Unit 5: Physical Properties

Exploring the observable characteristics of acids, bases, and salts.

5.4 Solubility of Acids, Bases & Salts

Solubility, the ability of a substance to dissolve in a solvent like water, is a key physical property that helps differentiate acids, bases, and salts.

  • Acids: Most common inorganic acids (e.g., $HCl$, $H_2SO_4$, $HNO_3$) are highly soluble in water. Some larger organic acids are less soluble.
  • Bases: The solubility of bases varies significantly.
    • Group 1 hydroxides (like $NaOH$, $KOH$) are all soluble, making them strong alkalis.
    • Most other metal hydroxides (like $Cu(OH)_2$, $Fe(OH)_3$) are insoluble.
    • Ammonia ($NH_3$) is very soluble in water.
    • Most metal carbonates and oxides are insoluble, except for those of Group 1 metals and ammonium.
  • Salts: Many salts are soluble, but there are important exceptions. The following rules are a helpful guide:
    • All nitrate ($NO_3^-$) salts are soluble.
    • All salts containing Group 1 cations ($Li^+$, $Na^+$, $K^+$, etc.) and the ammonium ion ($NH_4^+$) are soluble.
    • Most chloride ($Cl^-$) salts are soluble, except for silver chloride ($AgCl$) and lead(II) chloride ($PbCl_2$).
    • Most sulphate ($SO_4^{2-}$) salts are soluble, except for barium sulphate ($BaSO_4$), lead(II) sulphate ($PbSO_4$), and calcium sulphate ($CaSO_4$).
    • Most carbonate ($CO_3^{2-}$) and hydroxide ($OH^-$) salts are insoluble (except for those of Group 1 and ammonium).
Solved Examples:
  1. Is potassium carbonate ($K_2CO_3$) soluble in water? Explain.
    Solution: Yes, it is soluble because all Group 1 salts are soluble.
  2. A student wants to make a solution of a base. Should they choose copper(II) hydroxide or sodium hydroxide?
    Solution: Sodium hydroxide, because it is soluble in water (it's an alkali), whereas copper(II) hydroxide is insoluble.
  3. Predict the solubility of silver nitrate ($AgNO_3$).
    Solution: Soluble. All nitrate salts are soluble.
  4. Predict the solubility of barium sulphate ($BaSO_4$).
    Solution: Insoluble. It is a key exception to the sulphate solubility rule.
  5. Why is it incorrect to refer to iron(III) oxide as an alkali?
    Solution: Because it is a base that is insoluble in water. An alkali must be a soluble base.
  6. Name a soluble chloride salt and an insoluble chloride salt.
    Solution: Soluble: Sodium chloride ($NaCl$). Insoluble: Silver chloride ($AgCl$).
  7. Is ammonium carbonate, $(NH_4)_2CO_3$, soluble?
    Solution: Yes, because all ammonium salts are soluble.
  8. A reaction produces lead(II) sulphate. Would this appear as a solution or a precipitate?
    Solution: A precipitate, because lead(II) sulphate is insoluble in water.
  9. Which of these bases is an alkali: $Mg(OH)_2$ or $KOH$?
    Solution: $KOH$ (potassium hydroxide) is an alkali because it's a soluble Group 1 hydroxide. $Mg(OH)_2$ is largely insoluble.
  10. Is calcium carbonate ($CaCO_3$) soluble in water?
    Solution: No, it is insoluble. Most carbonate salts are insoluble.

5.5 Electrical Conductivity

An electrolyte is a substance that produces an electrically conducting solution when dissolved in a polar solvent, such as water. The dissolved substance dissociates into mobile ions, which are free to move and carry an electric current.

Aqueous solutions of acids, bases, and salts are all electrolytes because they contain mobile ions.

  • Acids dissociate to produce $H^+$ ions and an anion (e.g., $Cl^-$).
  • Bases (alkalis) dissociate to produce $OH^-$ ions and a cation (e.g., $Na^+$).
  • Salts dissociate to produce a cation and an anion (e.g., $Na^+$ and $Cl^-$).

The conductivity of a solution can be tested with a simple circuit containing a power source, a bulb, and two electrodes. If the bulb lights up when the electrodes are placed in the solution, the solution is conductive and contains an electrolyte. The brightness of the bulb can give a qualitative measure of the concentration of ions.

