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Chemistry: Changes to materials

Science Stage 6 Chapter Notes

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Chemistry: Changes to materials
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In this chapter

  1. 1Reversible and Irreversible Changes
  2. 2Mixtures and Their Properties
  3. 3Solutions, Solutes and Solvents
  4. 4Suspensions and Insoluble Substances
  5. 5Factors Affecting Rate of Dissolving

1. Reversible and Irreversible Changes

A change to a material can be either reversible or irreversible. A reversible change is a physical change that can be undone, returning the material to its original state. A common example is a change of state, like ice (solid water) melting into liquid water, which can be frozen back into ice. No new substance is created. An irreversible change is a chemical change that cannot be easily undone. During an irreversible change, new substances with different properties are formed. For example, burning a wooden match creates ash and smoke; you cannot turn the ash back into wood.

Key term

Irreversible Change: A change that results in the formation of one or more new substances and cannot be easily reversed.

Examiner insight

Examiners award marks for clearly stating whether a new substance is formed when justifying if a change is reversible or irreversible.

Common pitfall

Confusing 'dissolving' with 'melting'. Melting is a change of state for a single substance due to heat, while dissolving involves a substance (solute) mixing into another (solvent).

Worked example 13 marks

When you mix boiling water with jelly powder, the mixture is a liquid. In the fridge, it becomes a solid gel. Can we make the jelly change back to a liquid? Is this a reversible or irreversible change?

  1. 1
    1. Yes, the solid jelly can be changed back to a liquid by gently heating it.
  2. 2
    1. Because the change can be undone (solid to liquid and back to solid), it is a reversible change.
  3. 3
    1. No new substance is permanently formed; the process is a physical change involving temperature.

Worked example 22 marks

Does boiling an egg cause a reversible or irreversible change? Explain your answer.

  1. 1
    1. Boiling an egg causes an irreversible change.
  2. 2
    1. This is because the liquid egg white and yolk turn solid and change colour and texture. These are new properties, indicating new substances have formed through a chemical reaction (denaturation of proteins).
  3. 3
    1. You cannot cool the boiled egg to turn it back into a raw, liquid egg.

Recap

  • Reversible changes are physical changes that can be undone, like melting or freezing.
  • In a reversible change, no new substances are formed.
  • Irreversible changes are chemical changes that cannot be easily undone.
  • In an irreversible change, new substances with new properties are formed.
  • Burning, cooking, and rusting are common examples of irreversible changes.

Quick check

  1. Is dissolving sugar in water a reversible or irreversible change? Explain how you know.2 marks

2. Mixtures and Their Properties

A mixture is made when two or more substances are combined, but they do not chemically react with each other. This means the individual substances in the mixture keep their original properties. For example, if you mix sand and salt, you can still see the individual grains, and the salt is still salty and the sand is still sandy. Because they are not chemically bonded, mixtures can often be separated using physical methods like sieving, filtering, or magnetism.

Key term

Mixture: A substance made by combining two or more different materials in such a way that no chemical reaction occurs.

Fun fact

The air you breathe is a mixture of gases, primarily nitrogen (≈78%), oxygen (≈21%), argon (≈0.9%), and carbon dioxide (≈0.04%), plus trace amounts of others.

Worked example 13 marks

A student makes a mixture of small iron filings and yellow sulfur powder. The mixture looks like a yellow-grey powder. A magnet is passed over the mixture. The iron filings are attracted to the magnet, leaving the yellow sulfur powder behind. Is this a mixture or a new compound? Explain.

  1. 1
    1. This is a mixture.
  2. 2
    1. The iron filings and sulfur powder have not chemically reacted; they are just physically combined.
  3. 3
    1. We know this because the iron keeps its magnetic property and the sulfur keeps its yellow colour.
  4. 4
    1. The ability to separate them with a magnet (a physical method) proves it is a mixture.

Recap

  • A mixture contains two or more substances that are not chemically bonded.
  • The substances in a mixture retain their individual properties.
  • Mixtures can be separated by physical means.
  • No new substance is formed when a mixture is made.

Quick check

  1. You mix rice and flour together. Have the substances changed in any way? Explain.2 marks

3. Solutions, Solutes and Solvents

A solution is a special type of mixture where one substance dissolves completely into another. The substance that dissolves is called the solute. The substance it dissolves in is called the solvent. When a solute dissolves, its particles spread out and mix completely with the solvent particles, becoming so small that they cannot be seen. The resulting mixture is transparent (clear) and is called a solution. For example, in saltwater, salt is the solute and water is the solvent.

Solute + Solvent → Solution

Key term

Solution: A homogeneous mixture in which the minor component (the solute) is uniformly distributed within the major component (the solvent).

Worked example 13 marks

When you make jelly, you mix jelly powder with hot water to make a solution.a) Name the solute.b) Name the solvent.c) Why is the resulting liquid called a solution?

  1. 1

    a) The solute is the jelly powder (which contains gelatin and sugar).

  2. 2

    b) The solvent is the hot water.

  3. 3

    c) It is called a solution because the jelly powder dissolves completely in the water, creating a clear liquid where you cannot see the individual powder particles.

Worked example 23 marks

Draw a simple diagram to show how particles of salt (solute) are arranged in water (solvent) to form a salt solution.

  1. 1
    1. Draw a beaker or box representing the solution.
  2. 2
    1. Draw many particles representing the water (solvent), spread throughout the container.
  3. 3
    1. Draw fewer particles representing the salt (solute), scattered evenly between the water particles.
  4. 4
    1. Add labels: 'Water particles (solvent)' and 'Salt particles (solute)'. Ensure the solute particles are shown individually and not in clumps.

