Properties of Materials Chapter Notes | Year 8 Science IGCSE (Cambridge) PDF Download

Dissolving

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What is a solution?

• Dissolving occurs when a substance, such as a lump of sugar, is placed in a liquid like water, and it gradually seems to disappear.
• The substance that dissolves is called the solute.
• The substance that the solute dissolves into is called the solvent.
• A solution is a mixture of the solute and solvent, e.g., a colourless solution of sugar and water.
• Even though the solute (e.g., sugar) seems to disappear, its particles are still present, spread out among the solvent (e.g., water) particles.
• In the sugar-water example, sugar is the solute, water is the solvent, and the resulting mixture is the colourless solution.
• The process of dissolving involves the solute particles separating and mixing with the solvent particles.

Particle explanation of dissolving

• A sugar crystal is visible because it consists of tightly packed, vibrating groups of particles.
• Water particles, which vibrate and slide past one another, bump into the sugar particles, causing them to separate.
• The movement of water particles helps mix the separated sugar particles with the water particles.
• Eventually, all sugar particles are separated by water particles, becoming too small to be seen individually.
• All solutions are transparent, meaning you can see through them, though they are not necessarily colourless.
• For example, dissolving a coloured salt like copper sulfate in water forms a blue, transparent solution.
• In contrast, liquids like milk are opaque (you cannot see through them) and are not solutions.

Dissolving vs. Melting:

• Dissolving requires two substances: a solute and a solvent (e.g., sugar dissolving in water).
• Melting involves a single substance changing from a solid to a liquid due to heat (e.g., butter melting in a pan).

Examples of dissolving:

• Sugar (solute) in black tea (solvent).
• Instant coffee (solute) in hot water (solvent).
• Nail polish (solute) in nail polish remover (solvent).

Examples of melting:

• Butter in a frying pan.
• Ice cream on a warm day.
• Candle wax as the candle burns.

Mass conservation in dissolving:

• When a solute (e.g., salt) dissolves in a solvent (e.g., water), the solute particles remain in the solution.
• The mass of the solution equals the sum of the mass of the solute and the mass of the solvent.
• Formula: mass of solute + mass of solvent = mass of solution.
• No mass is lost during dissolving; the mass is conserved.

Solutions and Solubility

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Solutions

• A solution is formed when a solute dissolves in a solvent.
• Concentrated solution: Contains a large number of solute particles dissolved in the solvent (e.g., a sugar solution with many sugar particles).
• Dilute solution: Contains fewer solute particles dissolved in the solvent (e.g., a sugar solution with fewer sugar particles).

Solubility

• A solid that dissolves in a solvent, such as water, is described as soluble (e.g., sodium chloride and sugar).
• A solid that does not dissolve in a solvent is described as insoluble (e.g., iron filings in water).
• When a soluble solid is continuously added to a solvent, a point is reached where no more solute can dissolve, forming a saturated solution.
• Solubility refers to the amount of solute that can dissolve in a given amount of solvent at a specific temperature.
• Different solutes have varying solubility levels: For example, sodium chloride is highly soluble in water, while lead chloride has low solubility.

Comparing solubility

To compare solubility, measure how much of each solute dissolves in a known amount of solvent under the same conditions (e.g., temperature).

Temperature and Solubility

• Most solutes dissolve more quickly and easily in hot water compared to cold water due to the increased energy of particles.
• With more energy, particles vibrate and move more, facilitating the dissolving process.
• Higher temperatures allow a greater mass of solute to dissolve in the same volume of solvent.
• As temperature increases, the solubility of most solutes also increases.
• Example: In 100 g of water at 20 °C, 204 g of sugar can dissolve, while at 80 °C, 362 g of sugar can dissolve.

Comparing the solubility of different salts

Other Solvents

• Water is not the only solvent; other liquids can also act as solvents for specific solutes.
• Some substances that are insoluble in water can dissolve in alternative solvents.
• Example: Oil paint, which is insoluble in water, dissolves in methanol (methylated spirits), used to clean paint brushes.
• Example: Nail polish, which does not dissolve in water, dissolves in nail polish remover containing propanone (acetone) as the solvent.

Planning a Solubility Investigation

Dissolving Salt in Water

• An investigation can be designed to study how temperature affects the amount of salt (sodium chloride) that dissolves in water.
• Variables in the investigation:
  • Independent variable: The variable that is changed, e.g., the temperature of the water.
  • Dependent variable: The variable that is measured, e.g., the amount of salt (number of spatulas) that dissolves.
  • Control variables: Variables that are kept constant to ensure a fair test, e.g., the volume of water.
• When plotting results:
  • The independent variable (e.g., temperature) is plotted on the horizontal axis.
  • The dependent variable (e.g., amount of salt dissolved) is plotted on the vertical axis.

Paper Chromatography

• Paper chromatography is a technique used to separate dissolved substances in a mixture.
• It involves using a solvent to move substances along a piece of paper, separating them based on their solubility and interaction with the paper.
• The result of paper chromatography is called a chromatogram, which shows the separated substances as distinct spots or bands.
• Solvent front: The furthest point reached by the solvent on the chromatography paper.
• Permanent: Refers to substances (e.g., inks or dyes) that do not break down or change during the chromatography process.

Colours in Ink

• Black ink appears as a single colour but is actually a mixture of different coloured inks.
• Paper chromatography is a technique used to separate the different coloured inks within a mixture, such as black ink.
• Special paper, similar to filter paper, is used in paper chromatography.

Process of paper chromatography

• A small drop of ink (e.g., black ink) is placed on the chromatography paper.
• The paper is placed in a beaker with a solvent (e.g., water) so that the solvent soaks up into the paper.
• As the solvent moves up the paper, it carries the ink particles along, separating the different coloured inks that make up the original ink.
• The resulting image on the paper, showing the separated colours, is called a chromatogram.

Mechanism of separation

• The solvent (e.g., water) dissolves the ink particles.
• As the solvent moves up the paper, it carries the ink particles different distances based on their solubility.
• Ink particles with higher solubility are carried further up the paper, while less soluble particles are left behind sooner.
• Different inks (e.g., green, black, red) produce distinct chromatograms, revealing the various coloured components within each ink.
• Some inks, such as those in permanent marker pens, are not soluble in water and require a different solvent, such as alcohol, to separate their components.

Applications of chromatography

• Scientists use chromatography to study dyes in food to determine if they are pure substances or mixtures.
• A pure substance produces a single spot on the chromatogram, while a mixture produces multiple spots.
• Identifying the composition of food dyes is important to ensure they are safe and do not contain harmful substances that could be toxic or cause allergic reactions.
• Public health scientists use chromatography to check the safety of colourings in products like hair dye or pen ink by comparing chromatograms of the product with those of permitted dyes.