Category Science

WHAT IS DENSITY?

The amount of space that a substance takes up is called its volume. It is measured in cubic units. For example, a cube measuring one metre on each side has a volume of one cubic metre or 1m3. But a cubic metre of lead has a much greater mass than a cubic metre of wood. That is because the lead has a much higher density than the wood. Its particles are more tightly packed together. The density of an object is calculated by dividing its mass by its volume and is expressed as kilograms per cubic metre (kg/m3) or pounds per cubic foot (lb/ft3).

Density is a measure of how compact the mass in a substance or object is. The density of an object or substance can be calculated from this equation: density in kilograms per meter cubed is equal to mass in kilograms, divided by volume in meters cubed. Or in other words, density is mass spread out over a volume. Or in other, other words, it’s the number of kilograms that 1 meter cubed of the substance weights. If each meter cubed weighs more, the substance is denser.

As we’ll discuss in other lessons, density is super important because it relates to whether things rise or sink. Less dense materials tend to rise above more dense materials, particularly in the case of liquids and gases. So understanding density has major implications for the motions of materials and gases in the atmosphere and objects floating (or sinking) in water. Density is the reason some objects sink and other objects float. And it’s the reason that some clouds are high in the sky, while others are low down.

Density means that if you take two cubes of the same size made out of different materials and weigh them, they usually won’t weigh the same. It also means that a huge cube of Styrofoam can weigh the same as a tiny cube of lead.

Examples of dense materials include iron, lead, or platinum. Many kinds of metal and rock are highly dense. Dense materials are more likely to ‘feel’ heavy or hard. Although a sparse material (sparse is the opposite of dense) can feel heavy if it’s really big. Examples of sparse materials would be Styrofoam, glass, soft woods like bamboo, or light metals like aluminum.

In general, gases are less dense than liquids and liquids are less dense than solids. This is because solids have densely-packed particles, whereas liquids are materials where particles can slide around one another, and gases have particles free to move all over the place.

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WHAT IS MASS?

The mass of a substance is the amount of matter it contains. This is different from its weight, which is a measurement of the pull of gravity on this mass. For example, an astronaut would have the same mass on Earth as on the Moon, but his weight would be much less in the Moon’s gravity than in the Earth’s.

We use the word mass to talk about how much matter there is in something. (Matter is anything you can touch physically.) On Earth, we weigh things to figure out how much mass there is. The more matter there is, the more something will weigh. Often, the amount of mass something has is related to its size, but not always. A balloon blown up bigger than your head will still have less matter inside it than your head (for most people, anyhow) and therefore less mass.

The difference between mass and weight is that weight is determined by how much something is pulled by gravity. If we are comparing two different things to each other on Earth, they are pulled the same by gravity and so the one with more mass weighs more. But in space, where the pull of gravity is very small, something can have almost no weight. It still has matter in it, though, so it still has mass.

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IS HEATING THE ONLY WAY TO CAUSE CHANGES OF STATE?

As well as heating or cooling, changing the pressure acting on a substance can also cause it to change state. If the pressure on the molecules in a substance is increased, it becomes harder for them to move apart from each other, so the temperature at which they become a liquid is increased. Similarly, at low pressure, changes happen at lower temperatures. It is impossible to make a good cup of tea or coffee’ at the top of Everest, for. example, because water boils at a temperature almost 30°C (50°F) less than at sea level.

All matter can move from one state to another. It may require extreme temperatures or extreme pressures, but it can be done. Sometimes a substance doesn’t want to change states. You have to use all of your tricks when that happens. To create a solid, you might have to decrease the temperature by a huge amount and then add pressure. For example, oxygen (O2) will solidify at -361.8 degrees Fahrenheit (-218.8 degrees Celsius) at standard pressure. However, it will freeze at warmer temperatures when the pressure is increased.

Some of you know about liquid nitrogen (N2). It is nitrogen from the atmosphere in a liquid form and it has to be super cold to stay a liquid. What if you wanted to turn it into a solid but couldn’t make it cold enough to solidify? You could increase the pressure in a sealed chamber. Eventually you would reach a point where the liquid became a solid. If you have liquid water (H2O) at room temperature and you wanted water vapor (gas), you could use a combination of high temperatures or low pressures to solve your problem.

