1-2: Properties of Matter

Physical vs. Chemical Properties

How would you describe this object?

Here are a few of its properties, or defining traits/things about it:

• it's a solid

• it's dark grey in most places, but has some other colours on it too

• it's a few centimetres long, and a couple of centimetres wide

• if you put a drop of hydrochloric acid on it, nothing will happen

• if you tried to scratch it with your fingernail, your fingernail would get worn down but the object wouldn't

There are two main ways you can describe an object or substance:

Physical properties: how an object or substance acts on its own, without forming any new materials

Chemical properties: how an object or substance combines (or reacts) with others to make new materials

As you can imagine, given the short bullet-point list above, there are many different properties we can use when describing something. A few properties are listed below; you might have seen some of these before. Some of them are obvious, but some really aren't.

Some Physical Properties

Colour: This one's one of the obvious ones.

State: Is it a solid, liquid or gas? (There are more states of matter than this, but these are the big three we usually talk about.)

Hardness: There's a scale of 1 to 10, called the Mohs hardness scale, which has many common solids on it.

Malleability: A substance's ability to be hammered/shaped into a thin sheet. Gold is so malleable, one gram of it can be made into a sheet which is a square metre (1 m × 1 m)!

Solubility: The ability of a substance to dissolve in a liquid, usually water. Salt is very soluble in water, but pepper is not.

Ductility: The ability to be drawn out into a long, thin wire. It's similar to malleability, but not exactly the same. Copper is great at this.

Electrical conductivity: Can electricity easily pass through a substance? Copper is also great at this.

Melting point: The temperature at which a solid changes into a liquid. Note that this is not the same as dissolving! (That is a common mistake and misconception.) Gallium is an element that has a very low melting point, around 30°C, so if you have warm hands they will melt solid gallium into liquid gallium.

Boiling point: Similar to melting point, the temperature at which a liquid changes into a gas. Under normal conditions, water boils at 100°C.

Some Chemical Properties

Reactivity with acid: Some substances, when you mix them with certain kinds of acid, will react somehow, either by turning colour or giving off gas bubbles. Here's a video showing calcium carbonate bubbling when hydrochloric acid is added. It goes into a lot of extra detail here which you'll only really learn about if you take Grade 11 Chemistry, but it shows the chemical reaction pretty well.

Reactivity with oxygen: Whether it's quickly in an explosion or quietly over years, some substances combine with oxygen in the air to make something new. Below are steel nails that have rusted: the rust is a new compound of iron in the steel, plus oxygen in the air.

Flammability: Related to the above, this describes how well something burns in oxygen in the presence of heat. Some things, like matches, you want to be flammable; some things, like materials making up a mattress, should be as non-flammable as possible. Below, a mattress is tested for its flammability.

Qualitative vs. Quantitative Properties

Some of the above properties can only be described in words, and not numbers. If someone asked you what colour the sky was, answering with "42" wouldn't make any sense.

But if you wanted to know how soluble a substance was in one litre of water, "you can dissolve sort-of a lot" isn't precise enough. You'd want to know how many grams of the substance you can dissolve in that water.

This means we can also split properties into two other types of categories: qualitative properties don't use numbers, but quantitative properties do use numbers. (Think about the word quantity, which describes how many of something there are.)

Density

This giant block of Styrofoam on a construction site has a mass of 38 kg.

One of these small stones has a mass of 0.1 kg.

Yes, the block of Styrofoam has a larger mass. But there's something obviously different about each of these materials: their density, or the mass of a given volume of a substance.

Think of it this way: let's look at blocks of Styrofoam and stone which have the same volume.

Now that we have the same volume of them, we can see that their masses are very different! We can now easily see what their densities are, and we usually express them in one of two ways:

• if it's a small sample, we can talk about how many grams per cubic centimetre, g/cm³ the density is

• if it's a larger object, the unit of kilograms per cubic metre, kg/m³ is probably more useful to us

(For the record, people in chemistry usually use g/cm³, but people in physics usually use kg/m³. This isn't a hard-and-fast rule, though.)

Calculating Density

To calculate density, we use a simple formula:

As you can see, we use D for density, m for mass, and V for volume. (Always use the proper symbol, and don't use d instead of D; it matters!) In science, you always leave units in the calculations -- this is very different from math class, but it's a good habit to get into now.

Example: A block of wood has a volume of 9.3 cm³, and has a mass of 11.9 g. What is its density?

When you're doing calculations like this, try to follow these general ideas:

• Write out the given information first.

• Clearly state what you want to find in the end.

• Use the proper symbols, don't use words.

• Never forget the units. Numbers shouldn't just be on their own.

• Write out the formula you're going to use.

• If you need to, rearrange the formula so that what you want to find is on one side, and all your given information is on the other side.

• Substitute the numbers you know into the formula.

• Do the calculation, and round appropriately. (More on this later.)

• Include units in your final answer.

