General and Characteristic Properties

In Physics and Chemistry class, you measured these properties of several materials: mass, volume, density, melting point, hardness, and electrical conductivity. Classify them into two groups: general properties (those that all matter possesses) and characteristic properties (those used to distinguish one substance from another). Briefly justify your classification.

You inflate a balloon and see that it takes up space that it did not occupy before; furthermore, on a very precise balance, the full balloon weighs a little more than the empty balloon, as shown in the diagram.

Which two general properties of matter does air demonstrate with this experiment? Name them and explain which observation corresponds to each.

At the greengrocer's, the shopkeeper places a melon on the scales and they read $2.5\ \text{kg}$.

1. a) Which property of the melon is being measured?
2. b) Is it a general or characteristic property? Justify your answer.
3. c) What is the unit of this quantity in the International System of Units?

To measure the volume of a metal keychain, Noa pours $50\ \text{mL}$ of water into a graduated cylinder; when she submerges the keychain, the level rises to $68\ \text{mL}$, as shown in the diagram.

1. a) What is the volume of the keychain, in $\text{cm}^3$?
2. b) Express it in liters.
3. c) Why could Noa not measure the volume of this object with a ruler?

Complete the sentence with the missing words: '$\dots$ properties are possessed by all materials, such as mass and volume; $\dots$ properties make it possible to distinguish one substance from another, such as density or melting point'. Then, explain how you would use a characteristic property to distinguish salt from sugar without tasting them.

Marcos has two marbles that weigh exactly $20\ \text{g}$ each: one is made of glass and the other of steel, but they are painted the same color and have got mixed up.

1. a) Does knowing their mass help him tell which is which? Why?
2. b) Which characteristic property could he measure to distinguish them? Explain how he would do it.

You break a bar of chocolate into two exact halves.

1. a) What happens to the mass and volume of each half compared to those of the whole bar?
2. b) What happens to the density of the chocolate?
3. c) Use this example to explain the difference between a general property and a characteristic property.

An iron beam at a building site weighs $790\ \text{kg}$ and a screw made of the same iron weighs $7.9\ \text{g}$. The density of both is $7.9\ \text{g/cm}^3$.

1. a) Calculate the volume of the screw and that of the beam (express the latter in $\text{cm}^3$; remember that $1\ \text{kg} = 1000\ \text{g}$).
2. b) Do they have the same mass? The same volume? The same density?
3. c) Using this example, explain how general properties differ from characteristic properties, and reason why density does not change whether the piece of iron is large or small.

In the laboratory, Lucía pours water into a measuring cylinder up to the $100\ \text{mL}$ mark, as shown in the diagram.

1. a) Which property of water is she measuring with the measuring cylinder?
2. b) Express this quantity in $\text{cm}^3$ (remember: $1\ \text{mL} = 1\ \text{cm}^3$).
3. c) Is it a general or characteristic property?

Water always boils at $100\ \celsius$ (at sea level) and alcohol at $78\ \celsius$.

1. a) What is the property referred to by these data called?
2. b) Is it a general or characteristic property?
3. c) How would this property help you determine which of the two liquids is in a bottle that has lost its label?

The teacher heats an unknown white substance and observes that it melts at exactly $80\ \celsius$. The laboratory table lists these melting points: sugar, $186\ \celsius$; naphthalene, $80\ \celsius$; table salt, $801\ \celsius$.

1. a) Which of the three substances is the unknown one?
2. b) Explain why the melting point helps to identify it, whereas the white color does not.

A substance has a melting point of $-10\ \celsius$ and a boiling point of $60\ \celsius$, as shown in the diagram.

1. a) In which state of matter is it at $20\ \celsius$?
2. b) What about at $-20\ \celsius$?
3. c) What about at $80\ \celsius$?
4. d) Justify each answer by comparing the temperature with the two characteristic points, and explain why these points can be used to predict the state of the substance at any temperature.

With a coin, you can scratch the wood of a pencil, but no matter how much you rub the wood against the coin, you cannot scratch it.

1. a) Which characteristic property are you comparing with this test?
2. b) Which of the two materials has a higher value for it? Explain how you know.

Frying pans are made of metal because metal conducts heat from the stove very well to food, but the handle is usually made of wood or plastic, as shown in the diagram.

Explain, using the term 'thermal conductivity', why the frying pan and handle are not made from a single piece of metal.

At a mineral museum they let you do a test with three samples: mineral A scratches mineral B, and mineral C scratches mineral A. In addition, the sign on the display case tells you that the hardness of mineral C on the Mohs scale is 7 (for comparison, window glass has a hardness of 5.5).

