1.7 Physical and Chemical Properties

Physical Property and Physical Change

Have you ever wondered why ice floats in your drink or why some metals stick to magnets? These behaviors are all about physical properties – the special characteristics that make each substance unique. Physical properties are like a substance’s ID card, telling us things like its color, how hard it is, or whether it conducts electricity. Some of these properties, like color, are easy to spot just by looking. Others, like melting point, only show up when we change the substance’s state – think of ice cream melting on a hot day! Scientifically speaking, a physical property is a characteristic of matter that could be observed or measured without a change in its chemical composition.

Now, let’s talk about physical changes. Imagine you’re making a snowman. When the snow melts, it turns into water – that’s a physical change! The water is still H₂O, just in a different form. A physical change is a change in the state or properties of matter without any accompanying change in the chemical identities of the substances contained in the matter. Physical changes are observed when wax melts, when sugar dissolves in coffee, and when steam condenses into liquid water (Figure 1). Other examples of physical changes include magnetizing and demagnetizing metals (as is done with common antitheft security tags) and grinding solids into powders (which can sometimes yield noticeable changes in color). In each of these examples, there is a change in the physical state, form, or properties of the substance, but no change in its chemical composition.

Figure 1. (a) Wax undergoes a physical change when solid wax is heated and forms liquid wax. (b) Steam condensing inside a cooking pot is a physical change, as water vapor is changed into liquid water. (credit a: modification of work by “95jb14”/Wikimedia Commons; credit b: modification of work by “mjneuby”/Flickr)

Chemical Property and Chemical Change

In addition to physical properties, substances have another set of characteristics called chemical properties. These are all about how a substance can change into something completely different – or how stubbornly it refuses to change!

Think of chemical properties as a substance’s “personality traits.” Some substances are real daredevils, ready to transform at a moment’s notice. Others are more like couch potatoes, hardly reacting to anything. Here are some examples:

1. Flammability: This is why we’re careful with gasoline but not so much with water.
2. Toxicity: It’s the reason we can drink water but not bleach.
3. Reactivity: Some substances are social butterflies, always ready to mingle and change. Others are wallflowers, keeping to themselves.

Chemical properties could be closely connected to our life. Iron, for example, combines with oxygen in the presence of water to form rust; chromium does not oxidize (Figure 2). Nitroglycerin is very dangerous because it explodes easily; neon poses almost no hazard because it is very unreactive.

Figure 2. (a) One of the chemical properties of iron is that it rusts; (b) one of the chemical properties of chromium is that it does not. (credit a: modification of work by Tony Hisgett; credit b: modification of work by “Atoma”/Wikimedia Commons)

Now, let’s dive into the world of chemical changes. Unlike physical changes, chemical changes create brand new substances. It’s like magic, but it’s science! A chemical change occurs when one type of matter transforms into a different type of matter. This transformation creates new substances with different properties. Let’s explore some everyday examples:

• Rust Formation: When iron objects are exposed to air and water, they form rust. Rust is a new substance, different from the original iron.
• Cooking Food: When you cook an egg, the clear, runny egg white becomes white and solid. This is a chemical change!
• Burning Wood: When wood burns, it turns into ash and releases gases. These are new substances, very different from the original wood.
• Digesting Food: Our bodies chemically change the food we eat into nutrients we can use.

Most chemical changes are not as dramatic as exploding fireworks, so how can you tell whether a chemical change has occurred? There are usually clues to look out for:

• Bubbles forming
• Color changes
• New odors appearing
• Heat or light being released
• Loud sounds (in some cases)

Remember, these clues don’t always mean a chemical change has occurred, but they’re good indicators to start investigating!

Figure 3. (a) Copper and nitric acid undergo a chemical change to form copper nitrate and brown, gaseous nitrogen dioxide. (b) During the combustion of a match, cellulose in the match and oxygen from the air undergo a chemical change to form carbon dioxide and water vapor. (c) Cooking red meat causes a number of chemical changes, including the oxidation of iron in myoglobin that results in the familiar red-to-brown color change. (d) A banana turning brown is a chemical change as new, darker (and less tasty) substances form.

Can Chemical Changes Be Reversed?

Because chemical changes produce new substances, they often cannot be undone. For example, you can’t change ashes from burning logs back into wood. Some chemical changes can be reversed, but only by other chemical changes. For example, to undo tarnish on copper pennies, you can place them in vinegar. The acid in the vinegar combines with the copper oxide of the tarnish. This changes the copper oxide back to copper and oxygen, making the pennies reddish brown again. You can try this at home to see how well it works.

Extensive vs. Intensive Properties

Properties of matter fall into one of two categories. If the property depends on the amount of matter present, it is an extensive property. The mass and volume of a substance are examples of extensive properties; for instance, a gallon of milk has a larger mass than a cup of milk. The value of an extensive property is directly proportional to the amount of matter in question. If the property of a sample of matter does not depend on the amount of matter present, it is an intensive property. Other intensive properties include color, temperature, density, and solubility.

Figure 4. Differences between extensive and intensive properties

CHEMISTRY IN EVERYDAY LIFE

Hazard Diamond

You may have seen the symbol shown in Figure 5. on containers of chemicals in a laboratory or workplace. Sometimes called a “fire diamond” or “hazard diamond,” this chemical hazard diamond provides valuable information that briefly summarizes the various dangers of which to be aware when working with a particular substance.

Figure 5. The National Fire Protection Agency (NFPA) hazard diamond summarizes the major hazards of a chemical substance.

The National Fire Protection Agency (NFPA) 704 Hazard Identification System was developed by NFPA to provide safety information about certain substances. The system details flammability, reactivity, health, and other hazards. Within the overall diamond symbol, the top (red) diamond specifies the level of fire hazard (temperature range for flash point). The blue (left) diamond indicates the level of health hazard. The yellow (right) diamond describes reactivity hazards, such as how readily the substance will undergo detonation or a violent chemical change. The white (bottom) diamond points out special hazards, such as if it is an oxidizer (which allows the substance to burn in the absence of air/oxygen), undergoes an unusual or dangerous reaction with water, is corrosive, acidic, alkaline, a biological hazard, radioactive, and so on. Each hazard is rated on a scale from 0 to 4, with 0 being no hazard and 4 being extremely hazardous.

While many elements differ dramatically in their chemical and physical properties, some elements have similar properties. For example, many elements conduct heat and electricity well, whereas others are poor conductors. These properties can be used to sort the elements into three classes: metals (elements that conduct well), nonmetals (elements that conduct poorly), and metalloids (elements that have intermediate conductivities).