Physical and Chemical Changes

Learning Objective

In this lesson we will compare physical and chemical changes, and learn how to differentiate between them.

Learning Outcomes

By the end of this lesson you will be able to:

  • Differentiate between a physical change and a chemical change.

  • Give examples of different types of physical change.

  • Identify signs of a chemical change.

  • Compare the energy transfer and reversibility of physical and chemical changes.

 

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Introduction

  • Physical and chemical changes both involve changes in the appearance of substances.
  • The key difference is whether a new substance is formed or not.
    A chemical change involves the formation of one or more new substances.
    A physical change does not result in the formation of new substances.

 
pizza bread baking oven physical chemical change

What types of changes are involved when a pizza is baked in an oven?.

(Image: Jared Tarbell, Wikimedia Commons)

 

Types of Physical Change

  • Physical changes can involve one or more of the following alterations in appearance.
    In all cases described, the substances have changed their appearance, but they are still the same substance.

 

Change in Shape

  • Twisting, bending, tearing, shredding, breaking, crushing, pulverising, stretching and squashing are all ways that the shape of a substance can be altered by the application of a force.
    They may result in substances looking very different, but they remain the same substances, just in a different form.
  • Examples:
    Tearing paper, crushing cans, stretching plastic, grinding flour, moulding clay.

 

shred paper physical change shape    crush can physical change shape    mould clay physical change shape

Shredding paper, crushing cans and moulding clay are examples of physical changes involving a change in shape or form.

(Images: stux, Pixabay; Panhard, Wikimedia Commons; pxhere)

 

Change in State of Matter

  • The six types of state changes are: melting, freezing, vaporisation, condensation, sublimation and deposition.
    These result from changes in temperature (and/or pressure).
  • Examples:
    Melting of ice cream, condensation of water on a mirror, sublimation of dry ice.

 

melting physical state change    condensation physical state change    sublimation physical state change

Changes of state, such as melting, condensation and sublimation, are physical changes.

(Images: Couleur, Pixabay; DerNaut, Pixabay; KarolinaHalatek, Wikimedia Commons)

 

Dissolving and Crystallising

  • Dissolution occurs when a soluble substance (solute) separates and disperses throughout another substance (solvent).
  • Crystallisation occurs when a solid solute comes out of a solution, due to evaporation of solvent from a saturated solution, or cooling of a supersaturated solution.
  • Examples:
    Dissolving of salt in water, formation of calcite crystals in caves, ‘growing’ of copper sulfate crystals in a laboratory.

 

dissolving dissolution physical state change    crystallising crystallisation physical state change

Dissolving and crystallising are physical changes.

(Images: Chris 73, Wikimedia Commons; Tess Watson, Wikimedia Commons)

 

Formation of Mixtures

  • The dissolving of salt mentioned above is an example of the formation of a type of mixture called a solution.
    The formation of any type of mixture is a physical process as all the original substances are still present.
  • Examples:
    Whisking an egg with milk, preparing a salad, creating potting mix.

 

whisking mixture physical change    salad mix physical process    soil mixture physical process

The formation of a mixture is a physical process.

(Images: congerdesign, PIXNIO; pxhere; pxhere)

 

Signs of a Chemical Change

  • There are many types of chemical change, but it is not always easy to distinguish whether a chemical change has taken place or whether a change in appearance is purely physical.
    This is because chemical changes will usually involve physical changes as well.
    For example, the burning of a candle is a chemical change. It involves the conversion of wax and oxygen to carbon dioxide and water. However, this is accompanied by physical changes as well, such as melting of the wax.

 
candle burn melt physical chemical change

The burning of a candle involves physical and chemical changes.

(Image: skeeze, Pixabay)

 

  • There are certain signs which indicate that a chemical change may have taken place.
    Although they are not proof of a chemical change, chemical changes usually involve one or more of the following indicators.

 

Colour Change

  • A colour change may indicate that a new substance has been formed.
  • Examples:
    Rusting of iron, ripening of fruit, bleaching of hair, burning of toast.
  • Not all colour changes are chemical changes.
    For example, the mixing of blue and yellow paint to make green paint is a physical change, as separate blue and yellow pigments still exist.

 

ripening fruit colour chemical change    corrosion rust colour chemical change    toast baking chemical colour change

A colour change may indicate that a chemical change has taken place.

(Images: pxhere; moshehar, Pixabay; CordMediaStuttgart, Pixabay)

 

Formation of Bubbles or Odours

  • Bubbles or odours may indicate that a new substance – a gas – has been produced.
  • Examples:
    Fizzing when acid comes into contact with metal, the smell of rotting rubbish, the fumes from a car exhaust.
  • Not all bubbling and odours are chemical changes.
    For example, the bubbling that occurs when a substance boils, or the smell of a substance when it vaporises, are both the result of changes of state.

 

chemical reaction acid hydrogen gas bubbles    fermentation reaction carbon dioxide gas bubbles    water pollution gas bubbles

The formation of bubbles or an odour may indicate that a chemical change has taken place.