One winter day, you sit by a window inside your warm home. You watch the snow pile up on the ground. You see small animals slide across a frozen pond in your backyard. You can see their hot breath as steam clouds in the cold air. You are drinking a cup of cocoa. You see steam rising from the mug, and you know it is too hot to drink. So you add an ice cube to the cup and wait for the melting ice to cool your cocoa. Solids, liquids, and gases are all around you. The solid ice in the pond, the liquid cocoa, and the steamy air are different states of matter. What is matter? How are solids, liquids, and gases different? Why did the solid ice cube melt into liquid when you put it into your cocoa?

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WHAT HAPPENS WHEN MOLECULES ARE HEATED?

When molecules are heated, they gain heat energy in addition to the kinetic energy they already have. If the molecules in a solid gain enough energy, they can break free of each other and become liquid. This is called melting. If they gain even more heat energy, the liquid becomes a gas.

Water freezes, or becomes solid, at temperature of 0oC (32oF) or below. If the temperature outside drops to this level, the water on the surface of ponds and lakes will freeze, although the water below may hold enough heat to remain liquid.

When solid water (ice) is heated, it melts to become liquid. Generally speaking, we think of water as being liquid at a “room temperature” of 200C (68oF), or, in other words, under normal conditions, Copper, however, is a solid under such conditions, because it needs a temperature of 1083oC (1981oF) to melt into a liquid.

When water is heated and boils, it turns into a gas. We can see this when a kettle boils. In fact, it is not the billowing steam that is the gas – that is the water turning back in tiny droplets of liquid as it comes into contact with cool air. The real steam is invisible. It can be “seen” in the gap between the spout of the kettle and the visible vapour.

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WHAT ARE MOLECULES?

In nature, it is rare to find one atom on its own. Atoms are usually grouped together in larger structures called molecules. A molecule is the smallest particle of a substance that can exist by itself. The atoms in a molecule are chemically bonded together. They may be atoms of the same element or they may be of different elements. A molecule of carbon dioxide, for example, has two atoms of oxygen and one of carbon.

For millennia, scientists have pondered the mystery of life – namely, what goes into making it? According to most ancient cultures, life and all existence was made up of the basic elements of nature – i.e. Earth, Air, Wind, Water, and Fire. However, in time, many philosophers began to put forth the notion that all things were composed of tiny, indivisible things that could neither be created nor destroyed (i.e. particles).

However, this was a largely philosophical notion, and it was not until the emergence of atomic theory and modern chemistry that scientists began to postulate that particles, when taken in combination, produced the basic building blocks of all things. Molecules, they called them, taken from the Latin “moles” (which means “mass” or “barrier”). But used in the context of modern particle theory, the term refers to small units of mass.

By its classical definition, a molecule is the smallest particle of a substance that retains the chemical and physical properties of that substance. They are composed of two or more atoms, a group of like or different atoms held together by chemical forces.

It may consist of atoms of a single chemical element, as with oxygen (O2), or of different elements, as with water (H2O). As components of matter, molecules are common in organic substances (and therefore biochemistry) and are what allow for life-giving elements, like liquid water and breathable atmospheres.

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HOW DO SOLIDS, LIQUIDS AND GASES DIFFER?

Many substances can exist in three different states of matter: as solids, liquids and gases. In each state, the substance has the same chemical make-up — the elements in its molecules have not changed, but the way in which they are connected to each other has. Scientists think of all matter as being constantly in motion. The atoms and molecules of which it is made have energy, called kinetic energy.

Solids have a set shape and volume. In a solid, the particles are more closely arranged. The particles have less net movement. Solids are rigid, meaning that the particles are “locked” into place. Solids are not easily compressible, given that they have a set volume. Solids do not flow easily, because the particles are rigidly connected and do not move past one another. In solids, the energy is not strong enough for the particles to break free of the attraction they have for each other. It is as though they are vibrating but not moving from their positions.

Liquids have a set volume, but they can change shape depending on what container they are in. The particles in a liquid move more freely within the set volume. Liquids do not form structures like solids, because of the intermolecular movement. Liquids are also not easily compressible, because of the set volume. Liquids flow easily because the particles are able to move past each other easily. The molecules in a liquid have more energy and can move away from neigh-boring molecules, so that a liquid will flow to cover as wide an area as it can.

Gases can change shape and volume. Gas particles will spread out as far as they need to in order to fill the container. They will take the shape and volume of whatever container they’re in. They are easy to compress because there is a lot of space between the particles. It also flows easily because of the amount of space between particles. The molecules of a gas have most kinetic energy. They will move apart from each other until they fill the space in which they are contained.

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