Author Archives: Anne Helmenstine

Why Milk Curdles

Milk curdles when its pH drops, allowing protein molecules in milk to clump together. (Tess Watson)

Milk curdles when its pH drops, allowing protein molecules in milk to clump together. (Tess Watson)

Curdled milk is what you get when lumps form in smooth milk. Although the clumps form in spoiled milk, the chemical reaction that causes curdling also occurs in fresh milk, under the right conditions. Intentional curdling of milk is used to produce foods, such as yogurt, cheese, and buttermilk. Here’s a closer look at how curdling happens:

Curdling Chemical Reaction

Fresh milk is an example of a colloid, consisting of fat and protein particles floating in a water-based solution. The colloidal suspension scatters light, causing milk to appear white. The protein molecules, mainly casein, repel each other so they naturally distribute evenly through the liquid. Milk is slightly acidic. When the pH is lowered even more by the addition of another acidic ingredient, the protein molecules stop repelling each other. This allows them to stick together or coagulate into the clumps known as curds. The watery liquid that remains is called whey.

How Sour Milk Curdles

When milk goes “off” or turns sour, it is because acids produced by bacteria lower the pH of milk so the proteins can clump together. The increased acidity of the milk also causes it to taste sour. The bacteria living in milk naturally produce lactic acid as they digest lactose so they can grow and reproduce. This occurs whether milk is fresh or pasteurized. You won’t notice the effect on flavor until enough acid has been produced. Refrigerating milk slows the growth of bacteria. Similarly, warm milk helps bacteria thrive and also increases the rate of the clumping reaction.

Curdling Milk in Coffee and Tea

If you enjoy milk in your coffee or tea, you may have noticed sometimes milk immediately curdles when added to the hot beverage. Except for the chunkiness, the drink may taste perfectly fine. This is because coffee and tea contain just enough acidity to tip the pH of milk to the point of curdling. The effect is most often seen in milk that is close to going sour or when adding milk to very hot coffee or tea, since the high temperature can coagulate casein.

Intentional Curdling of Milk

No one wants to drink chunky milk straight from the fridge, but the chemical reaction that causes curdling isn’t always bad. The same reaction produces buttermilk, cheese, and yogurt.

Adding lemon juice or vinegar to fresh milk is an easy way to make homemade buttermilk. Why isn’t buttermilk clumpy? It would be, if you added the acidic ingredient to hot milk. However, adding acid to cold milk allows casein to coagulate more slowly. Rather than forming clumps, the chemical reaction simply thickens the liquid. The ingredient also affects the flavor of the buttermilk, adding a tangy note.

Yogurt and cheese are slightly more complicated because you usually control the type of bacteria (the bacterial culture) used to make a product with a pleasing flavor and texture. However, fresh cheeses, such as ricotta, is very simply made by heating milk, adding an acidic ingredient, and straining the curd.

Color Change Violets Chemistry Demonstration

The color change violets project works because violets are natural pH indicators.  (Robert & Pat Rogers)

The color change violets project works because violets are natural pH indicators. (Robert & Pat Rogers)

You can change violets from violet to green by exposing them to a household chemical. This is a fun chemistry demonstration, especially for the home.

Color Change Violet Materials

  • violets
  • household ammonia

Change the Color!

Really, nothing could be easier. Hold violets over ammonia and watch them change from blue or violet to green. You can speed the reaction if you use warmed ammonia or you can dip the flowers in the ammonia. The demo used to be performed as a sort of ‘trick’ using ammonia gas, which produces a quick and dramatic color change. With ordinary household ammonia, you can expect the color change to take a couple of minutes.

How It Works

Flowers, including violets, get their color from anthocyanins. This is a class of pigment molecules that changes color in response to pH. Ammonia is very basic, so it changes the blue color of violets to green. Violets come in other colors besides… well… violet, so expect a color change for these flowers too. The final color might not be green. For example, I tried this project using orange pansies (a form of violet) and got sort of a dull greenish-yellow color as a final result. Also, you can experiment with other flowers to get a color change in response to ammonia.

Learn More

Odor of Violets Chem Demo
Make a pH Rainbow Tube

Actinium Facts – Atomic Number 89 or Ac

These are vials of glowing actinium-225. Oak Ridge National Laboratory, US Department of Energy

These are vials of glowing actinium-225. Oak Ridge National Laboratory, US Department of Energy

Element Name: Actinium

Symbol: Ac

Atomic Number: 89

Atomic Weight: 227.0278

Electron Configuration: [Rn] 6d1 7s2

Element Classification: Radioactive Rare Earth (Actinide Series)

Discovery: Andre-Louis Debierne 1899 (France)

Word Origin: Greek: akis, aktinos (ray).

Melting Point (K): 1320

Boiling Point (K): 3470

Appearance: heavy, silvery-white metal that is highly radioactive – glows with a pale blue light

Atomic Radius (pm): 188

Atomic Volume (cc/mol): 22.54

Ionic Radius: 118 (+3e)

Fusion Heat (kJ/mol): (10.5)

Evaporation Heat (kJ/mol): (292.9)

Pauling Negativity Number: 1.1

First Ionizing Energy (kJ/mol): 665.5

Oxidation States: 3

Lattice Structure: Face-Centered Cubic

Lattice Constant (Å): 5.310

References: Los Alamos National Laboratory (2001), Crescent Chemical Company (2001), Lange’s Handbook of Chemistry (1952), CRC Handbook of Chemistry & Physics (18th Ed.)

