Gold is a precious metal that has been valued by humans for thousands of years. It has a distinctive yellowish color in its natural state that is familiar to most people. However, when gold is heated to high temperatures, its color can change dramatically. Understanding the science behind these color changes can reveal interesting facts about gold’s chemical properties.
Gold’s Natural Color
In its pure form, gold is a bright, shiny yellow color. This color is produced by the specific arrangement of gold’s electrons, which absorb blue and violet light while reflecting back yellow and red light. The particular yellow color that gold possesses is known as golden yellow or gold yellow. It has a hue angle of about 50 degrees on the color wheel.
Gold’s famous yellow color has led to its use for centuries in jewelry, art, decoration, and coinage. The Latin word for gold – aurum – even means “shining dawn,” referencing gold’s radiant yellow brilliance. Gold’s color stays constant and does not rust, tarnish, or change over time. This is just one of the attributes that has made gold so prized throughout human history.
Effects of Heat
When gold is heated to certain high temperatures, however, its color can shift noticeably. Gold is an excellent conductor of electricity and heat. Adding thermal energy excites the electrons in the gold atoms, causing subtle shifts in the way light is absorbed and reflected.
As gold is initially heated, it maintains its shiny yellow color but becomes slightly lighter and less saturated. At around 1,064°C (1,948°F), gold begins to melt and turn orange in color. The orange color results from increased vibration of the gold atoms and subtle changes in the electronic structure of the material.
Heating gold even further causes more dramatic color changes. At around 2,500°C (4,500°F), the vibration of the gold atoms becomes so excited that the electrons emit a significant amount of thermal radiation. This causes the color to shift to a bright whitish yellow.
Purple Gold
One of the most interesting color changes occurs when gold is heated to around 2,850°C (5,150°F). At this temperature, the electrons in the gold atoms shift position, absorbing and reflecting light in a new way. This causes gold to take on a vivid purple color.
The purple color results from a change in the electron configuration of the gold atoms. Some of gold’s outer electrons shift to higher energy levels, leading to absorption of green and yellow light. The remaining reflected light takes on a predominant purple or violet hue.
This vivid purple version of gold is sometimes called amethyst gold, due to its resemblance to the purple color of amethyst gemstones. The color change is only temporary, however – if the gold cools down, the electrons return to their original lower energy levels and the familiar yellow color reappears.
Blue and Red Gold
At even higher temperatures between 3,000-4,000°C (5,400-7,200°F), gold can take on a red, orange-red, or even blue color. This is again caused by shifts in the gold’s energetic electron structure.
The blue color results when gold absorbs the entire visible spectrum except for blue light. This gives the reflective color a rich blue appearance. Red gold occurs when the opposite effect takes place – gold absorbs blue light while reflecting longer red wavelengths of light.
These colors arise due to gold’s unique optical properties. In very specific temperature ranges, gold’s electrons can exist in configurations that select for blue or red wavelengths of visible light. A mere 50°C difference in temperature can flip gold between appearing red or blue.
Conclusion
Heating gold to varying high temperatures can reveal a rainbow of colors that differ wildly from its ordinary yellow hue. While pure gold generally appears some shade of yellow at typical temperatures, intense heat provides enough energy to excite gold’s electrons into new arrangements that reflect and absorb light in unconventional ways.
The sequence of color changes includes first lighter yellows and oranges, then temporary purple phases, followed by blue and red shades at extreme temperatures. While these colors are often fleeting effects, they demonstrate gold’s chemical versatility and reveal insights into the intricate quantum physics governing how atoms interact with light.
Frequently Asked Questions
Why does gold change color when heated?
Gold changes color when heated because heating provides energy to the electrons in the gold atoms, causing them to move to higher energy levels. This changes how light interacts with the electrons, producing different colors.
What temperature does gold turn purple?
Gold turns a vivid purple color at around 2,850°C (5,150°F). This purple color results when some electrons shift position within the gold atoms, absorbing green and yellow light.
Does gold change back to yellow when cooled?
Yes, gold reverts back to its original yellow color when cooled down after being heated. The excited electrons return to their normal stable configuration, reflecting yellow light again.
Can gold turn blue or red?
Yes, at very high temperatures between 3,000-4,000°C, gold can briefly appear blue or red before melting. These colors are produced when gold’s electron configuration causes it to absorb or reflect only certain wavelengths.
