Category Chemistry

Elements do not usually exist on their own. In the natural world, they are found in combination with other elements. By understanding how elements combine, scientists have been able to make new combinations, creating molecules that are not found in nature. These combinations are not made simply by mixing two or more substances together. Brown sugar and salt can be stirred together, for example, but this does not create a new substance. Each little particle is either a grain of sugar or a grain of salt — they have remained separate. Mixtures can usually be separated again, but when elements are chemically joined together, they are said to be bonded and have created a new substance.

Bonding is caused by a chemical reaction. Most chemical reactions need some form of energy to start them. Usually, this energy is supplied in the form of heat. Many compounds are made by heating two or more substances together until their molecules are moving so fast that they react with each other.

Energy plays a key role in chemical processes. According to the modern view of chemical reactions, bonds between atoms in the reactants must be broken, and the atoms or pieces of molecules are reassembled into products by forming new bonds. Energy is absorbed to break bonds, and energy is evolved as bonds are made. In some reactions the energy required to break bonds is larger than the energy evolved on making new bonds, and the net result is the absorption of energy. Such a reaction is said to be endothermic if the energy is in the form of heat. The opposite of endothermic is exothermic; in an exothermic reaction, energy as heat is evolved. The more general terms exoergic (energy evolved) and endoergic (energy required) are used when forms of energy other than heat are involved.

 A great many common reactions are exothermic. The formation of compounds from the constituent elements is almost always exothermic. Formation of water from molecular hydrogen and oxygen and the formation of a metal oxide such as calcium oxide (CaO) from calcium metal and oxygen gas are examples. Among widely recognizable exothermic reactions is the combustion of fuels (such as the reaction of methane with oxygen mentioned previously).

The formation of slaked lime (calcium hydroxide, Ca (OH)2) when water is added to lime (CaO) is exothermic. This reaction occurs when water is added to dry Portland cement to make concrete, and heat evolution of energy as heat is evident because the mixture becomes warm.

Not all reactions are exothermic (or exoergic). A few compounds, such as nitric oxide (NO) and hydrazine (N2H4), require energy input when they are formed from the elements. The decomposition of limestone (CaCO3) to make lime (CaO) is also an endothermic process; it is necessary to heat limestone to a high temperature for this reaction to occur. The decomposition of water into its elements by the process of electrolysis is another endoergic process. Electrical energy is used rather than heat energy to carry out this reaction.

Generally, evolution of heat in a reaction favours the conversion of reactants to products. However, entropy is important in determining the favorability of a reaction. Entropy is a measure of the number of ways in which energy can be distributed in any system. Entropy accounts for the fact that not all energy available in a process can be manipulated to do work.

A chemical reaction will favour the formation of products if the sum of the changes in entropy for the reaction system and its surroundings is positive. An example is burning wood. Wood has low entropy. When wood burns, it produces ash as well as the high-entropy substances carbon dioxide gas and water vapour. The entropy of the reacting system increases during combustion. Just as important, the heat energy transferred by the combustion to its surroundings increases the entropy in the surroundings. The total of entropy changes for the substances in the reaction and the surroundings is positive, and the reaction is product-favoured.

When we cook food, chemical reactions take place as het energy is supplied to the ingredients. New compounds are formed, so that the cooked dish usually has a different appearance, texture and taste from the mixed raw ingredients.

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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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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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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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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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