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Problems with Classifying Solids, Liquids and Gases

This focus idea is explored through:

Contrasting student and scientific views

Student everyday experiences

2 students are squelching hair gel and are squeezing toothpaste onto their handsStudents’ understandings of solids, liquids and gases have been well researched and confirm that their early conception of these terms is shaped by their everyday use of these words. Students are frequently reported as using the word ‘solid’ as an adjective rather than to describe a class of substances. Typically, when students are asked for examples of each state, they are able to provide numerous examples of solids, less of liquids and only a few of gases - which reflects their common experiences. Solids are typically identified as objects that can be held, liquids as ‘dishwashing liquids’ that are ‘runny’ or ‘wetting’ and gases as LPG gas or propane gas that are combustible. Everyday language appears to strongly influence early student identification. For example, solid steel, liquid detergent and camping gas are frequently provided when students are asked for examples of substances in each state.

Research: Jones (1984), Krnel, Watson & Glazar (1998)

 

Student ideas about gases are further explored in the focus idea A gas is matter.

Some students strongly believe that to be a solid the substance must be very hard and clearly consolidated into unbreakable lumps. Substances which appear as powders or in fine granules like sand or talc are often identified as liquids because they are viewed by students as easily shaped or freely poured. Other students are comfortable with seeing powder as a solid because it will not ‘wet’ immersed objects. Water and water based liquids (for example milk, sea water, cordial and lemonade) are recurring examples of liquids identified by students. Non-water based liquids like cooking oils, kerosene, mineral turpentine, paraffin oil and oil based paints are less frequently identified. Evidence suggests that students freely associate liquids with water or assume they all contain some water because they are a liquid.

Research: McGuigan, Qualter & Schilling (1993), Krnel, Watson & Glazar (1998)

 

Scientific view

The classification of matter into one of three states (solid, liquid or gas) is a convenient way in lower levels to identify when substances change state (i.e. melt, boil, evaporate or freeze). However because this is a simple system of classification and the structure of matter is complex, it is not without difficulties. Many substances cannot be classified easily and many not at all. For example, hair gel, toothpaste, mayonnaise, play dough and Oobleck (created from a mixture of corn starch and water) provide ‘fuzzy’ examples which resist easy classification. Given the limitation of this classification system, some acceptable ideas when defining solids, liquids and gases are:

  • Solids are best defined as occupying a constant volume and retaining their shape when moderate forces are applied to them.
  • Liquids also occupy a constant volume but easily change shape to match that of their container by flowing to form a horizontal surface. They are said to ‘flow’ easily or be ‘runny’ or ‘wetting’ and can withstand moderate compressive forces.
  • A gas can occupy any size container, is also able to ‘flow’ and can be easily compressed with moderate forces.

A change in temperature can cause a substance to change state; however this may also be achieved by a change in pressure. Some substances like butter and chocolate are much more difficult to describe because they soften over a range of temperatures, compared with melting at a single melting point. Gels, colloids, immersions and many other substances defy simple classification because they contain mixtures of substances in different states over a range of temperatures.

 

Critical teaching ideas

  • The purpose of classification is to identify objects with common or similar properties.
  • Solids, liquids and gases provide a simple means of classifying the state of matter but they are not the only groupings used by scientists.
  • Some substances are very difficult to ‘classify’.
  • Classifying states of matter has limitations but can still be useful.
  • A change in temperature can cause a substance to change state.
 

Explore the relationships between ideas about states of matter in the Concept Development Maps: States of Matter

When teaching about changes of state it is important to emphasise that although a substance has moved from one state to another (for example, melted from a solid to a liquid), it still remains the same substance. Students frequently believe that a change of state creates a new substance with entirely new properties. This is understandable given the obvious differences between the properties of the various states. The choice of teacher language during discussion is important in reassuring students that the substance remains the same although it appears to behave differently.

Some observant students may question why we do not observe frozen carbon dioxide (dry ice) in a liquid state. Carbon dioxide changes from a frozen solid (dry ice is solid below -79°C) to a gas (a process called sublimation) without appearing to form pools of liquid. This is because it requires a pressure of approximately 60 times normal atmospheric pressure to remain in the liquid state. Students can consider that as it melts from a solid to a liquid it instantly boils to form a gas. Naphthalene (used to manufacture moth balls) is another substance which also sublimes at room temperature.

 

Teaching activities

Practise using and build perceived usefulness of a scientific model or idea

Image of a pile of clothes buttons of various sizes and colours.Provide small groups of students with a range of common objects (such as mixed buttons) composed of different substances and set them the challenge of devising a system of classifying them in ways which help to identify common features or properties. Their systems could be based on colour, hardness, whether they are natural, how they feel or how useful they are. It may be sufficient for students to be asked to identify just three common features for all items in the grouping. Students could draw up tables, or cut and paste images onto sheets to group them.

Careful consideration should be given to selecting items with obvious common properties for this task. Through class discussion, aim to promote the view that all of the features used to group the items are correct, however some groupings (systems of classifying) may be more helpful than others in identifying useful common features.

 

Open up discussion via shared experiences

During class discussions encourage students to consider a wide range of suitable contexts which have strong connections to their everyday experiences. Consider scenarios for matter changing states such as drying clothes, melting butter and dripping icy poles. Aim to extend student thinking beyond the common examples of water, ice water and water vapour. Discuss melting chocolate, candle wax, sugar and experiences that some children will have had with frozen carbon dioxide (dry ice).

Use examples of dry ice and naphthalene in a Predict-Observe-Explain (POE) demonstration showing that some substances can change state from a solid to a gas without becoming a liquid. Investigate how students could detect that a gas is being released from both of these solid substances. Students can detect the naphthalene gas by smell. Spoon several dry ice pellets into an empty balloon and seal it by tying a knot in the neck. The balloon will inflate as the solid changes into a gas. Alternatively, drop dry ice pellets into a glass of water to see the production of gas bubbles. (Note that the fog produced is not carbon dioxide gas but tiny liquid droplets of water made visible by the cold gas mixing with moist air. This is a second change of state associated with this phenomenon).

 

Challenge students existing ideas

Provide students with difficult to classify substances which challenge their definitions and force them to question their current understandings of the system of classification. At this level it is appropriate for students to see that some substances are more easily handled and don’t require containers for them to be passed around (solids). Some substances are ‘runny’ and do require open containers for them to be handled (liquids) and some cannot be handled at all and require closed containers (gases). Once students have developed a strong understanding of this system of obvious classification they could be presented with substances which provide greater challenges, such as toothpaste, sand, hair gel, Ooblek or granulated sugar. Classifying these items will help to challenge the view that all substances can be easily classified.