In the context of relationships, chemistry is a simple "emotion" that two people get when they share a special connection. It is not necessarily sexual. It is the impulse making one think "I need to see this [other] person again" - that feeling of "we click". It is very early in one's relationship that they can intuitively work out whether they have positive or negative chemistry.
While the actual definition of chemistry, its components, and its manifestations are fairly vague, this is a well documented concept. Some people describe chemistry in metaphorical terms, such as "like cookie dough and vanilla ice cream", or "like a performance". It can be described in the terms of mutual feelings - "a connection, a bond or common feeling between two people", or as a chemical process - "[it] stimulates love or sexual attraction...brain chemicals are definitely involved". While chemistry has been described as "that romantic spark between [two people]", the term "spark" in the context of relationships is as vague as "chemistry", and therefore is not particularly useful in a definition. It has also been described as "intangible, unspoken [and] energetic". Chemistry is an unconscious decision, informed by a complex blend of criteria.
Some of the core components of chemistry are: "non-judgment, similarity, mystery, attraction, mutual trust, and effortless communication" Chemistry can be described as the combination of "love, lust, infatuation, and a desire to be involved intimately with someone".
Research by Kelly Campbell, Ph.D., suggests that "not everyone experiences chemistry". She decided that "chemistry occurred most often between people who are down-to-earth and sincere". This is because "if a person is comfortable with themselves, they are better able to express their true self to the world, which makes it easier to get to know them...even if perspectives on important matters differed." Sharing similarities is also deemed essential to chemistry as "feeling understood is essential to forming relational bonds."
In general terms, there are 3 main types of chemistry, which are defined in terms of the nature of the rapport between the respective people:
- Good chemistry - good rapport
- No chemistry - find it hard to create rapport
- Bad chemistry - have no rapport, or have negative rapport
The various manifestations of chemistry are: sexual chemistry, romantic chemistry, emotional chemistry, activity chemistry, team performance chemistry, creative chemistry, intellectual chemistry, and empowerment chemistry".
There are various psychological, physical and emotional symptoms of having good chemistry with another person. It has been described as a "combination of basic psychological arousal combined with a feeling of pleasure". The nervous system gets aroused, causing one to get adrenaline in the form of "rapid heartbeat, shortness of breath, and sensations of excitement that are often similar to sensations associated with danger". Other physical symptoms include "blood pressure go[ing] up a little, the skin...flush[ing], the face and ears...turn[ing] red and...[a] feeling of weakness in the knees". One can feel a sense of obsession over the other person, longing for "the day [when they return] to that person". One can also uncontrollably smile whenever thinking about the other person.
There is some debate over whether one can artificially create chemistry if they are "not initially feeling it". While some people hold that it is something that you "can't learn and can't teach...[and you] either have...or you don't", others hold that chemistry is a process rather than a moment, "build[ing] up and adds up and eventually you get this kind of chemical bonding". Some people, while believing it is possible to artificially create chemistry, think that it is better to let chemistry hit them spontaneously.
In Western Society, chemistry is generally considered the "igniter [and] catalyst for the relationship", i.e., without this chemistry, there can be no relationship. Having chemistry "can be the difference between a relationship being romantic or platonic". Chemistry "can cause people to act sexually impulsively or unwisely". It can also be the difference between someone remaining faithful in their relationship, and seeking one night stands and affairs.
Romantic chemistry can be one of the most dangerous and self destructive emotions if left unchecked. Some people will enter relationships with incompatible mates blinded by chemistry. Chemistry often seems to have the power to blind us. Chemistry is the reason the saying, "Love is blind," exists. Chemistry can make otherwise rational people ignore serious problems and issues in an individual and relationship. Chemistry often blinds people to warning signs that a person or relationship is not healthy or the right one for them.— Elizabeth Baldwin, 
Importance to relationship satisfaction
Dating coach Evan Marc Katz suggests that "chemistry is one of the most misleading indicators of a future relationship. Chemistry predicts nothing but chemistry." This is because chemistry can make people blind to actual incompatibilities or warning signs. Psychologist Laurie Betito notes that arranged marriages actually do quite well in terms of relationship satisfaction, and this is because "a spark can build based on what you have in common. You can grow into love, but you grow out of lust."
Neil Clark Warren argues that physical chemistry is important because "couples who don't share strong chemistry may have additional problems during the ups and downs of a life together." Like Betito, he suggests not ruling someone out on the first date due to lack of chemistry. "But," he adds, "if by the second or third date you don’t feel a strong inclination to kiss the other person, be near him, or hold his hand, you’re probably never going to feel it." April Masini likewise says that chemistry is a strong predictor of relationship success. She suggests that chemistry comes and goes, and it's important to actively cultivate it because it can help couples deal with future conflicts.
- Baldwin, Elizabeth (November 21, 2007). "Spark of Chemistry in a Romantic Relationship: Organic or Developed?". Retrieved September 11, 2012.
- Neumann, Kimberly Dawn. "How Much Does Chemistry Count?". Retrieved September 11, 2012.
- Campbell, Kelly. "More Than Chemistry". Retrieved September 11, 2012.
- "What's the Definition of Chemistry in a Relationship – Is it Love?". Retrieved September 11, 2012.
- "The 3 C's of Relationship: Chemistry, Compatibility & Commitment". Retrieved September 11, 2012.
- Livingwell, Joy (June 9, 2010). "Relationship chemistry: What is it? How does it work?". Retrieved September 11, 2012.
- Johnston, Susan. "No Spark? Give It Another Chance!". Happen Magazine. Retrieved 30 May 2014.
- Warren, Neil Clark. "How long should I wait for chemistry?". eHarmony Advice. Retrieved 30 May 2014.
- Mandell, Judy. "Does Chemistry Equal Relationship Compatibility?". Happen Magazine. Retrieved 30 May 2014.