1. a) Order the three minerals from softest to hardest.

1. b) Explain the reasoning you have followed.

1. c) What would happen to a window glass if you rubbed it with mineral C? Justify it by comparing their hardnesses.

In winter, walking barefoot on the marble bathroom floor feels much colder than walking on the hallway carpet, even though both are at the same temperature.

1. a) Which of the two materials has higher thermal conductivity?
2. b) Using that fact, explain what happens to the heat from your foot in each case and why the sensation is so different.

Good kitchen knives are made of steel and not aluminum, which is a much lighter metal.

1. a) Which characteristic property makes steel a better material for a knife?
2. b) Explain why, in this case, lightness (a small mass) is not the deciding property.
3. c) Give another example of an everyday object for which the material is chosen because of that same property you indicated in part a).

In Design and Technology, you set up a circuit with a battery and a bulb: if you complete the circuit with a conducting material, the bulb lights up, and if it is insulating, it does not, as shown in the diagram.

You test four objects: an iron key, an eraser, pencil lead (graphite), and a plastic ruler.

1. a) Predict with which ones the bulb will light up.
2. b) Classify the four materials into conductors and insulators according to their electrical conductivity.
3. c) Explain why electricians use screwdrivers with plastic handles and what would happen if the handle were made of metal.

A brand is designing the glass for a sports watch and is choosing between three materials: plastic (very cheap, scratches easily), glass (hardly scratches, but can break with a hard impact), and synthetic sapphire (almost impossible to scratch, very expensive).

1. a) Which properties of the materials appear in this decision? Indicate which ones are characteristic.
2. b) Choose a material for the watch of a $10$-year-old child who wears it to play in the playground, and justify your choice.
3. c) Which material would you choose for a luxury watch? Justify your choice by comparing the properties.

At a street market, a seller claims a bracelet is made of silver. Salma weighs it at home: $31\ \text{g}$. It turns out that a steel teaspoon from her kitchen also weighs $31\ \text{g}$.

1. a) Explain why the mass does not help Salma check whether the bracelet is made of silver.
2. b) The density of silver is $10.5\ \text{g/cm}^3$ and that of steel is $7.9\ \text{g/cm}^3$. Describe which two measurements Salma must take and what calculation she must perform to check this.

A laboratory bottle has lost its label and contains a colourless liquid. You heat it and it boils at $78\ \celsius$. The candidates are: water (boiling point $100\ \celsius$), alcohol ($78\ \celsius$) and acetone ($56\ \celsius$).

1. a) Which substance does it appear to be, based on its boiling point?
2. b) You measure its density and get $0.79\ \text{g/cm}^3$; that of alcohol is $0.79\ \text{g/cm}^3$ and that of acetone is $0.78\ \text{g/cm}^3$. Does this measurement resolve the doubt between alcohol and acetone? Justify your answer.
3. c) Which property turned out to be decisive in this case, and why is it advisable to measure more than one characteristic property to identify a substance?

In a storehouse, there are two identical closed bottles, one full of water and the other full of oil, but they have lost their labels and cannot be opened.

1. a) Which of the two will feel heavier when picked up?
2. b) Justify your answer using their densities (water, $1\ \text{g/cm}^3$; oil, $0.9\ \text{g/cm}^3$) and the fact that both bottles contain the same volume of liquid.
3. c) Why is looking at the size of the bottles not helpful in deciding?

An unlabelled piece of shiny metal arrives at the laboratory. You perform three tests and obtain: density $7.9\ \text{g/cm}^3$; melting point $1535\ \celsius$; conducts electric current very well. The candidates are: aluminum ($2.7\ \text{g/cm}^3$, $660\ \celsius$), iron ($7.9\ \text{g/cm}^3$, $1535\ \celsius$) and lead ($11.3\ \text{g/cm}^3$, $327\ \celsius$).

1. a) Which metal is the piece?
2. b) Explain which candidates, if any, are ruled out by each of the three tests.
3. c) Reason why it is advisable to check several characteristic properties instead of settling for just one.

Your young cousin claims that air 'is not matter, because it weighs nothing'.

1. a) Design an experiment using two balloons and a balance to show him that air has mass: describe what you would do and what he would observe.
2. b) Describe another simple experiment that demonstrates that air occupies volume (think, for example, of an upside-down glass submerged in a basin of water).
3. c) Conclude by explaining why air is matter, using the concept of general properties.