Is the color change permanent?
No, the color changes in heated gold are temporary effects that only occur at certain temperature ranges. Once gold is cooled, its original yellow color returns as the electrons return to their ground state.
Scientific Explanation
The color changes that gold undergoes when heated can be explained by principles of physics and chemistry:
Electron Excitation
Heating gold provides thermal energy that excites electrons in the gold atoms to move to higher energy levels further from the atomic nucleus. This affects how light interacts with the electrons.
Photon Absorption/Reflection
Excited electrons can absorb different wavelengths of photon light energy. The absorbed colors do not reflect back to our eyes. The remaining reflected light gives gold its heated color.
Band Gap Shifting
In metals like gold, heating causes electron band gaps to shift based on temperature. This changes the energy required for an electron to transition between bands, altering light absorption.
Lattice Vibration
Thermal vibration of the crystalline gold lattice also contributes to electron excitation and band gaps shifting, facilitating color change.
Quantum Effects
Color change results from intricate quantum interactions between gold’s electrons and photons of light. Quantum physics governs how matter and light behaved at the atomic scale.
Chemical Composition
Pure gold is composed of the elemental metal gold with the chemical symbol Au and atomic number 79 on the periodic table of elements. Gold has an electron configuration of [Xe]4f14 5d10 6s1.
When heated, these 79 electrons in the gold atoms become excited to higher energy levels, changing how the electrons interact with visible light to produce different colors.
Key Properties
- Atomic number – 79
- Atomic weight – 196.967 amu
- Melting point – 1064°C
- Boiling point – 2808°C
- Density – 19.32 g/cm3
- Phase at room temperature – Solid
Heating Methods
There are several ways that gold can be heated to achieve the high temperatures necessary to produce color change:
Bunsen Burner Flame
A simple Bunsen burner laboratory flame can reach over 1,000°C and melt gold into an orange color.
Torch Heating
Handheld propane or butane micro-torches can heat gold over 2,000°C to achieve purple or blue tones.
Electric Heating
Electric furnaces, kilns, and induction heaters allow precise temperature control to elicit gold’s full range of colors.
Laser Heating
High-powered lasers can instantaneously heat a gold surface beyond its boiling point, causing color shifts.
| Heating Method | Typical Temperature Range | Color Achieved |
|---|---|---|
| Bunsen Burner | 500-1500°C | Orange |
| Torch Heating | 1000-2500°C | Purple |
| Electric Furnace | 2000-4000°C | Blue/Red |
| Laser Heating | 3000°C+ | Full Range |
Industrial Applications
The color changes displayed by heated gold have some useful industrial applications, including:
Jewelry Annealing
Jewelers commonly heat gold alloy jewelry to a faint orange color to soften or anneal the material for further working and shaping.
Gold Refining
Heating gold ore to high temperatures is an essential step in smelting and melting out pure gold for extraction and refining.
Glass Coloration
Purples and reds produced by heated gold can be used to tint glass or create ruby-colored stained glass.
Temperature Measurement
The sequence of color changes can act as a visual gauge to estimate the temperature of molten gold.
Gold Synthesis
Intense heating is sometimes used in chemical reactions that synthetically produce elemental gold.
Appearance in Nature
While pure natural gold generally retains its yellow color, there are some rare forms found in nature where gold takes on hints of other colors due to impurities or crystal structures:
Rose Gold
A gold and copper alloy with a rosy pink hue due to the copper content.
Black Gold
Natural gold containing black manganese oxides that give it a dark, sooty black appearance.
Blue Gold
Tiny amounts of iron in the crystal structure of gold can give it a bluish cast.
Purple Gold
Trace amounts of aluminum or iridium impurities may impart a purple tint.
Red Gold
Iron oxide minerals mixed with gold after volcanic activity can color it red.
Key Takeaways
- Heating gold causes color changes from lighter yellows to vivid oranges, purples, blues and reds due to electron excitations.
- These colors result from changes in how gold’s electrons absorb and reflect light at different temperatures.
- The colors are temporary effects and gold reverts back to yellow when cooled.
- Color changes start above 1,000°C and become most dramatic over 2,500°C.
- Quantum physics governs how heated gold interacts differently with photons of light.