(Images: Chemicalinterest, Wikimedia Commons; Jim Champion, Wikimedia Commons; Jemzo, Pixabay)

 

Formation of a Solid

  • The formation of a solid may indicate that a new substance has been formed.
    When the solid results from the mixing of two liquids, or a gas and a liquid, it is called a precipitate.
  • Examples:
    Turning clear lime water cloudy by breathing into it through a straw, formation of solid silver chloride when mixing silver nitrate and sodium chloride solution, formation of soot in chimneys.
  • Solids aren’t always produced from chemical changes.
    For example, the formation of ice on a cold windscreen, or the formation of crystals in a cave, are a result of physical changes.

 

precipitate chemical reaction    precipitation chemical change    solid formation chemical change sign

The formation of a solid may indicate that a chemical change has taken place.

(Images: PRHaney, Wikimedia Commons; Cychr, Wikimedia Commons; Der Kreole, Wikimedia Commons)

 

Temperature Change

  • Chemical changes involve the transfer of heat energy, resulting in an increase or decrease in temperature.
    Sometimes the release of heat is accompanied by light, and occasionally, sound.
  • Examples:
    Burning coal in a steam engine, the metabolic heat which keeps our bodies warm, igniting fireworks, instant cold packs.
  • Temperature changes aren’t always the result of chemical change.
    For example, heat caused by friction is not due to a chemical reaction, but the transfer of kinetic energy to heat energy.
    Similarly, the production of light in electric devices, such as light globes, is due to the conversion of electrical energy to light energy, not as a result of a chemical reaction.

 

steam engine heat chemical reaction    metabolic heat chemical reaction    thermite heat light chemical reaction

A temperature change may indicate that a chemical change has taken place.

(Images: hpgruesen, Pixabay; Master Sgt. Michel Sauret, US Army; CaesiumFluoride, Wikimedia Commons)

 

The Process of Chemical Change

  • Chemical change involves the breaking and forming of chemical bonds between atoms.
  • Atoms and molecules are rearranged, but the number and type of each atom remains the same.
  • Consequently, the total mass of reactants is equal to the total mass of products.
    This is known known as the conservation of mass.

 
chemical reaction conservation of mass

Chemical changes involve the breaking and forming of chemical bonds, but the number and type of each atom, and therefore total mass, is the same before and after a chemical reaction.

 

Energy Transfer During Physical and Chemical Changes

  • All chemical changes involve energy changes and the transfer of heat energy.
    Depending on the particular example, energy can be released or absorbed.
  • When energy is released, it is known as an exothermic chemical change.
    Energy is released in the form of heat, but occasionally also as light, or even sound.
    As a result, the surrounding environment experiences an increase in temperature.
    Since energy is released, the new substances contain less stored chemical energy than the original substances.
  • When energy is absorbed, it is known as an endothermic chemical change.
    Heat energy is absorbed from the surrounding environment.
    As a result, the surrounding environment experiences a decreases in temperature.
    Since energy is absorbed, the new substances contain more stored chemical energy than the original substances.
  • Physical changes can also be exothermic or endothermic.
    For example, dissolving calcium chloride in water is an exothermic process, whereas dissolving ammonium nitrate is endothermic.

 

combustion fire exothermic chemical reaction    instant cold pack endothermic reaction

Exothermic changes, such as burning wood, release heat energy, heating the surrounding environment.
Endothermic changes, such as those that take place in instant cold packs, absorb heat energy, cooling the surrounding environment.

(Images: StockSnap, Pixabay; OpenStax, Wikimedia Commons)

 

Reversibility of Physical and Chemical Changes

  • Most physical and chemical changes are reversible.
  • Generally, physical changes are easier to reverse than chemical changes.
    This is because physical processes do not require as much energy as chemical processes, as they do not involve the breaking of chemical bonds.
    For example, dents in metal can be beaten out, melted ice cream can be re-frozen, dissolved salt can be recovered by evaporation and shredded paper can be recycled.
    On the hand, the formation of rust, the burning of propane gas and the fermentation of sugar into alcohol could all be reversed, but would require complex chemical processes and much more energy.

 

car metal dent physical change reversible    car metal rust chemical change irreversible

Physical changes are generally easier to reverse than chemical changes.

(Images: Hans, Pixabay; Skitterphoto, Pixabay)

 

Summary

  • A physical change involves changes in the appearance of a substance, but the type of substance does not change.
  • A chemical change involves the formation of one or more new substances.
  • Physical changes can involve one or more of the following:
    • Change in shape.
    • Change in state of matter.
    • Dissolution or crystallisation.
    • Formation of mixtures.
  • Chemical changes can involve one or more of the following (although they are not proof of a chemical change):
    • Colour change.
    • Formation of bubbles or odours.
    • Formation of a solid precipitate.
    • Release/absorption of heat.
  • Chemical change involves the breaking and forming of chemical bonds between atoms.
    Atoms and molecules are rearranged, but the number and type of each atom remains the same.
    In other words, there is conservation of mass.
  • Exothermic changes release heat energy; endothermic changes absorb heat energy.
  • Most physical changes and many chemical changes are reversible.
    Generally, physical changes are easier to reverse than chemical changes as they don’t require the breaking of chemical bonds.

 
signs of physical and chemical change

(Image: Counselling, Pixabay)

 

(Header image: Gaertringen, Pixabay)

 

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