Recap

  • A solution is a mixture formed when a solute dissolves in a solvent.
  • The solute is the substance that dissolves.
  • The solvent is the substance that does the dissolving.
  • Solutions are transparent because the solute particles are too small to see.
  • Substances that dissolve are called soluble.

Quick check

  1. In a cup of sugary coffee, identify the solute and the solvent.2 marks

4. Suspensions and Insoluble Substances

Some substances do not dissolve in a liquid. These are called insoluble substances. When you mix an insoluble solid with a liquid, it does not form a solution. Instead, it forms a suspension. In a suspension, the solid particles are large enough to be seen and they are dispersed (spread out) throughout the liquid, making it cloudy or opaque. If left to stand, the solid particles will often settle at the bottom of the container due to gravity. Sand in water is a classic example of a suspension.

Key term

Suspension: A heterogeneous mixture containing solid particles sufficiently large for sedimentation, which are dispersed in a liquid but not dissolved.

Examiner insight

Students who can clearly describe the visual differences between a solution (clear) and a suspension (cloudy, settles out) score well.

Worked example 13 marks

A student stirs a spoonful of flour into a glass of water.a) Is the flour soluble or insoluble in water?b) Is the resulting mixture a solution or a suspension?c) How do you know?

  1. 1

    a) The flour is insoluble in water.

  2. 2

    b) The mixture is a suspension.

  3. 3

    c) I know this because the flour does not dissolve. The water becomes cloudy/milky, and if left to stand, the flour will settle at the bottom.

Recap

  • Substances that do not dissolve in a liquid are called insoluble.
  • An insoluble solid mixed with a liquid forms a suspension.
  • Suspensions are cloudy or opaque because the solid particles are visible.
  • The solid particles in a suspension can settle out over time.
  • Sand in water and muddy water are examples of suspensions.

Quick check

  1. What is the main difference in appearance between a solution and a suspension?1 mark

5. Factors Affecting Rate of Dissolving

The 'rate of dissolving' refers to how fast a solute dissolves in a solvent. We can speed up this process by changing certain conditions. There are three main factors: 1. Temperature: Increasing the temperature of the solvent gives its particles more kinetic energy, causing them to move faster and collide more frequently and forcefully with the solute particles, breaking them apart more quickly. 2. Surface Area: Crushing a solute into smaller pieces (like using granulated sugar instead of a sugar cube) increases its total surface area. This allows more of the solute to be in contact with the solvent at once, leading to faster dissolving. 3. Stirring (Agitation): Stirring or shaking the mixture moves the solute and solvent particles around, bringing fresh solvent into contact with the solute surface and moving dissolved solute away. This maintains a high concentration gradient at the surface, speeding up the process.

Key term

Rate of Dissolving: The speed at which a solute dissolves in a solvent to form a solution.

Common pitfall

Stating that 'more dissolves' at a higher temperature, instead of 'it dissolves faster'. While solubility often increases with temperature, the question is about the rate.

Fun fact

Effervescent tablets, like some vitamins, dissolve quickly because the bubbles of carbon dioxide they produce act as a self-stirring mechanism, increasing the rate of dissolving.

Worked example 14 marks

When making a cup of instant coffee, people usually use hot water and stir with a spoon. Using your knowledge of dissolving, explain why these two actions are taken.

  1. 1
    1. Hot water is used because increasing the temperature increases the rate of dissolving. The high-energy water particles collide with the coffee granules more often and with more force, causing them to dissolve faster.
  2. 2
    1. Stirring with a spoon also increases the rate of dissolving. It brings fresh water into contact with the coffee granules and moves the already dissolved coffee away from the surface of the granules, allowing more to dissolve quickly.

Recap

  • The rate of dissolving is how fast a solute dissolves.
  • Increasing the temperature of the solvent increases the rate of dissolving.
  • Increasing the surface area of the solute (e.g., by crushing it) increases the rate of dissolving.
  • Stirring or agitating the mixture increases the rate of dissolving.
  • These factors increase the frequency of effective collisions between solute and solvent particles.

Quick check

  1. A sugar cube is dropped into a cold drink. State two different ways to make it dissolve faster.2 marks

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. For each of the following, state whether it is a reversible or irreversible change, and give a brief reason: a) Melting chocolate b) Frying an egg c) A puddle of water evaporating.3 marks
  2. Define the terms 'solute', 'solvent', and 'solution' using the example of sugar dissolving in tea.3 marks
  3. Sunil heated some popcorn kernels in hot oil. They popped to make popcorn. a) Is this a reversible or irreversible change? b) What makes the kernels change? c) Did a new substance form? Explain your reasoning.4 marks
  4. A student has a mixture of salt and sand. Both are white solids. Describe the steps the student should take to separate the salt and the sand, and obtain dry samples of each. Explain the property used in each step.5 marks
  5. A student wants to investigate if stirring affects the time it takes for a sugar cube to dissolve in water. Briefly outline the steps of a fair experiment. Name the independent, dependent, and two control variables.5 marks
  6. Is a glass of milk a solution or a suspension? Explain your answer by describing its appearance and behaviour.3 marks
  7. When a metal like magnesium is dropped into acid, it fizzes and the test tube gets warm. Is this a chemical or physical change? Give two pieces of evidence from the description to support your answer.3 marks
  8. Draw two simple diagrams labelled 'A' and 'B'. Diagram A should represent particles in a solution. Diagram B should represent particles in a suspension that has been left to stand for five minutes. Label your diagrams clearly.4 marks
  9. Why does granulated sugar dissolve faster in hot tea than a sugar cube of the same mass? Explain your answer in terms of both surface area and temperature.4 marks
  10. Acid rain can damage buildings made of limestone. This is an irreversible chemical change. Explain what is meant by an 'irreversible chemical change' in this context.2 marks

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