Conductivity Testing Circuit
Solved Examples:
  1. Why does a solution of sodium chloride conduct electricity, but solid sodium chloride does not?
    Solution: In the solid state, the ions ($Na^+$ and $Cl^-$) are held in a fixed crystal lattice and cannot move. When dissolved in water, the ions are free to move and carry charge.
  2. Would a solution of sugar (a molecular compound) conduct electricity?
    Solution: No. Sugar dissolves as neutral molecules, not ions. Therefore, there are no mobile charge carriers, and it is a non-electrolyte.
  3. Which solution would conduct electricity more strongly, $1.0 M$ $HCl$ or $1.0 M$ $CH_3COOH$? Explain.
    Solution: $1.0 M$ $HCl$. $HCl$ is a strong acid and dissociates completely into ions. Ethanoic acid ($CH_3COOH$) is a weak acid and only partially dissociates, resulting in a much lower concentration of mobile ions.
  4. What particles are responsible for carrying current in a molten salt?
    Solution: Mobile positive and negative ions.
  5. Is pure, deionised water a good conductor of electricity?
    Solution: No, it is a very poor conductor. Water self-ionises to a very small extent ($H_2O \rightleftharpoons H^+ + OH^-$), so the concentration of ions is extremely low.
  6. Name the charge carriers in an aqueous solution of potassium hydroxide ($KOH$).
    Solution: Potassium ions ($K^+$) and hydroxide ions ($OH^-$).
  7. A student tests a solution with a conductivity meter, and the bulb glows brightly. What can be concluded about the solute?
    Solution: The solute is a strong electrolyte, meaning it is likely a strong acid, a strong alkali, or a soluble salt.
  8. Why are acids, bases, and salts classified as electrolytes?
    Solution: Because they all form mobile ions when dissolved in water, allowing the solution to conduct electricity.
  9. If you mix a solution of $Ba(OH)_2$ with a solution of $H_2SO_4$, the conductivity decreases initially. Why?
    Solution: The reaction forms insoluble barium sulphate ($BaSO_4$) and water, removing mobile ions ($Ba^{2+}$, $SO_4^{2-}$, $H^+$, $OH^-$) from the solution.
  10. Does molten lead(II) bromide conduct electricity?
    Solution: Yes. It is an ionic compound, and in the molten state, its ions ($Pb^{2+}$ and $Br^-$) are free to move.

5.6 Taste

One of the classical (though not recommended for safety reasons in a lab) physical properties of acids and bases is their taste.

  • Acids have a characteristic sour taste. This is due to the presence of hydrogen ions ($H^+$). Many foods we eat are acidic, such as lemons and limes (citric acid), vinegar (ethanoic acid), and yogurt (lactic acid).
  • Alkalis (soluble bases) have a bitter taste and a characteristic soapy or slippery feel. This is caused by the hydroxide ions ($OH^-$). Unsweetened cocoa and baking soda are examples of bitter-tasting basic substances.
  • Salts can have a variety of tastes, but the most common is salty, a taste primarily associated with sodium chloride ($NaCl$).

Safety Warning: You should never taste chemicals in a laboratory to identify them. This is a historical classification and is unsafe.

Solved Examples:
  1. What ion is responsible for the sour taste of lemons?
    Solution: The hydrogen ion ($H^+$), from citric acid.
  2. A substance feels soapy to the touch. Is it likely to be an acid or an alkali?
    Solution: An alkali.
  3. What is the characteristic taste of acidic substances?
    Solution: Sour.
  4. Baking soda ($NaHCO_3$) has a slightly bitter taste. What does this suggest about its chemical nature?
    Solution: It suggests that baking soda is a basic substance.
  5. Why do chemists no longer use taste as a method for identifying chemicals?
    Solution: Because many chemicals are toxic, corrosive, or poisonous, making it extremely dangerous.
  6. What gives vinegar its characteristic sour taste?
    Solution: Ethanoic acid (acetic acid).
  7. Which class of compound is associated with a bitter taste?
    Solution: Bases (specifically alkalis).
  8. Table salt ($NaCl$) tastes salty. Is it an acid, base, or salt?
    Solution: It is a salt.
  9. A drink is advertised as "tangy and sour." Is it likely to have a pH above or below 7?
    Solution: Below 7, as sourness is a property of acids.
  10. What physical property, other than taste, is characteristic of alkalis?
    Solution: A soapy or slippery feel.

5.7 Hygroscopic, Deliquescent & Efflorescent Substances

Some solid compounds have a strong affinity for water and can interact with atmospheric moisture in different ways.