 

Americium Facts – Element 95 or Am

This is a disk of americium-241 viewed under a microscope. Bionerd, Creative Commons License

This is a disk of americium-241 viewed under a microscope. Bionerd, Creative Commons License

Element Name: Americium

Atomic Number: 95

Symbol: Am

Atomic Weight: 243.0614

Electron Configuration: [Rn] 5f7 7s2

Atomic Number: 95

Element Classification: Radioactive Rare Earth Element (Actinide Series)

Discovered By: G.T.Seaborg, R.A.James, L.O.Morgan, A.Ghiorso

Discovery Date: 1945 (United States)

Name Origin: named for the American continent, similar to the naming of the element europium.

Density (g/cc): 13.67

Melting Point (K): 1267

Boiling Point (K): 2880

Appearance: silvery-white, radioactive metal

Atomic Radius (pm): 173

Atomic Volume (cc/mol): 20.8

Ionic Radius: 92 (+4e) 107 (+3e)

Fusion Heat (kJ/mol): (10.0)

Evaporation Heat (kJ/mol): 238.5

Pauling Negativity Number: 1.3

Oxidation States: 6, 5, 4, 3

References: Los Alamos National Laboratory (2001), Crescent Chemical Company (2001), Lange’s Handbook of Chemistry (1952), CRC Handbook of Chemistry & Physics (18th Ed.)

 

Bohrium Facts – Element 107 or Bh

Element 107 - Bohrium

Element 107 – Bohrium

Element Name: Bohrium (formerly Nielsbohrium)

Atomic Number: 107

Symbol: Bh

Atomic Weight: [264]

Discovery: Dubna (Russia) 1975 or Heavy Ion Research Lab (Germany) 1976

Electron Configuration: [Rn] 7s2 5f14 6d5

Word Origin: Formerly Ns, Nielsbohrium. Named in honor of Niels Bohr.

Sources: Bohrium is a man-made element. In 1976, Soviet scientists at Dubna announced they had synthesized element 107 by bombarding Bi-204 with heavy nuclei of Cr-54. There are reports that the element was ‘glimpsed’ for 2/1000 s in 1975. Bohrium was produced by bombarding a rapidly rotating cylinder target, coated with a thin layer of bismuth metal, with a stream of tangentially fired Cr-54 ions. West German physicists at the Heavy Ion Research Laboratory at Darmstadt verified the existence of element 107, creating and identifying six nuclei of Bohrium.

Element Classification: Transition Metal

Appearance: radioactive synthetic metal

References: Los Alamos National Laboratory (2001), Crescent Chemical Company (2001), Lange’s Handbook of Chemistry (1952), CRC Handbook of Chemistry & Physics (18th Ed.)

 

Dysprosium Facts – Element 66 or Dy

Dysprosium is a silver-colored rare earth element. This is a chunk of purified dysprosium metal. Tomihahndorf

Dysprosium is a silver-colored rare earth element. This is a chunk of purified dysprosium metal. Tomihahndorf

Element Name: Dysprosium

Atomic Number: 66

Symbol: Dy

Atomic Weight: 162.50

Discovered By: Paul-Emile Lecoq de Boisbaudran

Discovery Date: 1886 (France)

Electronic Configuration: [Xe] 4f10 6s2

Atomic Weight: 162.50

Element Classification: Rare Earth (Lanthanide Series)

Name Origin: Greek: dysprositos (hard to get at)

Density (g/cc): 8.55

Melting Point (°K): 1685

Boiling Point (°K): 2835

Appearance: soft, lustrous, silvery metal

Atomic Radius (pm): 180

Atomic Volume (cc/mol): 19.0

Covalent Radius (pm): 159

Ionic Radius: 90.8 (+3e)

Specific Heat (@20°C J/g mol): 0.173

Evaporation Heat (kJ/mol): 291

First Ionizing Energy (kJ/mol): 567

Oxidation States: 3

Lattice Structure: Hexagonal

Lattice Constant (Å): 3.590

Lattice C/A Ratio: 1.573

Sources: Los Alamos National Laboratory (2001), Crescent Chemical Company (2001), Lange’s Handbook of Chemistry (1952)

Return to the Periodic Table

Mendelevium Facts – Element 101 or Md

Mendelevium is a radioactive metal.

Mendelevium is a radioactive metal.

Element Name: Mendelevium

Atomic Number: 101

Symbol: Md

Atomic Weight: 258.1

Discovery: G.T. Seaborg, S.G. Tompson, A. Ghiorso, K. Street Jr. (1955, United States)

Electron Configuration: [Rn] 7s2 5f13

Word Origin: Named in honor of Dmitri Mendeleev, who devised a periodic table of the elements

Isotopes: Fourteen isotopes have been recognized.

Properties: Mendelevium is the ninth transuranium element of the actinide series. Experiments indicate that mendelevium has a moderately stable dipositive (II) oxidation state in addition to the tripositive (III) oxidation state, as is characteristic of the actinide elements.

Uses: Md-256 has been used to determine some of the chemical properties of mendelevium in aqueous solution.

Sources: Mendelevium first was produced during the bombardment of Es-253 with helium ions in the Berkeley 60-inch cyclotron. The isotope produced was Md-256, with a half-life of 76 min. Md-256 was synthesized on a one-atom-at-a-time basis.

Element Classification: Radioactive Rare Earth Element (Actinide Series)

Melting Point (K): 1100

Appearance: Radioactive, synthetic metal

Atomic Radius (pm): 287

Pauling Negativity Number: 1.3

First Ionizing Energy (kJ/mol): (635)

Oxidation States: 3

References: Los Alamos National Laboratory (2001), Crescent Chemical Company (2001), Lange’s Handbook of Chemistry (1952), CRC Handbook of Chemistry & Physics (18th Ed.)