This might sound like a lot, but after a while all of this should become pretty automatic. Here's how you might want to solve this:

It's better to write the unit as a tops-and-bottoms fraction, rather than just as g/cm³. The fraction really helps to remind you that you're dividing one thing by another. If you only just write it out as g/cm³, it's easy to forget that. (However, it's much easier to type here, so you should be able to recognize both forms.)

Some teachers will ask you to have a concluding sentence, containing your final answer. Some will only just ask you to put a rectangular box around the final answer.

Significant Digits

This is an idea you will see a lot more in later grades. And it might take a little while to get the hang of thinking of numbers this way. But, we have to be able to answer the question, Where do you round off a number in an answer?

In the above worked-out example, when dividing 11.9 by 9.3, a calculator will give you 1.27956989. DO NOT EVER WRITE ALL OF THIS OUT!!! It's a waste of time, effort, and ink or pencil-lead.

You need to think about the number of significant digits that are in each of the values that went into the calculation:

• since m = 11.9 g, there are three significant digits: the 1, the other 1, and the 9

• since V = 9.3 m³, there are two significant digits: the 9 and the 3

These seem pretty easy, and they are. But keep in mind that if the mass was, let's say, 0.73 g, that would only have two significant digits, because the zero out in front doesn't count. (There are rules for all of this.)

We don't focus a lot on significant digits (SDs) in Grade 9 Science, but consider this general idea: round your final answer to the least number of significant digits of any value that goes into the calculation.

In the above calculation, there was a value with 3 SDs, and another with only 2. This means you round the final number to 2 SDs, because that's the lower number. (If they were both 3 SDs, then round the final answer to three.)

So now we look at the value our calculator gives us, 1.27956989. Again, never write this all out. But now we have to round it to 2 SDs:

• the 1 is our first SD, so that one stays

• the second SD is the next number down, and it's a 2

• however... since the next digit is a 7, and that's "5 or over," the 2 gets rounded up to a 3

• this means the final value, properly rounded, is 1.3

This will take some getting used-to, and it's different from math class where you're always asked to round to the nearest tenth, hundredth, or some other place-value. But while mathematics and science both use numbers a lot, they use them in very different ways.

All numbers in science come from measurements, and all measurements have some amount of uncertainty in them. The idea of significant digits takes this into account: if the mass is 11.9 g, that means you measured it on a scale that only gives you the mass to the nearest 0.1 g. The volume is 9.3 cm³, which means that it's somewhere between 9.25 cm³ and 9.34 cm³, if you knew it more precisely... but you don't.

We won't be spending a lot of time on this in Grade 9. If you're ever in doubt, just round to either 2 or 3 significant digits and you're probably somewhere close.

Rearranging Formulas and/or Formula Triangles

Example: The density of iron is 7.87 g/cm³, and you have a block of it with a volume of 5.6 cm³. What is its mass?

Given: D = 7.87 g/cm³, V = 5.6 cm³, m = ?

Now you run into a problem, because what you want to find isn't density, but it's buried inside the formla.

Before you solve this problem, you're either going to have to rearrange this formula to solve for mass, or use a Formula Triangle to get that new formula. The preferred way is to rearrange the formula (you learn this in Grade 9 Math), but if that's a lot of trouble, we can just use the Triangle.

First, the "proper" way, by rearranging:

Ultimately, you should be able to rearrange a formula like this to solve for either m or V. However, if you can't quite do that yet, you can use a device called a formula triangle which can help you along. For the density formula, the triangle looks like this:

Here's how to use it:

• figure out which of the three quantities you want to solve for

• put your finger over the quantity you want to solve for

• look at what the other two quantities in triangle look like... do they look like things beside each other that you'd multiply, or does it look like a fraction that you'd divide?

For example, say you wanted to solve for mass (m), like in the above question. Put your finger over the m and see what it looks like:

What's left looks like D V; in math, when two symbols are squished together, it means you mutliple them. This means you get m = DV or m = D × V (either is fine), and then you do the calculation as above.

Practice

The Basics

1. Make a chart with four columns, like the one below.

For the definitions, write them in your own words, and use as few words as possible. If your first language is something other than English, you may want to add another "Definition" column which is in that language. If you need to do some research to find other examples, use reliable sources.

2. Explain the difference between a qualitative and a quantitative property, and give an example of each.

3. Find the density of a block of plastic that has a volume of 26 cm³ and a mass of 31 g. (Hint: set up the calculation, but before you actually do the number-crunching, ask yourself... is this going to be bigger than 1, or less than 1? That should help guide you, especially if you punch it into a calculator incorrectly.)

4. Find the mass of a piece of sulphur that has a density of 2.07 g/cm³ and a volume of 18 cm³.

5. The density of gold is 19.3 g/cm³ (it's a very dense metal). A ring looks like gold, but it has a mass of 11.2 g and a volume of 0.66 cm³. Is the ring made of gold, or is it actually made of something else? Use a density calculation to help you answer this question.

Extensions

1. The density of solid water (ice) is less than that of liquid water. How did that change the fate of the Titanic?

2. Silver is also an extremely ductile element. So why aren't the wires in our walls made of silver, and not copper?