  • Hygroscopic: A substance is hygroscopic if it readily absorbs moisture from the atmosphere. Concentrated sulphuric acid and solid sodium hydroxide are common examples. They are often used as drying agents (desiccants) in laboratories.
  • Deliquescent: This is an extreme form of hygroscopy. A deliquescent substance absorbs so much moisture from the air that it dissolves in the water it has absorbed, eventually forming a solution. Solid sodium hydroxide and calcium chloride are deliquescent.
  • Water of Crystallisation: Many ionic solids, when crystallised from an aqueous solution, incorporate a fixed number of water molecules into their crystal lattice structure. This water is called the water of crystallisation, and the solid is said to be hydrated. An example is hydrated copper(II) sulphate, $CuSO_4 \cdot 5H_2O$.
  • Efflorescent: An efflorescent substance is a hydrated solid that loses its water of crystallisation to the atmosphere upon standing. This often happens when the air is dry. An example is hydrated sodium carbonate ($Na_2CO_3 \cdot 10H_2O$), which crumbles to a powder as it loses water.
Solved Examples:
  1. What is the difference between a hygroscopic and a deliquescent substance?
    Solution: Both absorb moisture, but a deliquescent substance absorbs enough to dissolve itself, while a hygroscopic substance simply becomes damp or wet.
  2. Why is concentrated sulphuric acid used as a drying agent?
    Solution: Because it is strongly hygroscopic and readily removes water from other substances or the air.
  3. Calculate the molar mass of hydrated magnesium sulphate, $MgSO_4 \cdot 7H_2O$. (Mg=24.3, S=32.1, O=16, H=1)
    Solution: Molar mass = (24.3 + 32.1 + 4*16) + 7*(2*1 + 16) = 120.4 + 7*(18) = 120.4 + 126 = 246.4 g/mol.
  4. A blue crystal of $CuSO_4 \cdot 5H_2O$ is heated. What is observed?
    Solution: The crystal loses its water of crystallisation and turns into a white powder (anhydrous $CuSO_4$).
  5. Define efflorescence.
    Solution: The process by which a hydrated crystal loses its water of crystallisation to the atmosphere.
  6. A student leaves a dish of solid calcium chloride on a bench. The next day, it has turned into a puddle of liquid. What property does this demonstrate?
    Solution: Deliquescence.
  7. What is the term for a salt that contains no water of crystallisation?
    Solution: Anhydrous.
  8. A sample of hydrated sodium sulphate is found to have a molar mass of 322.1 g/mol. Find the value of x in its formula, $Na_2SO_4 \cdot xH_2O$. (Na=23, S=32.1, O=16, H=1)
    Solution: Molar mass of $Na_2SO_4$ = 2*23 + 32.1 + 4*16 = 46 + 32.1 + 64 = 142.1 g/mol. Mass of water = 322.1 - 142.1 = 180 g/mol. Molar mass of $H_2O$ = 18 g/mol. x = 180 / 18 = 10. The formula is $Na_2SO_4 \cdot 10H_2O$.
  9. Name a common deliquescent salt.
    Solution: Calcium chloride ($CaCl_2$) or sodium hydroxide ($NaOH$).
  10. Washing soda crystals ($Na_2CO_3 \cdot 10H_2O$) are left in a dry room and turn into a powder. What is this process called?
    Solution: Efflorescence.

Knowledge Check (20 Questions)

Answer: $AgCl$ (silver chloride).

Answer: A substance that forms mobile ions when dissolved in water, allowing the solution to conduct electricity.

Answer: Deliquescence.

Answer: Sour.

Answer: Yes.

Answer: Water of crystallisation.

Answer: No, it is a molecular compound and does not form ions in solution.

Answer: Efflorescence.

Answer: Soluble, because all nitrates are soluble.

Answer: A soapy or slippery feel.

Answer: It readily absorbs moisture from the atmosphere.

Answer: It contains mobile ions ($Na^+$ and $Cl^-$).

Answer: Insoluble (or sparingly soluble). It's an exception to the rule.

Answer: $CuSO_4 \cdot 5H_2O$.

Answer: Bitter.

Answer: Concentrated sulphuric acid or anhydrous calcium chloride.

Answer: ...does not contain water of crystallisation.

Answer: Insoluble (most carbonates are insoluble).

Answer: Mobile hydrogen ions ($H^+$) and the corresponding anions.

Answer: Yes, it reacts with water to form ions ($NH_4^+$ and $OH^-$), so it is a (weak) electrolyte.
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