electrons than protons, which gives them a net negative electrical charge.
Question 3
What type of covalent bond is formed between the two nitrogen atoms in a nitrogen molecule (N2)?
Answer: C Triple bond
Why
Nitrogen has 5 valence electrons and needs 3 more to fulfill the octet rule. The two nitrogen atoms share three pairs of electrons, forming a triple covalent bond.
Question 4
On the Linus Pauling scale, which element is the most electronegative with a value of 4.0?
Answer: D Fluorine
Why
Fluorine is the most electronegative element with a value of 4.0 on the Pauling scale, while cesium is the least electronegative at 0.7.
Question 5
A student in Adama tests solid table salt (NaCl) and dissolved table salt in water for electrical conductivity. Why does the dissolved salt conduct electricity while solid salt does not?
Answer: B Dissolving frees the cations and anions so they can move toward electrodes
Why
In solid NaCl, ions are held tightly in fixed crystal positions. When dissolved in water, the sodium cations and chloride anions become free to move toward the electrodes under an electric field, conducting electricity.
Question 6
Why are ionic crystals, such as sodium chloride, hard but brittle when struck with a hammer?
Answer: B The mechanical force shifts ion layers so like charges face each other and repel
Why
Striking an ionic crystal shifts one layer of ions relative to its neighbor. This brings ions of the same charge next to each other, and the resulting strong repulsive forces cause the crystal to shatter.
Question 7
According to the electronegativity difference guide in the textbook, what kind of bond forms when the electronegativity difference between two bonded atoms is in the range of 0.5 to 2.0?
Answer: B Polar covalent bond
Why
The rough guide states that an electronegativity difference of 2.0 or more results in an ionic bond, 0.5 to 2.0 forms a polar covalent bond, and below 0.5 forms a covalent bond with little or no polarity.
Question 8
Carbon dioxide (CO2) contains polar C-O bonds, but CO2 is a non-polar molecule overall. What is the reason for this?
Answer: B The linear shape causes the partial charges on opposite oxygen atoms to cancel out
Why
CO2 has a linear geometry (O=C=O). Because the two polar C-O bonds point in opposite directions, the dipole charges cancel each other out, making the overall molecule non-polar.
Question 9
What distinguishes a coordinate covalent (dative) bond from an ordinary covalent bond?
Answer: C The shared electron pair is provided by only one of the bonded atoms
Why
A coordinate covalent bond (dative bond) is a type of covalent bond where the pair of electrons is donated by only one of the bonded atoms, but shared by both atoms after the bond forms.
Question 10
Which of the following describes how an ammonium ion (NH4+) is formed from ammonia (NH3) and a hydrogen ion (H+)?
Answer: B NH<sub>3</sub> shares its lone pair of electrons with the empty valence shell of H<sup>+</sup>
Why
The hydrogen ion (H+) has no electrons in its valence shell. Ammonia (NH3) shares its lone pair of electrons with H+ to form a coordinate covalent bond, completing the duet for hydrogen.
The app holds 12 questions on this unit, marked as you go. These are the first 10.
Unit 5 runs from page 143 to page 180 of the Ministry of Education edition.
Ministry of Education edition, English, 186 pages
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