Atoms, Bonds & Water — Practice · Grade 10 Science

Chapter: Investigating the Chemistry of Life

What this practice covers

By the end of Atoms, Bonds & Water you should be able to:

10 questions, multiple choice, numeric and written answer, each with a worked explanation.

Vocabulary used: atom, element, covalent bond, ionic bond, hydrogen bond, polar molecule, pH.

Problem 1

Which type of bond forms when two atoms share a pair of electrons?
  1. Metallic bond
  2. Ionic bond
  3. Covalent bond
  4. Hydrogen bond
Answer and explanation

Covalent bond

In a covalent bond, two atoms share a pair of electrons — as oxygen does with two hydrogens in water.

Problem 2

Water is described as a 'polar molecule' because
  1. it is made of only one element
  2. it carries a full electrical charge overall
  3. it cannot dissolve any other substances
  4. it shares its electrons unequally, giving it a slightly positive end and a slightly negative end
Answer and explanation

it shares its electrons unequally, giving it a slightly positive end and a slightly negative end

Oxygen pulls the shared electrons closer, leaving the oxygen end slightly negative and the hydrogen ends slightly positive — that charge separation makes water polar.

Problem 3

How many times more acidic is a solution of pH 2 than a solution of pH 5?
Answer and explanation

1000 times

Each pH unit is a factor of 10. The difference is 5 − 2 = 3 units, so 10³ = 1000 times more acidic.

Problem 4

Explain how hydrogen bonds give water its cohesion, and give one example of cohesion in nature.
Answer and explanation

Hydrogen bonds attract water molecules to one another, so they stick together — this is cohesion. It creates surface tension that lets a water strider walk on a pond and helps tall trees pull an unbroken column of water up from their roots to their leaves.

Problem 5

A student claims, 'Salt dissolves in water because water breaks the covalent bonds inside salt.' What is the main error?
  1. Salt is held together by ionic bonds, not covalent bonds; water surrounds and separates its ions without breaking any covalent bonds
  2. Water cannot dissolve salt at all
  3. Salt has no chemical bonds to break
  4. Water is nonpolar, so it can never dissolve salt
Answer and explanation

Salt is held together by ionic bonds, not covalent bonds; water surrounds and separates its ions without breaking any covalent bonds

Salt (NaCl) is held together by ionic bonds. Polar water molecules surround the Na⁺ and Cl⁻ ions and pull them apart — it separates ions rather than breaking covalent bonds.

Problem 6

Why do oil and water tend to separate while salt can dissolve in water? Explain the reasoning in your own words.
Answer and explanation

Different molecular interactions explain the contrasting behavior; water does not dissolve everything.

Water interacts strongly with ions and many polar groups.

Nonpolar oil does not make similarly favorable interactions with water and separates in bulk.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 7

Why is water's oxygen region partially negative while hydrogen regions are partially positive? Show the reasoning, not just the conclusion.
Hints

Do not treat hydrogen bonds between molecules as the covalent bonds inside water.

Answer and explanation

Oxygen attracts shared electrons more strongly, creating an uneven charge distribution rather than full ionic charges.

Problem 8

Does melting ice break all O–H covalent bonds in water molecules?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

No. It mainly alters intermolecular organization; water molecules remain intact.

Problem 9

Equal energy additions need not give equal temperature changesEqual energy additions need not give equal temperature changesTemperature (°C)20253035400123Equal successive energy additionsSample A: water modelSample B: comparison
Original matched-mass heating example with invented records; the horizontal axis is an input count, not real calorimeter time or a measured water constant.
Interpret the supplied visual. What controlled conditions are needed to infer different heat capacities from these slopes?
Answer and explanation

Use matched masses, equal energy inputs, stated phases and comparable heat losses. The supplied water-model sample rises 6 °C, the comparison 18 °C.

Temperature describes a thermal state, while thermal energy also depends on amount and material. Specific heat capacity is energy per unit mass per degree of temperature change under stated conditions. Matched masses receiving equal energy can warm at different rates. Water’s interactions and degrees of freedom contribute to its relatively high heat capacity, moderating temperature changes in organisms and environments. This is different from evaporation, which transfers energy through a phase change. Heat-loss conditions, mass and phase must be comparable before assigning the observed slope only to a material property.

Use matched masses, equal energy inputs, stated phases and comparable heat losses. The supplied water-model sample rises 6 °C, the comparison 18 °C.

Response criteria

  • Read the structures, axes, directions and units correctly.
  • Connect the evidence with the biological mechanism.
  • Respect stated model assumptions and distinguish an observation from a further inference.

Problem 10

Compare hydrogen-ion concentrations at pH 5 and pH 8 using a tenfold change per unit. Explain why a buffer cannot be described as preventing every pH change.
Answer and explanation

The pH 5 sample has 10³ = 1,000 times the hydrogen-ion concentration of the pH 8 sample in the dilute-solution comparison. A buffer has a finite working range and capacity; sufficiently large additions can exceed it.

The pH 5 sample has 10³ = 1,000 times the hydrogen-ion concentration of the pH 8 sample in the dilute-solution comparison. A buffer has a finite working range and capacity; sufficiently large additions can exceed it.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the biological mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

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